Astro Navigation Demystified

Finding Your Latititude From The Midday Sun.

The following diagrams will help to explain how the latitude can be calculated from the Sun’s midday altitude and its declination.

 In the following diagram ,

NOS  represents the horizon and O represents the position of the observer,

X       represents the position of the Sun,

Z         represents an imaginary position exactly above the observer so that OZ is perpendicular to NOS,

OX     represents a line from the observer to the Sun,

angle XOS    represents the altitude of the Sun at midday,

angle XOZ  equals 90^o – Altitude.

Now consider the next diagram.

Imagine the Earth to be at the centre of the imaginary celestial sphere with the positions Z and X projected onto the surface of that sphere.

WOE  represents the projection of the Equator onto the surface of the sphere,

NOS   represents the projection of a line joining the North and South poles onto the surface of the sphere.

angle EOZ equals the latitude of the observer,

angle EOX equals the declination of the Sun,

angle XOZ         equals 90^o – Altitude

Therefore, Lat. = Declination + (90^o-Altitude).

In the case above, latitude and declination are in the same hemisphere but the latitude is greater than the declination. There are two other cases to consider:

In the next diagram, latitude and declination are in the same hemisphere but declination is greater than latitude.

In this case:

Declination  =  (90^o – Altitude) + Latitude

therefore Lat.  =  Dec. – (90^o – Alt.)

In the next case, latitude and declination are in opposite hemispheres as shown in this diagram:

In this case:

Latitude + Declination  =  (90^o – Altitude)

therefore, Lat.  =  (90^o – Alt.) – Dec.

We can summarise the rules for the three cases as follows:

(i)                Latitude and declination same names and latitude greater than declination:

LAT  =  DEC + (90^o – ALT)

(ii)             Latitude and declination same names and declination greater than latitude:

LAT  =  DEC – (90^o – ALT)

(iii)           Latitude and declination contrary names:

LAT  =  (90^o – ALT) – DEC

Examples:

1.   Using rule (i)

Scenario:  True altitude at midday: 72^o 30’.1     Sun’s declination: 23^o 21’.3

LAT  =  DEC + (90^o – ALT) (rule i)

= 23^o 21’.3 + (90^o – 72^o 30’.1)

= 23^o 21’.3 + 17^o 29’.9

= 40^o 51’.2 S.

2.   Using rule (ii)

Scenario:  True altitude at midday: 69^o 41’.3      Sun’s declination: 23^o 25’.6

LAT  =  DEC – (90^o – ALT) (rule ii)

= 23^o 25’.6 – (90^o – 69^o 41’.3)

= 23^o 25’.6 – 20^o 18’.7

= 3^o 06’.9 N.

3.   Using rule (iii)

Scenario:  True altitude at midday: 80^o 48’.01    Sun’s declination: 2^o 59’.0

LAT = (90^o – ALT) – DEC (rule iii)

= (90^o – 80^o 48’.01) – 2^o 59’.0

      = 9^o  11’.99 – 2^o 59’.0 = 6 ^o 12’.9N.

web: www.astronavigationdemystified.com