You may imagine the stars as being fairly fixed. But our view of the sky actually changes minute by minute, hour by hour and day by day. That is largely due to the movement of the Earth as it spins on its axis and revolves around the Sun.
From our position on the Earth, everything else in the universe seems fixed to the inside of a ball which slowly turns. Wherever we are on our planet, we see around half of the sky at a time.
This slow turning of the sky is, of course, really because the Earth is rotating in the opposite direction. This is most clearly shown by the Sun, which rises, crosses the sky and sets each day. What might not be so obvious is that all the other objects rotate across the heavens in a similar way.
How stars appear to cross the sky from a point in mid-latitudes (Illustration: Yoko Kikuta)
This heavenly ball is called the Celestial Sphere by astronomers. It appears to rotate around two points which lie above the Earth’s North and South Poles. This sphere can be fitted with an imaginary grid that equates to the lines of longitude and latitude on the Earth.
Lines running from one celestial pole to the other are called Right Ascension and are measured in hours and minutes. They are the equivalent to lines of longitude on Earth. Above the Equator runs an imaginary line termed the Celestial Equator. Lines parallel to the Celestial Equator, similar to our lines of latitude, are called Declination and are measured in degrees, running from 0 to +90° at the North Celestial Pole and to -90° at the South Celestial Pole.
Although many stars rise and set, not all do. If you stand at either pole, all the stars appear to move parallel to the horizon. If you are at the Equator, they all rise vertically cross the sky and sink vertically below the opposite horizon.
If you live anywhere else in the northern hemisphere, the North Celestial Pole will lie at an altitude in the sky equal to your own latitude. Stars close to the North Celestial Pole will never set. The South Celestial Pole will always be hidden below the horizon and surrounding stars will never rise. For those in the southern hemisphere, the situation is reversed.
It takes a little under 24 hours – 23h 56m to be precise – for the celestial sphere to make one rotation and for every star to make one complete journey around it. We call this the Sidereal Period. So why is a day a full 24 hours long?
The extra four minutes are due to the Earth movement in its orbit around the Sun which makes the Sun appear to shift from day to day against the stars. From its highest point in the sky one day to the next takes 24 hours.
The Earth's axis tilted to its orbit produces seasons (Illustration: Yoko Kikuta)