The Astro Navigation Resource

See the latest post:  Pillars of the Sky (Learning from the Polynesians).

Although this website aims to promote the Astro Navigation Demystified series of books, it is hoped that it will also provide a useful resource for navigators, scholars and students of the subject.

A wealth of iEARTH AND SUN IN THE SPHEREnformation on the subject of astro navigation can be found under the various headings on the menu bar at the top of the page and in the archives listed down the right. The images below give links to various pages which may be of interest.

Why Astro Navigation?  There is rapidly growing interest in the subject of astro navigation or celestial navigation as it is also known. It is not surprising that, in a world that is increasingly dominated by technology and automation, there is an awakening of interest in traditional methods of using the celestial bodies to help us to navigate the oceans.

Astro navigation is not just for navigators; the subject is an interwoven mix of geography, astronomy, history and mathematics and should appeal to both mariners and scholars alike.

Russia is one of the few countries in the worlaltitude and azimuthd to acknowledge the educational value of astro navigation and to include it as an important part of the school curriculum. In other countries, institutions such as nautical schools and maritime colleges include the subject in their curricula as a subject in its own right while for some independent schools, it provides the perfect theme for integrated studies and open-ended project work.

The question is often asked: ‘how could seafarers navigate the oceans if the global positioning system (GPS) failed? The answer is quite simple; they could revert to the ‘fail-safe’ art of astro navigation. The problem here though, is that we have become so reliant on automated navigation systems that traditional methods are being forgotten.  Even so, there is a very realPZX TRIANGLE danger that the GPS could be destroyed.  During periods of increased solar activity, massive amounts of material erupt from the Sun. These eruptions are known as coronal mass ejections and when they impact with the Earth they cause disturbances to its magnetic field known as magnetic storms. Major magnetic storms have been known to destroy electricity grids; shut down the Internet, blank out communications networks and wipe out satellite systems (including the global positioninplot 3g system).

Couple this danger with that posed by cyber terrorists who could block GPS signals at any time, then it can easily be seen that navigators who rely solely on electronic navigation systems could be faced with serious problems.

cross

Unfortunately, many sea-goers are deterred from learning astro navigation because they perceive it to be a very difficult subject to learn. In fact, it is very interesting and easy to learn but sadly, some writers and teachers of the subject attempt to disguise its simplicity by cloaking it in an aura of mystery.

 

 

 

I am throughly enjoying working through the wonderful book, ‘Astro Navigation Demystified’. At last a well written book on the subject. I was also very pleased to find this accompanying website.

 

Where to buy books of the Astro Navigation Demystified series:

Astro Navigation Demystified at Amazon.com

Astro Navigation Demystified at Amazon.uk

fullkindlecover3

Applying Mathematics to Astro Navigation at Amazon .com

Applying Mathematics to Astro Navigation at Amazon .uk

http://www.amazon.com/Applying-Mathematics-Astro-Navigation-Demystified/dp/1496012062/ref=sr_1_2?s=books&ie=UTF8&qid=1393696809&sr=1-2&keywords=astro+navigation

 

 

 

 

Astronomy for Astro Navigation at Amazon.com

Astronomy for Astro Navigation at Amazon.uk

http://www.amazon.com/Astronomy-Astro-Navigation-Black-Demystified/dp/1511675594/ref=sr_1_2?s=books&ie=UTF8&qid=1446153840&sr=1-2&keywords=astro+navigation+demystified

 

 

 

 

Celestial Navigation at Amazon.com

Celestial Navigation at Amazon.uk

celestial_navigation_cover_for_kindle-1

web: http://www.astronavigationdemystified.com

email: astrodemystified@outlook.com

Posted in Uncategorized

Pillars of the Sky (Learning from the Polynesians).

The last post in this series discussed the Polynesian’s Star Compass and their use of ‘steering stars’ for direction finding.  Although the Polynesians made extensive use of a form of dead reckoning to estimate position, as far as we know, they did not have a method of fixing a vessel’s position at sea.  If they did, we will never know because their methods were closely guarded secrets which were known only to elite groups of navigators and were never recorded.  However, David Lewis, in his book ‘We The Navigators’ discusses how early Polynesian navigators pin-pointed the position of certain islands by what they called the ‘Star on Top’.

If the latitude of a certain island coincides with the declination of a star, it stands to reason that when the star crosses the meridian of that island, it will be immediately above it.  So, to an observer on the island, the star will be overhead when it reaches its zenith; in other words, it will be the ‘Star on Top’.  It was believed that the ‘on top’ stars for all the islands were held up in the sky by pillars and that the sky was supported by these pillars which were known as ‘Pillars of the Sky’.

So, how would the Polynesian navigators have used an island’s ‘star on top’ to help them to navigate towards it?  Let’s try an example:

Palmyra is a tropical Atoll located roughly half way between Hawaii and Samoa.  Although it has no indigenous population now, it may well have had in the past and its position would have made it a suitable waypoint for voyages between Hawaii and the South Pacific islands, particularly since it has an abundant supply of fresh water, coconut palms, many species of nesting birds and lagoons teeming with fish. It is highly likely therefore, that ancient Polynesian wayfarers would have made voyages between Palmyra and Hawaii in their large outrigger canoes. We will use such a voyage for this example.

When Polynesian exploration was at its height around one thousand years ago, the declination of Arcturus was just to the north of Hawaii but due to precession, it slowly moved south and now sits above the southern tip of Hawaii (19.2oN). So, for many centuries, Arcturus would have been the ‘star on top’ for Hawaii and could still serve that purpose today.

Hawaii lies about 1,000 miles to the north of Palmyra and so the obvious course to steer from Palmyra to Hawaii would seem to be north.  However, for two reasons this would not have been the chosen course.

Firstly, for the first part of the voyage, the Equatorial Counter Current would set the canoe eastwards, but from about half way, the North Equatorial Current would set it westwards again.  Assuming that the voyage was conducted during Hawaii’s summer months, the prevailing Trade Winds would also set the vessel westwards.  So, taking the winds and currents into account, the navigator would have to lay off a course to the east of North.

The second reason is this.  If the navigator sailed north until Arcturus was directly overhead at its zenith, all he would be able to tell from this would be that his canoe was on the same latitude as Hawaii but he would not know if he was to the east or the west of the island.  If his chosen course caused the boat to finish downwind of the island, he would then have the difficult task of beating against the wind and tide to reach his goal.  However, if he if he deliberately steered a course that would take him upwind of the islands, he would then be able to sail downwind while maintaining latitude by keeping Arcturus ‘on top’.  This technique of deliberately steering so as to finish upwind of the target island was called ‘Windward Landfall’.

A star compass which shows the rising and setting points of the stars which would likely to be of help for a voyage from Palmyra to Hawaii has been constructed below.

FANCY COMPASS2

The navigation plan for the first part of the voyage would probably be to sail from Polymyra on a course of roughly NNE keeping Polaris on the port bow with the Southern Cross on the starboard quarter during the hours of darkness. (Note that the Southern Cross is not circumpolar north of 34o South and that Acrux, its brightest star, would rise at roughly SSE). Rigel Kentaurus and Hadar in the constellation Centaurus rise on approximately the same bearing as Acrux but shortly after it.  These two stars are known as the ‘Pointers’ because an imaginary line from Rigel Kentaurus to Hadar will point towards the Southern Cross.

The direction from which the North East Trade winds blow fluctuates between NE and and ENE so with any luck, the canoe would be able to complete the whole of this leg of the journey on the starboard tack without the need to beat upwind.  The heading would be checked by aligning the canoe with the star Dubhe when it rose at approximately NNE. (Dubhe is in the constellation Ursa Major (Great Bear) and is not circumpolar south of 38o North).  At the time of aligning the canoe with Dubhe, the angle between the direction of the advancing waves and the fore and aft line of the canoe would be noted and the navigator would use this information for guidance during daylight hours.

When a point was reached where Arcturus (declination  19.2oN) was immediately overhead at its zenith, the course would be changed to westerly for the second part of the voyage to sail downwind to Hawaii.

You will see from the star compass that the stars Alnilam and Altair set approximately due west and so they would make suitable ‘steering stars’ for this part of the voyage.  Alnilam is a winter star and Altair is a summer star so one of them will always be visible at night. The plan for the second part of the voyage would probably be to use Alnilam or Altair as the ‘steering star’ while keeping Polaris on the starboard beam and the Southern Cross to port. The latitude of Hawaii would be maintained by keeping Arcturus ‘on top’.

