Pillars of the Sky (Learning from the Polynesians).


pillars

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

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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

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web: http://www.astronavigationdemystified.com

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