With our unique Star Calculator you can predict where any star will be, at any time, from anywhere in the world.
We have designed a DIY version you can print off and assemble yourself. If you are a reader of William's Night Sky series in Yachting Monthly,, click here to email us and we'll send your special edition of the design.
To navigate by the stars, we use an invented model of the universe called the 'Celestial Sphere'.
In this imaginary system, the world is at the centre of the universe and it spins on its axis once a day. Projected out from our watery world is the perfectly round celestial sphere, with its poles and equator in direct alignment with ours.
Every star, regardless of how far away it is in reality, has a fixed position on this sphere, like a jewel embedded in a glass orb. Although there are thousands of stars in the night sky, fifty-eight are officially used for navigation and each one can be found by intersecting two coordinates; Declination (Dec.) and Sidereal Hour Angle (SHA).
Declination is the celestial equivalent of latitude, measuring the angular distance up to 90 degrees north or south of the equator; this affects how high a star climbs in the sky. To intersect this, Sidereal Hour Angle is 'celestial' longitude and affects the time a star rises and sets each month. In the illustration above, Vega is shown with a Dec. of 39N and S.H.A of 081.
It takes a detailed knowledge of the celestial sphere and complicated maths to work out where a star like Vega will be on a particular night, and if it is even visible from your location. This is why the Star Calculator is so useful; by aligning a star's co-ordinates with the red arrows you can quickly see where it will be in the sky, from anywhere in the world.
Latitude (Lat.) is your position north or south of the equator.
Longitude (Lon.) is your position east or west of the Greenwich Meridian.
Greenwich Meridian is a line on the earth's surface from North to South Pole, passing through Greenwich Observatory, London.
Declination (Dec.). is a star's celestial latitude.
Sidereal Hour Angle (S.H.A) is a star's celestial longitude.
Culmination is the moment a star is highest in the sky.
Zenith (Zen.) is a point directly above your position, drawn by a line from the centre of the earth passing through your feet and projected onto the celestial sphere.
Angular Distance (A.D.) is the difference between your latitude and a star's declination. For example, if your Lat. is 50N and a star's Dec. is 30N, the A.D is 20 degrees.
Ecliptic is the journey of the sun around the celestial sphere every year. S.H.A is measured westwards from the position of the sun on its ecliptic at the March Equinox.
To work out where a star will be on a particular night, find its co-ordinates on the back of your Star Calculator and align the Sidereal Hour Angle and Declination with the red arrows.
In the example here, the co-ordinates of Arcturus (S.H.A 146 / Dec. 19N) are aligned with the arrows. The circle tells us the time it is highest in the sky every month (e.g 9pm in June). The ruler then tells us the direction and height it will be at that time, from any latitude (e.g at Lat. 50N, Arcturus will be due south and 59 degrees above the horizon at 9pm in June)
If you are keen to learn how to navigate by the stars, you might be interested in William's upcoming astro-nav series with Yachting Monthly.
The idea for the series is to explore a different star each month, applying a new technique on a passage and sharing ways that it can help you navigate. Starting in January 2021, the features will be available here