Quite how one can tell what point is ‘immediately overhead’ from a canoe which is rocking and rolling in a choppy sea is not clear but according to Lewis, the Polynesian navigators had several secret methods such as lying in the bottom of the canoe facing upwards.  Another method was the ‘floating cane’ which Lewis vaguely describes.  Apparently, a cane would be cut below two consecutive growth rings so that a short length of cane which was sealed at one end and open at the other was obtained.  A small weight would be attached to the sealed end and the cane would then be filled with water.  In theory, when the cane was placed in a container of water, it would remain vertical in spite of the movement of the canoe.

Probably, for Polynesian navigators, the first indication that the canoe was approaching land would be the sighting of certain birds such as terns, noddies, boobies and frigate birds which are land-based and therefore fly out from the land in the mornings and return to it in the evenings thereby giving the navigator indications not only of the nearness of land but also its direction.  Pelagic species such as the albatross which roam freely over the open ocean would obviously be of no navigational use and so it would have been important to have the ability to recognise the different species.

Clouds would have been another indication of approaching land.  David Lewis gives an in-depth discussion of ‘cloud lore’ which was developed by Polynesian sailors over many centuries.  There is no space here to discuss this topic fully but there are a few useful tips that modern day navigators could take from the Polynesians.  Firstly, although an island may be below the horizon, clouds above it may be visible.  Drifting clouds tend to slow down and ‘stick’ over an island for a while and then pick up speed again.  Some islands, particularly those with mountains or volcanoes will often appear to have a permanent cloud above them as moist air  rising above them condenses and then evaporates again as it descends.

*Ref. Lewis, David, 1972. ‘We the Navigators’, Honolulu: The University Press of Hawaii.

Where to buy books of the Astro Navigation Demystified series:

Astro Navigation Demystified at Amazon.com

Astro Navigation Demystified at Amazon.uk

fullkindlecover3

Applying Mathematics to Astro Navigation at Amazon .com

Applying Mathematics to Astro Navigation at Amazon .uk

http://www.amazon.com/Applying-Mathematics-Astro-Navigation-Demystified/dp/1496012062/ref=sr_1_2?s=books&ie=UTF8&qid=1393696809&sr=1-2&keywords=astro+navigation

Astronomy for Astro Navigation at Amazon.com

Astronomy for Astro Navigation at Amazon.uk

http://www.amazon.com/Astronomy-Astro-Navigation-Black-Demystified/dp/1511675594/ref=sr_1_2?s=books&ie=UTF8&qid=1446153840&sr=1-2&keywords=astro+navigation+demystified

Celestial Navigation at Amazon.com

Celestial Navigation at Amazon.uk

celestial_navigation_cover_for_kindle-1

web: http://www.astronavigationdemystified.com

email: astrodemystified@outlook.com

Posted in astro navigation, Astro Navigation Demystified, astronomy, celestial navigation, Polynesian Navigators, Survival | Tagged , , , , ,

Survival – The Star Compass (Learning from the Polynesians)

Updated version.

Know the stars and you will always have a compass”  (The Revenant) *

Nainoa Thompson tells us how that, for centuries before European sailors reached the Pacific Ocean, the South Sea Islanders accurately found their way from island to island without the aid of magnetic compasses, sextants or any other navigational equipment. They navigated the Pacific by using their knowledge of natural phenomena such as the directions of the winds and waves, the flight paths of birds, cloud formations and the colour of the sea.  Their most important technique however, was the ‘Star Compass’ which was not a physical tool but a mental construct based on their knowledge of the directions that certain stars would rise and set.

Such knowledge was acquired over many hundreds of years and passed down by word of mouth and example with each generation committing it to memory.  It is a tragedy that because the navigational knowledge and techniques of the early Polynesians and Melanesians were not recorded, they have largely been forgotten or would have been if it were not for the enthusiastic work of Nainoa Thompson,  Mau Piailug and others.

In modern times, the majority of navigators rely on GPS to find their way although many still keep the traditional art of Astro (Celestial) Navigation alive.  How many could navigate the oceans without either of these methods though?

It is well known that we could be deprived of the GPS at any moment for many reasons including the ever increasing threat of cyber attack, coronal mass ejections, equipment failure or shipwreck.  How would we cope in such situations?  Would it not be a good idea to construct and then memorise our own star compasses for the latitudes at which we sail?  This would not be as difficult as it may seem at first.  Let us try an example:

Sailor Jerry plans to conduct an experiment by sailing from Fogo Island, Newfoundland (49.8oN) to Mullion Cove (50.02oN) in the British Isles without the aid of GPS or any other navigational equipment, not even a magnetic compass (no jokes about rum line sailing please). He plans to to sail due East along parallel 50oN  using only a star compass until he makes landfall, hopefully at one of the following: the Scilly Isles (49.9oN), Lands End (50.1oN) or Lizard Point (49.95oN).  Once he has made landfall, he will be able to set a course for Mullion Cove.

Jerry is aware that strong ocean currents in the North Atlantic will make it difficult for him to stay on track; at the beginning of his voyage, he will be pushed south by the Labrador and Irminger currents, later he will be pushed east and then northeast by the North Atlantic Current and then he may get caught by the Canary Current which will push him south east. To add to his difficulties, he knows that although the star compass will provide him with directional information, it will not help him to find his position. To overcome these problems so that he can try to stay on track, he plans to frequently calculate his latitude from the North Star by the method explained here.  It is likely that he will not have a sextant in a survival situation so he plans to measure the altitude of the North Star using a home made clinometer for that purpose.

The method he uses to construct the star compass is simple.  Firstly, he needs to select several bright stars which he could use to guide him in the right direction (preferably, stars with a magnitude of 1 or less).  The lower the star is to the horizon, the better it is to indicate direction so his next step is to calculate the azimuth of the chosen stars when they rise and set from his latitude of 50oN.

He doesn’t have time to observe and memorise the directions of the stars as the South Sea Islanders did so he allows himself the luxury of a nautical almanac to help him with this task.The following table shows the data that Jerry collected to help him to construct his star compass. table2Notes:  1. To make his calculations of azimuth, he uses the method explained here).

2.To calculate the azimuth of a star when it is on or just above the horizon, Jerry uses an LHA of approximately 271o for dawn and 089o for dusk.  (The method of calculating a star’s LHA is shown at step 1  here).

3. Bearings of the Sun at sunrise and sunset are included in the table but only for the equinoxes and the solstices since these are the only four dates on which we can be sure of the Sun’s declination without the aid of an almanac. This information is not included in Jerry’s star compass on this occasion because he will be making the voyage in August.

Below is a diagram of the star compass that Jerry constructed.  Because he wants to steer an easterly course, he has selected stars that rise between northeast and southeast and set between northwest and southwest.  The North Star (Polaris) and Kochab in Ursa Minor (Little Dipper) are also shown to help him find the direction of north.  
star-compass3

Additional Links: Star Compass 1,    Survival Astro Navigation  Predicting rising and setting times of stars,  Locating the North Star,  What is Astro navigation?

*Reference for the “know the stars” quote: Michael Punk. 2002. The Revenant

Where to buy books of the Astro Navigation Demystified series:

Celestial Navigation at Amazon.com

Celestial Navigation at Amazon.uk

Astro Navigation Demystified at Amazon.com

Astro Navigation Demystified at Amazon.uk

Applying Mathematics to Astro Navigation at Amazon .com

Applying Mathematics to Astro Navigation at Amazon .uk

Astronomy for Astro Navigation at Amazon.com

Astronomy for Astro Navigation at Amazon.uk

web: http://www.astronavigationdemystified.com

e: astrodemystified@outlook.com

Posted in astro navigation, Astro Navigation Demystified, Astro Navigation Topics, astronomy, celestial navigation, earth science | Tagged , , , ,

Astro Navigation In A Nutshell Part Four

Part 4 – Full procedure for establishing an astronomical position line.

This post brings together all of the information from parts 1, 2 and 3 of this series to demonstrate the full procedure for establishing an astronomical position line.

.Links:  Astro Navigation In A Nutshell Part One

Astro Navigation In A Nutshell Part Two

Astro Navigation In A Nutshell Part Three

Please note.  There is not sufficient scope in this post to fully explain this topic; however, there are in-depth expositions in my books ‘Astro Navigation Demystified’ and ‘Celestial Navigation – The Ultimate Course’.

Demonstration of the procedure:

Scenario:

Date: 18 July

D.R. Position at Zone Time: 16h 44m:  52N   21o  43.1’W.

Time Zone +1

Deck Watch Time (DWT): 17h 50m 28s.

Deck Watch Error (DWE) 40s fast (-40s)

Body observed:  Sun lower limb.

Sextant Altitude at true position:  32o 10.’4  =  32o.173

Compass Bearing at true position: 261o  (for rough check on azimuth)

Index error:  +0’.54.     Ht. of eye:  8m.

Temperature:  28oC.  Pressure:  991mb.

Step 1. Note Lat and Long of DR Position.
Lat:   52o N
Long: 21o  43.1’W
Step 2. Calculate PZ. (90 – Lat).

