A Practical Guide to Navigating By The Stars

Polaris is the most useful star in the sky, guiding us true north more accurately than a compass (which points to the magnetic pole). But for its wonderful accuracy, the North Star has one great flaw; its brightness, or lack of. There are more than fifty brighter stars in the night sky, which means it doesn’t stand out at all when you look north. The trick to finding Polaris is to firstly locate the easily recognisable Plough, which famously looks like a saucepan.

Depending on the time, it could be either way round or even upside down, but regardless of its position the two outer stars – Dubhe and Merak – will always point towards the North Star (see Diagram 1). Simply extend an imaginary line five times the distance between the pointers and Polaris will be the only star in that area of the sky. True north is directly below the pinprick of light. As a compensation for the North Star’s dimness, it more than makes up for it by dutifully staying in one place (unlike every other star that revolves around the sky). The great advantage of this is that once you’ve found Polaris you don’t need to worry about losing it, because it’s not going anywhere. 

However, as you travel south, Polaris will drop one degree in the sky for every degree of latitude (60 nautical miles / 111 kilometres). At the North Pole it will be directly overhead, but if you were to continue all the way down to the equator, the North Star will be just a speck on the horizon and when you head into the Southern Hemisphere, it completely disappears from the sky. However, there is a technique the Polynesians developed to make use of Polaris in low southern latitudes, places like Tahiti; they would use the pointers in the Plough (which is visible part of the night) to gauge where Polaris is lying beneath the horizon. 

The Polynesians knew that from your location a particular star will always rise at a specific point on the horizon, move through the sky in a predetermined path and then set in the same place. To understand why this happens, it helps to understand a little background theory.

Firstly, we imagine all stars embedded into the ‘celestial sphere’ (see Diagram 3), which is like an orb that surrounds the Earth with its poles and equator in alignment with ours. Every star has a fixed position on the celestial sphere and its ‘celestial latitude’ (position north or south of the celestial equator) is called declination. This is what determines where a star rises; from the equator its horizon bearing north or south of east is equal to its declination. For example, Alnilam, the middle star in Orion’s Belt (see Diagram 4) has a declination of 1° north, so it rises almost due east (89°) and then sets in a mirror-image, almost due west (271°). Sirius, which is the brightest star in the sky found by following a line from Orion’s Belt, has a declination of 17° south, so it rises 17° south of east (107°) and sets 17° south of west (253°), when viewed from the equator.

The bearings change with your latitude, but from the Northern Hemisphere Sirius will always rise approximately E-S-E and set W-S-W. By understanding this theory, all you need to know is a star’s declination and you can predict where it will rise and set; to find this information, you can download a DIY Star Calculator from Tide School.

Quite simply, all navigators needed to do was memorise their favourite tunes and they had a chart of the Pacific

The Polynesians by heart the positions that dozens of stars rose and set, giving them a compass laid out in the sky every night, with shimmering lights guiding them to all the cardinal points, filling in the gaps between the pole stars. If that isn’t amazing enough, this knowledge created the foundations of an even more incredible achievement; they used star bearings to chart the entire Polynesian Triangle, mapping everything within the area between Hawaii, New Zealand and Easter Island. This was done by undertaking a multitude of voyages between islands over multiple generations, perfecting the star bearing of each island in relation to the others around it. Combined with an understanding of average sailing times, the Polynesians had a mental map of the whole South Pacific, a vast maze of volcanic islands, reefs and atolls. To simplify learning, the information was converted into songs, poems and chants. Quite simply, all navigators needed to do was memorise their favourite tunes and they had a chart of the Pacific. 

Naturally, you can only use the horizon star technique on stars that rise and set. Not all celestial bodies do; some stars are perpetually circling through the sky, neither rising nor setting. These are called ‘circumpolar stars’ and they can be found in both hemispheres. In the Northern Hemisphere, they all spiral anticlockwise around Polaris, like the Plough. In the Southern Hemisphere, they all spiral clockwise above the south pole, like the Southern Cross. Whether or not a star is circumpolar from your position depends on its declination and your latitude. Firstly, the star needs to be in the same hemisphere in the celestial sphere as your hemisphere on earth. Secondly, your latitude added to the star’s declination must exceed 90. For example, from my boat Luna in Catalonia, where the latitude is 42° north, any star with a declination higher than 48° north will be circumpolar. This means these stars will neither rise nor set, but perpetually circle Polaris, an effect not made by the stars actually moving but by the earth spinning on its axis, giving the appearance of movement in the heavens.