How to read a star map

The disc: the sky above you

Picture yourself lying on your back, head towards the north. The centre of the map is the spot in the sky straight above your head. The closer a star is to the edge of the disc, the lower it is on the horizon.

The outer circle is therefore the horizon: the map shows the half of the sky you can see at a given moment. The other half, below your feet, is hidden by the Earth.

The compass points: why east is on the left

On a street map, east is on the right. On a star map, it is on the left. That is not a mistake: a street map is read from above, looking down, while the sky is read from below, looking up. North is at the top and east on the left, exactly as you see them when you look up.

The initials of the compass points are written on the edge of the disc to help you orient yourself.

The stars: the bigger, the brighter

The size of a dot depends on the star’s brightness, which astronomers call its magnitude: the brightest stars are the biggest. The map shows the stars visible to the naked eye under a truly dark sky, down to magnitude 6. The catalogue we use holds 5,044 of them for the whole sky; the map draws those above the horizon at the chosen moment.

The constellations

The lines join the stars of the 88 official constellations defined by the International Astronomical Union. These figures are conventions: they do not exist in the sky, they simply help the eye find its way. Each star belongs to one constellation only.

What the map does not show

The map is an artistic rendering of real data. Stars fainter than magnitude 6, the planets and the Moon are not drawn.

Finding your sky outdoors

In a city, light pollution washes out the faint stars: do not expect to see thousands, but the brightest ones and the big constellations can still be picked out. The sky turns by about 15 degrees per hour; at a given date and time it is almost the same from one year to the next.