PZ = 90o – 52o  =  38o

 
Step 3.  Calculate Greenwich Date at time of observation.
Date:  18 July
Zone time:                          16h 44m
Zone correction:                +1h
Universal Time (GMT):     17h 44m
Deck watch time:               17h 50m 28s
Deck watch error:                            -40s
Greenwich date:          18d 17h 49m 48s July
Step 4. Calculate Greenwich Hour Angle and Declination.
Date: 18 July
                             GHA                Dec
UT 17h                73o 26’.1          N20o 54’.7 (d:0’.5 decreasing)
Inc. 49m 48s:    12o 27’.0                   -0’.4
                             85o 53’.1          N20o 54’.3
                         = 85o.885        = N20o.9
 
Step 5.  Determine if Lat and Dec are ‘Same’ or ‘Contrary’.
Lat = N

Dec = N

Therefore Lat and Dec are same.

Step 6. Calculate PX

(Lat and Dec Same therefore PX = 90 – Dec).

PX = 90o – Dec. =  90o – 20o.9  =  69o.1

 
Step 7. Calculate the Local Hour Angle (LHA). (Longitude combined with GHA should equal LHA as a whole number of degrees).
DR Long: 2143.1’W
GHA:                     85o.885
DR Long:              21o.718 West (-)
LHA:                      64o .167
Step 8. Determine Angle ZPX.

ZPX = LHA = 64o .167

 
Step 9. Calculate True Altitude at True Position (Observed altitude corrected for IE, Dip, Parallax and Refraction).
Sextant Altitude     =              32o 10′.40

Index error (IE)  =                        +0′.54

Observed Altitude =                32o 10′.94

Dip (ht. 8m.)         =                        -5′.00   (table 6a)

Apparent Altitude =                 32o 05′.94

Altitude correction  =                   +14′.50   (table 6d)

Added refraction (28o/991mb) =    +0′.10   (table 6c)

True Altitude =                        32o 20′.54  = 32o.34

Step 10. Calculate Zenith Distance at True Pos. (90o – Altitude).
Zenith Dist = 90o – 32o.34  = 57o.66  = 3459.6′
 
Step 11. Calculate Zenith Distance at DR Position. (ZX).
Lat. = 52oN

Declination =  N20o.9   (From Step 4)

Lat and Dec Same (From Step 5)

ZPX = 64o .167     (From Step 7)

PZ = 38o    (From Step 2)

PX = 69o.1  (From Step 5)

Cos (ZX) =  [Cos(PZ) x Cos(PX)] + [Sin(PZ) x Sin(PX) x Cos(ZPX)]

=  [Cos(38) x Cos(69.1)] + [Sin(38) x Sin(69.1) x Cos(64.167)]

= 0.53
ZX = Cos-1 (0.53)  =  57o.99

Zenith Distance at DR position =  57o.99 = 3479′.4

 
Step 12. Calculate Azimuth at DR Position (PZX)
Cos PZX = Cos(PX) – [Cos(ZX) x Cos(PZ]

[Sin(ZX) x Sin(PZ)]

Cos PZX = Cos(69.1) – [Cos(57.99) x Cos(38]

[Sin(57.99) x Sin(38)]

= -0.119
PZX = Cos-1(-0.119)  =  96o.8  ≈ 97o

Azimuth at DR position = 97o

 
Step 13. Convert azimuth angle to true bearing (ZN):
Rules for converting Azimuth (PZX) to True Bearing (Zn)
                Lat. North                              Lat. South
LHA>180o Zn = Z                Zn =  180o – Z
LHA<180o   Zn = 360o-Z      Zn =  180o + Z   
DR Lat.  =   52o.0N
Azimuth  (Z)   =   97o       (from step 12)
LHA                =   64o       (from step 7)
Therefore ZN = 360o – 97o  = 263o

Therefore true bearing of body at DR position = 263o

Compass Bearing at true position: 261(for rough check on azimuth)

 
Step. 14. Note observed compass bearing at true position and compare with true bearing at DR position for rough check.
Observed compass Bearing at true position: 261o

True Bearing of body at DR position = 263o

Step 15.  Calculate intercept.

Reminder: Subtract the ZD at the true position (a) from the ZD at the DR position (b).

  • If the result is positive, the intercept is towards the azimuth.
  • If the result is negative, the intercept is from the azimuth.
a. Zen. Dist. at DR Pos.        57o.99                                     (from step 11)
b. Zen. Dist. at True Pos.     57o.66                          (from step 10)
Intercept = a – b =                +0o.33                                 
Convert to minutes:               +19.8′
Azimuth:                                     263o
Intercept:                             19.8 nautical miles to 263o
Step 16.  Plot the position line.  (Reminder: Plot intercept from DR position along azimuth line).

Latitude:   52o 00’N

Longitude:  21o 53’W

Intercept  = 19.8 n.m. towards 263(From step 15)

Note. Drawing not drawn to scale.

INSERT PLOT

single-intercept2amendment

This topic is explained in far greater depth in my books ‘Astro Navigation Demystified’ and ‘Celestial Navigation – The Ultimate Course’.

Many thanks to Jeremy Parker for his help with this post.

Where to buy books of the Astro Navigation Demystified series:

Celestial Navigation at Amazon.com

Celestial Navigation at Amazon.uk

Astro Navigation Demystified at Amazon.com

Astro Navigation Demystified at Amazon.uk

Applying Mathematics to Astro Navigation at Amazon .com

Applying Mathematics to Astro Navigation at Amazon .uk

Astronomy for Astro Navigation at Amazon.com

Astronomy for Astro Navigation at Amazon.uk

web: http://www.astronavigationdemystified.com

e: astrodemystified@outlook.com

Posted in astro navigation, Astro Navigation Demystified, Astro Navigation Topics, celestial navigation, Marine Navigation, navigation | Tagged , , ,

Astro Navigation In A Nutshell Part 1

Part 1 – The Importance of Azimuth and Altitude.  The theory of astro navigation depends on the ability to solve the spherical triangle PZX in the diagram below.  The azimuth and altitude enable us to calculate the Local Hour Angle (LHA), declination and zenith distance of a celestial body which is the essential data that we need to calculate a position on the Earth’s surface.

diag23new drawing3

 

The Azimuth is the angle PZX in the diagram (it is, the angle between the observer’s celestial meridian and the vertical circle through the celestial body).  The Azimuth is similar to the bearing in that it is the angle between the observer’s meridian and the direction of the celestial body.  However, whereas bearings are measured clockwise from north from 0o to 360o, azimuth is measured from 0o to 180o from either north or south.  If the observer is in the northern hemisphere, the azimuth is measured from north and if in the southern hemisphere, it is measured from south.   For example, if the true bearing is 045o, in terms of azimuth it is either N45oE for an observer is in the northern hemisphere or S135oE for an observer in the southern hemisphere.

Local Hour Angle (LHA). LHA is the angle ZPX; that is the angle between the observer’s celestial meridian and the meridian of the celestial body.

Relationship between LHA and Azimuth.  When the LHA is less than 180o, the celestial body lies to the west of the observer’s meridian and when it is greater than 180o it lies to the east.  (Remember LHA is measured westwards from the observer’s meridian).     It follows that if the celestial body is to the west of the observer’s meridian, the azimuth must be west and when to the east, the azimuth must be east.                                                             So we have the rule:                                                                                                                                            LHA = 0o – 180o : Azimuth West.       LHA = 180o – 360o : Azimuth East

The Altitude is the angle AOX in the diagram; (it is the angle from the celestial horizon to the celestial body measured along the vertical circle through the celestial body).

The Zenith Distance is the angular distance ZX measured along the same vertical circle from the zenith to the celestial body; that is the angle XOZ.

 Relationship between Altitude and Zenith Distance   Since the celestial meridian is a vertical circle and is therefore, perpendicular to the celestial horizon, it follows that angle AOZ is a right angle and angles AOX and XOZ are complementary angles.  From this we can deduce that:                                                                                                                                          Zenith Distance = 90o – Altitude  and    Altitude = 90o – Zenith Distance

 Example.    Suppose that from our true position, the calculated altitude of a celestial body is 32o.34 and the compass bearing is 261 o.                                                                                               As explained above, Zenith Distance = 90o – Altitude                                                                   Therefore zenith distance = 90o – 32o.34 = 57o.66 = 3,459.6′

Since 1 minute of arc subtends a distance of 1 nautical mile on the Earth’s surface, we know that the distance of the true position from the geographical position of the celestial body will be 3,459.6 nautical miles in the approximate direction of the compass bearing 261 o which when converted to azimuth is N99oW for an observer in the northern hemisphere.  (More on this in the next post).

