THE NIGHT SKY
Vega and Polaris
Ancient Polynesian navigators could pinpoint tiny islands scattered across thousands of miles of empty oceans using visual observations of the stars (see YM Mar 21). This month, we will explore how you can apply that timeless technique to modern passages, using the star Vega as an example. But first, it’s worth recapping the vital element of declination.
This is the celestial equivalent of latitude, and Vega’s declination of
39N means it is located around the middle of the northern celestial hemisphere, making it highest in the sky around the mid-latitudes of Earth. But as you move north or south, Vega will sink in the sky until, at 51 south, it is no longer visible at all. Knowing why this happens is the key to unlocking the navigational secrets.
The basic concept is that every star except Polaris ascends and descends through the sky as the earth spins on its axis once a day. The moment a star is highest is called ‘culmination’ and this will be directly overhead if your latitude is the same as a star’s declination – so at 39 north, Vega will culminate directly overhead. But for every degree of latitude you travel south (or north), the observed altitude of Vega’s culmination will sink by one degree for every degree of latitude. To work out its altitude from any latitude, subtract the angular distance between your latitude and the star’s declination from 90. For example, if you were at the equator your angular distance with Vega would be 39º, so it would culminate 51 above the horizon (90-39=51). Although this relationship is beautifully synchronised it is easy to make a mistake with the maths, which is why I designed the Star Compass. With the star’s declination set (overhead at 39º), the red scale shows what its culmination will be at any latitude.
How do we put that knowledge into practice? A simple method is to use Polaris, hovering stationary above the North Pole and visible at all times of year. With a simple sextant (I started with a £50 Davis Mark III), all you need to do is measure its altitude above the horizon, align the Star Compass ruler so Polaris’ declination of 89N is aligned with the red arrow, and find the latitude that lines up with the observed altitude. This technique would work wonderfully on a passage from the Med back to Britain. At Gibraltar (36N) Polaris’ altitude would be 37, when it rises to 44 you will be passing La Corunna (43N) and at 51 you will be at the Western Approaches to the Channel. However, the downside of relying on Polaris is that it is only visible in the northern hemisphere and it is not particularly bright. The good news is that we can apply the same principle to any star.
Vega is ideal because it is the fifth brightest in the night sky, a dazzling bluewhite clearly visible in the constellation Lyra and making the famous summer triangle with Deneb and Altair. With a declination the same as the Azores, Vega culminates directly overhead, making it perfect for applying the Polynesian technique on transatlantic passages west-to-east. If your tech fails, the first step is to use Vega’s culmination to get on the right latitude. If it culminates north you are too far south and if it culminates south you are too far north. Once it culminates directly overhead you can be confident you are on the correct latitude, so head due east. The one variable to consider is that stars culminate two hours earlier each month, as per the yellow scale above, and many are only practical for navigation at certain times of year.