To navigate by the stars, we use an invented model of the universe called the 'Celestial Sphere'.

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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.