However, because of the impracticability of plotting a position line of such an immense distance on a large scale navigational chart, this does not tell us our exact position.

The next post in this series will explain how we can establish an astronomical position line by calculating the difference between the zenith distances of the DR position and the true position.

Please Note.  This topic is explained in far greater depth in my books ‘Astro Navigation Demystified’ and ‘Celestial Navigation – The Ultimate Course’.

Where to buy books of the Astro Navigation Demystified series:

Celestial Navigation at Amazon.com

Celestial Navigation at Amazon.uk

Astro Navigation Demystified at Amazon.com

Astro Navigation Demystified at Amazon.uk

Applying Mathematics to Astro Navigation at Amazon .com

Applying Mathematics to Astro Navigation at Amazon .uk

Astronomy for Astro Navigation at Amazon.com

Astronomy for Astro Navigation at Amazon.uk

web: http://www.astronavigationdemystified.com

e: astrodemystified@outlook.com

 

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Astro Navigation In A Nutshell Part 2.

Part 2 – The Intercept Method

Link:  Astro Navigation In A Nutshell Part One

Suppose we are in a yacht and we measure the altitude of the Sun and find it to be 35o; what does this tell us?  All that we know is that the yacht lies somewhere on the circumference of a circle centred at the geographical position of the Sun.  Such a circle is known as a ‘position circle’ since our position is known to lie somewhere on its circumference.  The diagram shows that, at any point on the circumference of the circle, the Sun’s altitude will be 35o and our distance from the GP will be equal to the radius of the circle.  The problem is to establish at which precise point on the position circle the yacht lays.

diag34 mod

At first, it might seem that all we need to do is to observe the bearing of the Sun at the same time that we measure its altitude and then draw the line of bearing on the chart along with the position circle.  In this way, it would seem that our true position would correspond to the intersection of these lines on the chart.  However, building on the work covered in Part One, we will find that there is a problem with this idea which makes it impracticable.  Because of the great distance of the Sun from the Earth, the radius of the position circle will be very large (approximately 3000 nautical miles or so).  A chart on which such a large circle could be drawn would require such a small scale that accurate position-fixing would be impracticable.

However there is another way of solving the problem.  We cannot physically measure the distance from the yacht to the GP but we can measure the altitude of the Sun at the true position and from that we can calculate the zenith distance as can be explained with the diagram below.

no-diag-no

The true position of the yacht is represented by A in the diagram

Z represents the zenith of the true position

X represents the position of the Sun

U represents the geographical position of the Sun

ZX is the zenith distance and AU is equal to the angular distance ZX in nautical miles.

We can see that the zenith distance is equal to  90o – Altitude

So, measuring the altitude gives us a method of calculating the zenith distance and the zenith distance gives us the distance AU in nautical miles.

The Intercept. Our aim is to calculate the azimuth and altitude of a celestial body from our DR position at the time that we accurately measure the altitude at the true position.  The azimuth will give us a line of direction between the DR position of the yacht and the geographical position of the celestial body.  (We use the azimuth rather than the compass bearing because a magnetic compass is not very accurate  whereas the azimuth can be accurately calculated. By finding the difference between the two altitudes and hence the difference between their zenith distances, we can calculate the intercept which is the distance from the DR position to the circumference of the true position’s position-circle.

Astronomical Position Line.  We draw the position line at a point where the intercept intersects with the position circle. Since the circumference of a circle at any point is at right-angles to the radius at that point, no accuracy will be lost by drawing the position line as a straight line at right-angles to the intercept.  The navigator will then be able to draw a short, straight line on the chart along which he knows his position lies; such a line is known as an astronomical position line.

In celestial navigation, just as in coastal navigation, we need to calculate the intersection of three or more position lines in order to obtain a position fix (or observed position as it is called in celestial navigation).

Please Note.  This topic is explained in far greater depth in my books ‘Astro Navigation Demystified’ and ‘Celestial Navigation – The Ultimate Course’.

Many thanks to Jeremy Parker for his help in updating this page.

Where to buy books of the Astro Navigation Demystified series:

Celestial Navigation at Amazon.com

Celestial Navigation at Amazon.uk

Astro Navigation Demystified at Amazon.com

Astro Navigation Demystified at Amazon.uk

Applying Mathematics to Astro Navigation at Amazon .com

Applying Mathematics to Astro Navigation at Amazon .uk

Astronomy for Astro Navigation at Amazon.com

Astronomy for Astro Navigation at Amazon.uk

web: http://www.astronavigationdemystified.com

e: astrodemystified@outlook.com

Posted in astro navigation, Astro Navigation Demystified, Astro Navigation Topics, astronomy, celestial navigation, Marine Navigation | Tagged , , , , ,

Astro Navigation in a Nutshell Part 3

Part 3 – Calculating Altitude and Azimuth at the Assumed Position by Spherical Trigonometry.

Links:  Astro Navigation In A Nutshell Part One

Astro Navigation In A Nutshell Part Two

There are several ways of calculating the azimuth and altitude at the assumed position; these include the use of sight reduction methods and software solutions. However, the traditional method is by the use of spherical trigonometry which is demonstrated below.

diag 3 no number

The PZX Triangle

 

 

 

 

 

 

 

 

 

In the diagram above,

PZ is the angular distance from the Celestial North Pole to the zenith of the observer and is equal to 90o – Lat.

PX is the angular distance from the Celestial North Pole to the celestial body and is equal to 90o – Dec.

ZX is the Zenith Distance and is equal to 90o – altitude.

Therefore, altitude is equal to 90o – ZX

The angle ZPX is equal to the Local Hour Angle of the Celestial Body with respect to the observer’s meridian.

The angle PZX is the azimuth of the body with respect to the observer’s meridian.

Summary.

PX = 90o – Dec.

PZ = 90o – Lat.

ZX = 90o – Alt.

Alt = 90o – ZX

<PZX = Azimuth.

<ZPX = Hour angle.

In order to calculate the azimuth and altitude of a celestial body we must solve the triangle PZX in the diagram above.  Specifically, we must calculate the angular distance of side ZX so that we can find the altitude and we must calculate the angle PZX so that we can find the azimuth.

However, because the triangle PZX is on the surface of an imaginary sphere, we cannot solve this triangle by the use of ‘straight line trigonometry’; instead we must resort to the use of ‘spherical trigonometry’ which is explained here.

Example of the use of spherical trigonometry to calculate the azimuth and altitude of celestial bodies.

Note.  Traditionally, the ‘half-haversine’ formula was used for this task but this formula does not lend itself well to solution by electronic calulator; therefore, the following solutions involve the cosine formula.

Example:  Star Sight.

Scenario:     Greenwich date: 30 June 18hrs 05 mins  33 secs

Assumed Position:  Lat. 30oN    Long. 45oW

Selected body: Alioth

SHA: 166

Declination: 56oN

GHA Aries:  250

  • Calculate LHA

SHA Alioth:   166

Add GHA Aries: 166 + 250 = 416

Subtract Long(W) = 416 – 45 = 371

Subtract 360 = 11

Therefore,  LHA  =  11W

(all results in degrees)

  • Calculate PZ/PX/ZPX

PZ = 90o – 30o = 60o      ∴PZ = 60o

PX=  90o – 56o = 34o     ∴PX = 34o

ZPX = LHA = 11west

  • Calculate Zenith Distance  (ZX).

As explained here, the formula for calculating side ZX is:

Cos (ZX) =  [Cos(PZ) . Cos(PX)] + [Sin(PZ) . Sin(PX) . Cos(ZPX)]

∴To calculate zenith distance of Alioth:

Cos (ZX) =  [Cos(PZ) . Cos(PX)] + [Sin(PZ) . Sin(PX) . Cos(ZPX)]

=  [Cos(60o) . Cos(34o)] + [Sin(60o) . Sin(34o) . Cos(11o)]

=  [0.5 x 0.829} + [0.866 x 0.559 x 0.982]

=  0.415 + 0.475

Cos (ZX) =  0.89

∴ ZX      =  Cos-1 (0.89)  =  27o

  • Calculate Altitude.

Altitude  = 90o – ZX   = 90o – 27o  = 63o

  • Calculate Azimuth (PZX)

As explained here the formula for calculating angle PZX is:

Cos PZX = Cos(PX) – [Cos(ZX) . Cos(PZ)]  /  [Sin(ZX) . Sin(PZ)]

∴To calculate azimuth of Alioth:

Cos PZX   = Cos(34) – [Cos(27) . Cos(60)]  /  [Sin(27) . Sin(60)]

= 0.829 – [0.89 x 0.5)]  /  [0.454 x 0.866)]

=  (0.829 – 0.445) / 0.393

=  0.384   / 0.393   = 0.977

Cos(PZX) = 0.977

∴  PZX   =  Cos-1(0.977)  = 12.31

∴  Azimuth = N12oW (since LHA is west)

In terms of bearing, the azimuth is 348o.

