The sky as a dome: horizon, zenith and the turning sky

The whole sky turns once in 23 hours 56 minutes, about 15° every hour.

Why the sky looks like a bowl

Stand in an open field at night and the sky looks like a huge upturned bowl resting on the ground all round you. The stars seem stuck to the inside of that bowl, all at the same distance. They are not: the nearest bright star, Alpha Centauri, is about 4.4 light-years away and many of the stars you see are hundreds of light-years off. But the eye cannot judge such distances, so for finding your way round the sky the bowl is a very useful picture. Astronomers call the full version the celestial sphere: an imaginary ball with Earth at the centre and every star pinned to its inside.

Only half of the sphere is above you at any moment. The ground hides the other half, and the line where sky meets ground is the horizon. The point straight overhead is the zenith. A line drawn from due north, through the zenith, down to due south is the meridian, and it splits the sky into an eastern half and a western half. These three ideas, horizon, zenith and meridian, belong to you: walk to another city and your zenith moves with you.

Two numbers for any point in the sky

To tell a friend where to look you need two angles. Altitude is the height above the horizon: 0° on the horizon, 90° at the zenith. Azimuth is the direction round the horizon, measured from north towards east: north is 0°, east 90°, south 180° and west 270°. A star at altitude 30°, azimuth 135° is a third of the way up the sky in the south-east. A fist held at arm's length covers roughly 10° of sky for most people, which is a handy ruler when you have no instruments.

Why the stars move

Watch for an hour and the bowl seems to turn. Stars in the east climb, stars in the south slide to the right, stars in the west sink and set. Nothing up there is turning: Earth spins from west to east, so the sky appears to turn the other way, from east to west. Earth turns once relative to the stars in 23 hours 56 minutes 4 seconds, the sidereal day. NASA's Glenn Research Center works it out as 360° plus the extra 0.9856° Earth needs each day because it has also moved along its orbit. Divide 360° by about 24 hours and you get 15° an hour, one and a half fists.

Press Play in the model above. Each second of play is one hour of sky. Stars rise along slanted paths in the east, reach their highest point when they cross the meridian, and set in the west. The card under the model shows the sidereal time, which tells you which part of the star map is crossing your meridian right now. Turn on Grid to see altitude circles at 30° and 60°.

The poles and the celestial equator

Extend Earth's axis into the sky and it hits the celestial sphere at two points, the north and south celestial poles. Everything turns round them. Extend Earth's equator outwards and it draws a great circle on the sky, the celestial equator, exactly halfway between the poles. Star positions on the sphere use two angles that work like latitude and longitude: declination is the angle north or south of the celestial equator, and right ascension is measured eastward along it in hours, from 0h to 24h. Unlike altitude and azimuth, these numbers belong to the star and hardly change from year to year, which is why star catalogues use them.