  • Summarize results.

LHA =  11west

Declination = 56oN

Azimuth at assumed position = N12oW

Altitude at assumed position = 63o

Please Note.  This topic is explained in far greater depth in my books ‘Astro Navigation Demystified’ and ‘Celestial Navigation – The Ultimate Course’.

The next post in this series will give a full example of calculating an astronomical position line.

Where to buy books of the Astro Navigation Demystified series:

Celestial Navigation at Amazon.com

Celestial Navigation at Amazon.uk

Astro Navigation Demystified at Amazon.com

Astro Navigation Demystified at Amazon.uk

Applying Mathematics to Astro Navigation at Amazon .com

Applying Mathematics to Astro Navigation at Amazon .uk

Astronomy for Astro Navigation at Amazon.com

Astronomy for Astro Navigation at Amazon.uk

web: http://www.astronavigationdemystified.com

e: astrodemystified@outlook.com

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Applying the Equation of Time when calculating longitude at the Sun’s Meridian Passage.

Author’s Note:  The original issue of this post was found to contain errors which have now been corrected.  Many thanks to Jeremy Parker for bringing the errors to my attention.

Although the imaginary Mean Time gives us an accurate measurement of time, it presents the navigator with a problem.   When fixing his position by an observation of the Sun, he measures the altitude of the True Sun which keeps apparent solar time.  However, he notes the time of the observation from a deck watch that keeps mean solar time.  To enable us to connect mean solar time with apparent solar time, we have the Equation of Time which is defined as follows:

Equation of Time = Mean solar time – Apparent Solar Time

In other words, the equation of time is the difference between apparent solar time and mean solar time taken at the same instant at one place.

The equation of time can be either positive or negative depending on the time of the year.

  • The values range from approximately +15 to -15 mins. but can be as much as +64 mins.
  • The values are positive from 15th April to 14th June and from 1st September to 24th
  • The values are negative from 15th June to 31st August and from 25th December to 14th

Nautical Almanac.

extractt-eot

The Equation of Time for 00h (lower meridian) and 12h (upper meridian) for each day is printed at the foot of the Nautical Almanac daily page as shown in this extract.  The Local Mean Time of the Sun’s Meridian Passage is shown in the column to the right of the EOT. (This is the apparent time of Mer Pas adjusted for EOT to give the LMT and rounded up to the nearest minute).

If the mean time of Mer. Pas is shown to be greater than 1200 then the EOT must be negative, indicating that apparent time is slow compared to mean time.  Conversely, if the mean time of Mer. pas. is less than 1200 then EOT is positive, indicating that apparent time is ahead of mean time.

To calculate longitude we simply find the difference between LMT of Mer. Pas. and the GMT of our observation of Mer. Pas. then, by converting the time difference to arc we are able to find the difference in degrees of longitude.

Let’s try an example:

Date: 22 June.  Zone Time: 1140 (+4).  DR Pos: 320 30’N.  610 55’W.  At meridian passage, the deck watch time was 16h 08m 25.1s  and the Deck Watch Error was  -05.0s.  The daily page for that date shows that the Eqn. of Time is 02m 02s and that Mer. Pas. is 1202 indicating that EOT is negative.

Procedure:

  • Calculate time difference.

Deck Watch Time:        16h 08m 25.1s

DWE:                                                    -05.0s

GMT:                                    16h 08m 20.1s

LMT Mer.Pas:                12h  02m 00s 

Time Diff:                        04h 06m 20.1s

(Longitude West, GMT Best)

  • Convert Time to Arc

4h = 4 x 15  =            60o  00’  00”

06m = 6 ÷ 4  =             1o 30’  00”

20.1s = 20.1 ÷ 4 =      0o  05’  01″.5

Therefore, Long = 61o  35’ 01″.5 W

The navigator seldom requires the time of meridian passage to accuracies greater than one minute. Therefore, use the time listed under the “Mer. Pass.” column unless extreme accuracy is required.

Alternative Method.  Notwithstanding the slight inaccuracy caused by the rounding up of the listed time of Mer. pas., practicing navigators may prefer the above method but for students and tutors of astro navigation, it provides very little understanding of the equation of time.  To overcome this problem, it may be more beneficial for students to calculate the time of Mer. Pas themselves by applying the equation of time.  There is also the benefit of a greater degree of accuracy.

So if, in the above example, we wished to recalculate the time difference by applying the equation of time to the local apparent noon (LAN) instead of simply using the published time of Mer. Pas., we would proceed as follows:

extractt-eot

On 22 June, the EOT is 01m 55s at 00h and it is 02m 02s at 12h so the hourly rate of change is (02m 02s – 01m 55s) ÷ 12 = 0.58s.  Therefore, at 16h 00m, the EOT will be 02m 02s + (4 x 0.58s) = 02m 04.32s which we can approximate to 02m 04s.  The table shows that Mer. Pas is greater than 1200 indicating that apparent time is slow compared to mean time.  This means that  EOT is negative and must be added to the apparent time to give mean time.

  • Calculate time difference.

Deck Watch Time:        16h 08m 25.1s

DWE:                                                    -05.0s

GMT Mer. Pas:               16h 08m 20.1s

LAN:                                 12h  00m 00s 

EOT:                                          02m 04s 

LMT Mer. Pas.:              12h  02m 04s 

Time Diff (GMT – LMT) = 04h 06m 16.1s

(Longitude West, GMT Best) 

  • Convert Time to Arc

4h = 4 x 15  =       60o  00’ 00”

06m = 06 ÷ 4  =       1o 30’  00”

16.1s = 16.1 ÷ 4 =  0o  04’  01″.5

Therefore, Long = 61o  34’ 01″.5 W

Links:  Understanding Meridian Passage,   Meridian Passage Short Method,  Meridian Passage Long Method    What is the point of meridian passage?     Zone Time,    Local Hour Angle and Greenwich Hour Angle,   Converting GMT to GHA ,  Altitude Corrections

Where to buy books of the Astro Navigation Demystified series:

Celestial Navigation at Amazon.com

Celestial Navigation at Amazon.uk

Astro Navigation Demystified at Amazon.com

Astro Navigation Demystified at Amazon.uk

Applying Mathematics to Astro Navigation at Amazon .com

Applying Mathematics to Astro Navigation at Amazon .uk

Astronomy for Astro Navigation at Amazon.com

Astronomy for Astro Navigation at Amazon.uk

web: http://www.astronavigationdemystified.com

e: astrodemystified@outlook.com

Posted in astro navigation, astronomy, celestial navigation, Equation of time, Marine Navigation, Meridian Passage, navigation | Tagged , , , , , ,

Short Distance Sailing And Rhumb Line Sailing

In my previous post about the meridian passage long method, I demonstrated how the ‘Short Distance Sailing Formulas’ are used to calculate a vessel’s position at meridian passage.  I have since received several questions asking how these formulas allow for the varying distances between the meridians of longitude.  Therefore, this post is dedicated to the task of explaining how the short distance sailing formulas are derived.

As I have done in all of my books, I will attempt to avoid over-complicated and stilted academic language and as far as possible, give my explanations in straightforward, plain English.

Links:  Understanding Meridian Passage,   Meridian Passage Short Method,  Meridian Passage Long Method    What is the point of meridian passage?     Zone Time,    Local Hour Angle and Greenwich Hour Angle,   Converting GMT to GHA ,  Altitude Corrections

 Measuring the Distance Between Meridians of Longitude Along a Parallel of Latitude.  In the diagram below, PBC and PAD lie on separate meridians of longitude.

diag15-mod

The arc BA is the distance between these meridians measured along a certain line of latitude.  The arc CD is the distance between the same meridians measured along the Equator.  Clearly, the distance CD is much greater than the distance BA

 To Calculate The Distance Between Two Meridians Along A Parallel Of Latitude.  The following formulas are used for calculating the difference in distance along a parallel of latitude (Ddist) corresponding to a difference in longitude (Dlong) and vice versa.

 Ddist = Dlong x Cos Lat.   and  Dlong = Ddist ÷ Cos Lat.

Since the secant is the inverse of the cosine, the formula for Dlong can be simplified to:  Dlong = Ddist x Sec Lat.

The  Rhumb Line.  If a ship were to steer a steady course, that is one on which her heading remains constant, her track would cut all meridians at the same angle, as the next diagram shows.  Such a line on the Earth’s surface is called a rhumb line.
diag-16-mod

When the rhumb line cuts all meridians at 90o, it will coincide with either a parallel of latitude or with the Equator.  When the angle is 0o, the rhumb line will be along a meridian of longitude.

A vessel’s course will always be a rhumb line; thus the course to be steered to travel from one place to another will refer to the angle between the rhumb line joining the places and any meridian.

 

Calculating the distance between two points along a rhumb line. In the next diagram, A, B, C, D and Z are meridians of longitude;  the lines aB, bC, and cD are different parallels of latitude; and the line ABCDZ is a rhumb line.  A series of right-angled triangles have been constructed along the rhumb line AZ and in each triangle, one short side lies along a meridian of longitude, one lies along a parallel of latitude and the hypotenuse lies along the rhumb line.

diag17-modIt can be seen from the diagram that the east-west distance between two points along the rhumb line is the sum of the distances along the parallels of latitude corresponding to the difference in longitude in each of the right-angled triangles.  This east-west distance is known as the departure

Middle Latitude. If we were to calculate the departure along each of the parallels of latitude aB, bC, cD, we would find that they would not be equal and so the task of calculating the total departure would be complicated.  In practice, the total departure is taken to be the east-west distance along the intermediate of these parallels which is known as the ‘middle latitude’.

By the formula established for Ddist above, we can derive a formula to calculate departure as follows:  Departure = d.long cos(middle latitude).

Mean Latitude.  In most cases, the arithmetic mean of the two latitudes can be used as the middle latitude without appreciable error, so the approximate formula dep.= d.long cos(mean lat) may be used.

When the difference of latitude is large (over 600 n.m.) or the latitudes are close to either of the poles, the middle latitude must be used instead of the mean latitude and in these cases, we have the more accurate formula:   Dep. = d.long cos(mid lat).

The difficulty lies in the task of calculating the middle latitude which involves finding the mean of the secants all the intermediate latitudes by integration.  Such methods are obviously impracticable in situations where courses and distances have to be calculated rapidly at sea.  For this reason, tables of corrections to be applied to the mean latitude are contained in various collections of nautical tables.  Since, celestial navigation involves short distance sailing calculations, it is not intended to copy middle latitude correction tables here; however, the following example demonstrates their use:

 Suppose a ship sails from position 50oN, 32oE., to 70oN., 15oE.

The d.long is 17o and the mean latitude is 60o.

The formula for calculating departure using the mean lat. is:  dep.= d.long cos(mean lat)

Using this formula we have:

Dep. = 17o cos(60)

= 1020’ cos(60)

= 510’ or 510 n.m.

In the tables for converting mean latitude to middle latitude, the correction for a mean latitude of 60o and a difference of latitude of 20o is +1o 09’.  So the middle latitude = 61o.15.

The formula for calculating departure using the middle latitude is:  Dep. = d.long cos(mid lat)  = 1020 cos (61.15)   = 492.17 n.m.

By comparing these results, we can see that there is a significant difference between calculations involving the mean latitude on one hand and the middle latitude on the other.

Summary of Formulas.  The formulas so far derived in this appendix are summarized below:

Ddist  =  Dlong x Cos Lat.

Dlong  =   Ddist ÷ Cos Lat = Ddist x Sec Lat.            .

dep.= d.long cos(mean lat) (for distances 600 n.m. or less).

Dep. = d.long cos(mid lat). (for distances over 600 n.m.).

 The Rhumb Line Formulas.   With the next diagram, we expand on the work above:

diag-18-mod

  • The rhumb line AZ is divided into a large number of equal parts AB, BC, CD, DZ.
  • aB, bC, cD… are arcs of parallels of latitude drawn through B, C, D…..
  • Pa’, Pb’, Pc’… are meridians of longitude.
  • Therefore, the angles at a, b, c….. are right angles.
  • If the divisions of AZ are made sufficiently small, the triangles ABa, BCb, CDc…..  will be small enough to be treated as plane triangles instead of spherical triangles.
  • Since the course angle is constant by the definition of a rhumb line, these small triangles are equal.

Consider triangle ABa in the diagram above:  AB is the distance made good, aB is the departure along a parallel of latitude, angle aAB is the course angle.

Therefore, Sin(course angle) = departure ÷ dist.  This formula applies to all of the small triangles since they are equal.

By transposition, the above formula becomes:   Dep = Dist x sin(course)

The departure between A and Z therefore, is the sum of the departures of all of the small triangles.   Therefore, by addition:

aB + bC + cD + …. = (AB + BC + CD + …. x sin(course)

i.e. Dep = Dist sin(course) 

If we again consider triangle ABa,  Aa = AB cos (course)

But Aa is the difference in Latitude (D.Lat) between A and B

So D.Lat = AB cos(course)

Again, this formula applies to all of the small triangles since they are equal.  Therefore, by addition, the total D.Lat corresponding to the total distance between A and Z becomes:

D.Lat = Dist cos(course)

We have established formulas to calculate Dep and D.Lat; we now need a formula to find the course.

If we return to triangle ABa, we can see that the course angle can be found by the formula:  tan(course) = Dep ÷ D.Lat.

As before, this formula applies to all of the small equal triangles.  So, by addition, the rhumb line course between A and Z can be found by the formula:

Tan(course) = Dep ÷ D.Lat 

Short Distance Sailing.   Short distance sailing is a term which is applied to sailing along a rhumb-line for distances less than 600 nautical miles.  From the formulas derived above, the following are used extensively in short distance sailing:

To Calculate Departure when the course is not known:   dep.= d.long cos(mean lat)

To Calculate Departure when the course is Known:   Dep  = Dist x Sin(course)

To Calculate Distance when departure and course are known:

Dist  =  Dep  ÷  Sin (course)

To Calculate Dlat when the distance and course are known:

DLat = Dist x Cos(course)

To Calculate Course to Steer (the rhumb line course between two points)

Tan(course) =   Dep ÷ D.Lat

To calculate Dlong (difference in longitude corresponding to the departure):

DLong.  =  Dep. x Sec(Mean.Lat)   or Dlong =  Dep  ÷ Cos(Mean.Lat)

Worked Example.  What is the rhumb line course to steer and the distance to travel from position 40o.5N, 43o.0W to position 42o.25N 41o.8W?

Solution:

Dlat         = 42o.25N – 40o.5N  = 1o.75N = 105’N

Mean Lat = 40o 30’N + 52’.5  = 41o 22’.5N

Dlong        = 43o.0W – 41o.8W  = 1o.2E = 72’E

Dep           = d.long x cos(mean lat)  = 72 cos(41.38)  = 54’.02

Tan(course)  = Dep ÷ D.Lat =  54.02 ÷ 105   = 0.51

Therefore course    = N27oE  =  027o

Dist         =     Dep  ÷ Sin (course)

=  54.02   ÷ Sin(27)  = 120’

Course to steer = 027

Distance to new position = 120 n.m.

Where to buy books of the Astro Navigation Demystified series:

Celestial Navigation at Amazon.com

Celestial Navigation at Amazon.uk

Astro Navigation Demystified at Amazon.com

Astro Navigation Demystified at Amazon.uk

Applying Mathematics to Astro Navigation at Amazon .com

Applying Mathematics to Astro Navigation at Amazon .uk

Astronomy for Astro Navigation at Amazon.com

Astronomy for Astro Navigation at Amazon.uk

web: http://www.astronavigationdemystified.com

e: astrodemystified@outlook.com

Posted in astro navigation, Astro Navigation Demystified, Astro Navigation Topics, celestial navigation, geometry, Marine Navigation, mathematics, navigation, Schools, spherical trigonometry, Topics For Schools, trigonometry | Tagged , , , , ,

Meridian Passage Long Method

Links:  Understanding Meridian Passage, Equation of Time,   Meridian Passage Short Method,  Short Distance Sailing Formulae,  What is the point of meridian passage?         Zone Time,    Local Hour Angle and Greenwich Hour Angle,   Converting GMT to GHA ,  Altitude Corrections

Outline Method

If a vessel is under way, its position at the time of meridian passage will not be known beforehand.  However, it is necessary to calculate the approximate time of meridian passage before it occurs so that the altitude measurements can begin in time.  This is achieved by calculating the time of meridian passage at the present D.R. position a few hours before noon and then estimating what the vessel’s new position will be at that time.  The approximate time of meridian passage can then be recalculated for the new position.

To calculate the new position, we use the Short distance sailing formulas which are fully explained in my book ‘Celestial Navigation – The Ultimate Course’. Short distance sailing is a term which is applied to sailing along a rhumb-line for distances less than 600 nautical miles.  Of course, we could simply extend our dead reckoning track for the required distance on a mercator chart.  However, mercator charts, which do not allow for the curvature of the Earth, are not very accurate for plotting tracks which cross high latitudes or which are predominately east-west.  Therefore, for the sake of accuracy, the meridian passage long method employs the short distance sailing formulas for calculating the new position.

Greenwich Mean Versus Universal Time.The nautical almanac lists the times of events such as mer. pas. in terms of universal time (UT); however, since we are dealing with the time that the Sun crosses the Greenwich Meridian, it is more helpful to refer to the time in terms of Greenwich Mean Time (GMT) instead. (Remember the terms Greenwich Mean Time and Universal Time are generally considered to be synonymous).

Refraction.  Since refraction is negligible when the Sun is at its zenith, additional altitude correction for non standard conditions is not necessary when calculating the true altitude at meridian passage.

 Relationship between Altitude and Zenith Distance.

Zenith Distance = 90o – Altitude

and Altitude = 90o – Zenith Distance

 Rules For Calculating Meridian Passage.

  • Latitude and declination same names but latitude greater than declination:

LAT  =  DEC + (90o – ALT)

  • Latitude and declination same names but declination greater than latitude:

LAT  =  DEC – (90o – ALT)

  • Latitude and declination opposite names:

LAT  =  (90o – ALT) – DEC

Nine-Step Proforma. This easy to the follow nine-step proforma has been devised provide a step by step method for calculating a vessel’s position by the Meridian Passage Long Method.  The method will become clear as you work your way through the guided examples below.  A blank copy of the proforma template can be found in appendix 10.

Outline of the 9-step Proforma.

Pre Planning.  At least one hour before noon, preferably two, calculate your vessel’s first DR position.  Busy navigators in ships travelling at speed will need to do this early so that they will have time to calculate what the ship’s  new position will be at the time of meridian passage.

  • Step 1. Using the nautical almanac daily page, find the time of meridian passage for the first DR position
  • Step 2.  Convert the time of meridian  passage from GMT to zone time.  (Remember, zone time will not correspond to the Sun’s apparent time so although meridian passage occurs at noon apparent time, the zone time is likely to be several minutes either side of this).
  • Step 3.  Calculate what the new position will be at the time of meridian passage as calculated at step 2.  (The short distance sailing formulas which are explained in note 15 should be used to calculate the new position).
  • Step 4.  Calculate the time of meridian passage at the new position.
  • Step 5.  Calculate the declination at the new time of local meridian passage.
  • Step 6.  Calculate the Meridian Altitude and note the deck-watch time.
  • Step 7.  Calculate the vessel’s latitude from the values of the meridian altitude calculated at step 6 and the declination calculated at step 5.
  • Step 8.  Calculate the vessel’s longitude from the deck watch time noted at step 6.
  • Step 9.  Summarise position at zone time of meridian altitude.

Example:    Use the Meridian Passage Long method to calculate the position of the vessel in the scenario below by following the nine-step proforma explained above.

Scenario.

Date: 17 December

(zone -9)

Mer. Pas.:  11h  56m

DR Position at 1000 (zone time): 41o 15’.0S. 134o 52’.0E.

Course: 030o  Speed: 15 knots.

Sextant Altitude at Mer. Pas. (Meridian Altitude):  72o 18’.2

Index error: +2’.1.

Height of eye: 12m

Deck Watch Time at Meridian Altitude:  02h 59m 10s

DWE 5s fast

Solution:

Pre Planning.

Date: 17 Dec.  Zone Time: 1000(-9).

DR Pos: 41o 15’.0S. 134o52’.0E.

Course: 030o  Speed: 15 knots.

Mer. Pas: 11 56

 
Step 1. Determine Time of Mer. Pas. at Greenwich.

From the Nautical Almanac Daily Page for 17 Dec,

Mer. Pas. at Greenwich =  1156 GMT.

Step 2. Estimate time of local mer.pas.at first D.R. Pos.

Starting Data: Long. 134o 52’.0E

 Calculations:

·      Convert Long. to time.

4 x 134o ÷ 60 =  8.93h               =  8h    55m    48s

4 x 52’.0 ÷ 60 = 3.46m                =  0h    03m    27.6s

= 8h   59m    15 .6s

·      Estimate zone time of local mer.pas.

Mer. Pas. Greenwich :                     11h   56m    00s

Long. (long east GMT least) :     –08h   59m    15.6s

Local Mer. Pas (GMT)    =               02h   56m   44.4s             

=               02h   57m (nearest minute)

Zone (-9) :                                        +09h             (- for GMT, + for ZT)

Zone time Mer. Pas.                        11h   57m

 
Step 3.  Calculate new position at estimated time of Mer. Pas.

Starting data:

Course = 030o Speed = 15 knots.

Zone time at first DR position = 1000

DR Position at 1000 = 41o 15’.0S. 134o 52’.0E.

Estimated zone time of Mer. Pas = 1157

Time elapsed since 1000 = 1h 57= 1.95h

Distance run at 15 knots in 1.95h = 15 x 1.95 = 29.25n.m.

Calculations:

Dep.            = Dist x Sin(course)

= 29.25 x Sin(30)  = 14’.6E

D.Lat.          = Dist x Cos(course)

= 29.25 x Cos(30)  = 25’.3N

New Lat.   = Lat – D.Lat

= 41o 15’.0S – 25’.3N  = 40o 49’.7S

Mid. Lat       = Lat – (D. Lat ÷ 2)

= 41o 15’.0S. – 12’.65N

= 41o 02’.35S.  = 41o.04S

D.Long.      = Dep. x Sec(M.Lat)

= 14’.62 x Sec(41o.04)

= 14′.62 x 1.3258 = 19’.4

New Long.  = 134o 52’.0E + 19’.4

= 135o 11’.4E

Summary:  New Position at 1157 = 40o 49’.7S. 135o 11’.4E.

Step 4.  Calculate time of Mer.Pas. at new position.

Starting data:

New Long. 135o 11’.4E (from step 3)

Calculations:

  • Convert Long. to time.

4 x 135o ÷ 60                       =  9h    00m    00.0s

4 x 11.4’ ÷ 60 = 0.76m           0h    00 m   45.6s

=   9h    00m   45.6s

  • Calculate new time of Mer. Pas.

Mer. Pas. Greenwich:       =   11h    56m   00s        (GMT)

Long (135o 11’.4E):             = –09h    00m   45.6s

Local Mer. Pas (GMT)       =  02h   55m   14.4s

Zone (-9)                               =+09h                   . 

Zone time Mer. Pas.            =  11h   55m    14.4s

=   11h 55(nearest minute)

Step 5. Determine declination at new time of local Mer.Pas.

Starting data:

Date: 17 Dec.

Local Mer. Pas (GMT):      02h  55m 14.4» 02h 55m

Calculations:

Dec Sun (02h):                S23o 21’.2  (d = 0’.1 increasing)

d Correction (55m):                      + 0’.1

Dec Sun (02h 55m):         S23o 21’.3

Step 6.  Calculate Meridian Altitude.

Starting data:

Sextant Altitude: 72o 18’.2

Index error: +2’.1.

Height of eye: 12m

Calculations:

Sextant Altitude:            72o 18’.2

I.E.                                          + 2’.1

Observed Altitude:        72o 20’.3

Dip (12m):                             – 6’.1   

Apparent Altitude:        72o 14’.2

Altitude Correction:         + 15’.9

True Altitude  =             72o 30’.1

(Note deck watch time: 02h 59m 10s (DWE -5s)) 

Step 7. Calculate Latitude

Starting Data:

Estimated Lat = 40o 49’.7S (From step 3)

Estimated Dec = S23o 21’.3 (From step 5)

Altitude = 72o 30’.1 (From step 6)

Rule: Same hemisphere Lat > Dec = rule i

Calculations:

LAT  =  DEC + (90o – ALT) (rule i)

= 23o 21’.3 + (90o – 72o 30’.1)

= 23o.36 + (90o – 72o.5)

= 23o.36 + 17o.5

= 40o.86

Calculated Latitude = 40o.86 = 40o 51’.6S. 

Step 8.  Calculate Longitude From Deck Watch Time.

Starting Data:

Estimated Longitude = 135o 11’.4E (from step 3)

Deck Watch Time = 02h 59m 10s   (from step 6)

DWE = -05s

Mer. Pas.:  11h  56m

Calculations:

  • Calculate time difference.

Deck Watch Time:        02h 59m 10s

DWE:                                              -05s

GMT/UT:                        02h 59m 05s (Longitude East, GMT Least)

Mer. Pas:                        11h  56m 00s  

Time Diff:                       11h  56m 00s   -02h 59m 05s  08h  56m 55s

(Longitude East, GMT Least)

  • Convert Time to Arc.

9=  8 x 15                  =      120o 00’  00”

56m  = 56 ÷ 4                      =        14o 00’  00”

55s = 55 ÷ 4                   =          0o  13’  45”

=     13413’  45”  =  13413’.75E

Calculated Longitude at 02h 59m 05s GMT = 13413’.75E 

Step 9.  Summarise position at zone time of meridian altitude.

Starting Data:

GMT of meridian altitude:  02h 59m 05(from step 6)

Zone: -9

Calculated latitude:  40o 51’.6S.  (from step 7)

Calculated longitude: 13413’.75E (from step 8)

Calculate zone time of meridian altitude:

GMT of meridian altitude             =    02h 59m 05s

Zone correction                               =  +09h

Zone time of meridian altitude    =    11h 59m 05s  ≈ 11h 59m

 Summary:

Observed Position at 1159 (zone time)  = 40o 51’.6S. 134o 13’.75 E

 Where to buy books of the Astro Navigation Demystified series:

Celestial Navigation at Amazon.com

Celestial Navigation at Amazon.uk

Astro Navigation Demystified at Amazon.com

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Applying Mathematics to Astro Navigation at Amazon .com

Applying Mathematics to Astro Navigation at Amazon .uk

Astronomy for Astro Navigation at Amazon.com

Astronomy for Astro Navigation at Amazon.uk

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Meridian Passage Short Method

Meridian Passage Methods. 

Short Method.  In the case of stationary or very slow moving vessels, it is acceptable to use the Meridian Passage Short Method which involves calculating the time of meridian passage at the current DR position.

Long Method.  For vessels that are making good headway, the long method should be used.  The long method involves calculating the time of meridian passage at the present position an hour or so before noon and then plotting a new DR position for that time.  In this way, the time of meridian passage at the new DR position can then be calculated in advance.

Note.  Since the calculations for position by meridian passage involve the Greenwich Hour Angle and the longitude, the base line for both of which is the Greenwich Meridian, it would seem appropriate to refer to Greenwich Mean Time instead of Universal Time in those calculations. Since the terms GMT and UT are generally considered to be synonymous, no loss of accuracy will arise.

Links:  Understanding Meridian Passage,   What is the point of meridian passage?           Applying the Equation of Time,   Meridian Passage Long Method,   Local Hour Angle and Greenwich Hour Angle,   Converting GMT to GHA ,  Altitude Corrections    Zone Time, 

Meridian Passage – Short Method.

As we learned in ‘Understanding Meridian Passage’, meridian passage (mer. pas.) occurs when a celestial body crosses the observer’s meridian of longitude and at that instant, it will reach its greatest altitude above the observer’s horizon.  We also learned that if we measure the Sun’s altitude at local meridian passage and use the result together with the Sun’s declination, we can calculate our latitude.

The short method is used for stationary or very slow moving vessels and involves calculating the time of meridian passage at the current DR position.

Rules For Calculating Latitude at Meridian Passage. The following rules were fully explained in ‘Understanding Meridian Passage’ but need to be reiterated here:

  1. Latitude and declination same names but latitude greater than declination:          LAT  =  DEC + (90o – ALT)
  2. Latitude and declination same names but declination greater than latitude:        LAT  =  DEC – (90o – ALT)
  3. Latitude and declination opposite names:        LAT  =  (90o – ALT) – DEC

 Short Method Outline. The short method involves calculating the time of local meridian passage at the vessel’s present geographical position shortly before noon and then measuring the Sun’s altitude as that time approaches.

Outline of the Six-Step Proforma for the Meridian Passage Short Method. This easy to follow six-step proforma can be used for calculating a vessel’s position by the Meridian Passage Short Method.

Pre Planning.  As the time approaches noon, calculate your vessel’s geographical position (DR or EP) and note the zone time at that position.

  • Step 1.  Using the nautical almanac daily page, find the time of meridian passage (mer. pas.) for today’s date (in GMT).
  • Step 2.  Convert the time of mer. pas. from GMT to zone time. (Remember that zone time will not correspond to the Sun’s apparent time so although mer. pas. occurs at noon apparent time, the zone time is likely to be several minutes either side of this).
  • Step 3.  From the nautical almanac daily page, find the Sun’s declination at the time of Mer. Pas.  (Note, when correcting declination for the ‘d’ increment, care should be taken to check the daily page to see if declination is increasing or decreasing).
  • Step 4.  Measure the altitude of the Sun’s lower limb at mer. pas. and calculate the corrected Meridian Altitude.
  • Step 5.  Calculate the vessel’s latitude from the meridian altitude and the Sun’s declination using the rules for Mer. Pas. as explained earlier.
  • Step 6.  Calculate the vessel’s longitude by converting the time difference between Mer. Pas. and GMT.

Example.  This example demonstrates the application of the above method of calculating latitude by the meridian passage short method.

Task.  Use the meridian passage short method to calculate the position of the vessel in the scenario below by following the six-step proforma above.

Scenario:  Date:  22 June

Zone Time: 1140 (+4)

Zone:  +4

Mer. Pas.: 1202

DR Position:  320 30’N.  610 55’W.

Speed negligible (Fishing vessel hauling nets).

Sextant altitude at Mer. Pas.: 80o 55’.8

Index Error (I.E.): -0′.2

Ht. of eye:  2.5m.

Deck Watch Time (DWT) at Meridian Altitude: 16h 08m 25.1s

Deck Watch Error (DWE): 5 sec fast (-5)

Solution.

Pre Planning.

Date: 22 June.  Zone Time: 1140 (+4).  DR Pos: 320 30’N.  610 55’W

Step 1. Determine Time of Mer. Pas. at Greenwich.
From the Nautical Almanac Daily Page for 22 June,

Mer. Pas. at Greenwich =  1202 GMT.

Step 2.  Calculate time of Mer.Pas.at D.R. Pos.

(DR Longitude = 610 55’W.)

  • Convert Longitude to time.

Long: 61o 55’W

4 x 61o ÷ 60  = 4.066h      =  4h    04m    57.6s

4 x 55’ ÷ 60 = 3.66m                 0h    03m   39.6s

=                                               4h    07m   37.2s (≈ 37s)

  • Estimate zone time of Mer. Pas.

Mer. Pas. Greenwich     =     12h   02m   00s (GMT) (from daily page)

Long (61o 55’W):             = + 04h   07m   37s (Longitude West, GMT Best(+))

Local Mer. Pas (GMT)    =    16h    09m   37s

Zone (+4)                           =  –04h                     (+4 for GMT but -4 for zone time).

Zone time Mer. Pas.         =   12h    09m   37s        ≈ 1209 (nearest minute)

Step 3. Determine Declination at Time of Local Mer.Pas.

Local Mer. Pas (in GMT): 16h  09m 37» 16h 10m

Dec Sun (16h)                        =    N23o 25′.9  (from Daily Page)  (d = 0′.0)

d correction                           =                0′.0 (from increments table)

Dec Sun (16h 10m GMT):      =    N23o 25′.9  (= N23o.43)

Step 4.  Calculate Meridian Altitude.

Sext. Altitude:                80o 55’.8

I.E.:                                         – 0’.2

Observed Altitude:        80o 55’.6

Dip (2.5m):                            – 2’.8   

Apparent Altitude:         80o 52’.8

Altitude Correction:           + 15’.8

True Altitude:                  81o 08’.6      (= 81o.143)

Step 5.  Determine Latitude.

DR Latitude = 320 30’N.

Declination = N23o.43 (from step 3)

Altitude = 81o.143 (from step 4)

(Lat and Dec same hemisphere; Lat > Dec = rule 1)

Therefore LAT  =  Dec + (90o – ALT) –   (rule 1)

= 23o.43 + (90o – 81o.143)

= 23o.43 + 8o.857

Therefore LAT  =32o.287N = 32o 17’ 13″.2 N

Step 6.  Calculate Longitude  (from DWT at meridian altitude).

DR Long = 610 55’W.

Mer. Pas.: 12 02

  • Calculate time difference.

Deck Watch Time             =    16h 08m 25.1s

DWE:                                   =                -05s

GMT/UT:                             =  16h 08m 20.1s

Mer Pas:                              =  12h  02m 00

Time Diff:                           =+04h 06m 20.1s   ( Longitude West = GMT Best)

  • Convert Time to Arc

4= 4 x 15  =        60o 00’  00”

06m = 6 ÷ 4  =         1o  30’  00”

20.1s = 20.1 ÷ 4 =  0o  05’  01″.5

= 61o  35’  01”.5

Therefore Long = 61o  35’ 01″.5 W

Therefore, observed position at zone time 12h 09m (local Mer Pas)

= 32o 17’13″.2N.      61o 35’ 01″.5W.

(For comparison DR Pos. at 11h 40m (zone time) was 320 30’N. 610 55’W.)

Where to buy books of the Astro Navigation Demystified series:

Celestial Navigation at Amazon.com

Celestial Navigation at Amazon.uk

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Applying Mathematics to Astro Navigation at Amazon .com

Applying Mathematics to Astro Navigation at Amazon .uk

Astronomy for Astro Navigation at Amazon.com

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