As most of us have noticed the evening sky
contains stars that differ from month to month.
On a daily basis we may think that all is stationary but this is not the case. our sky appears to rotate as the weeks and months progress through the year. In fact it is the Earth that is changing as we rotate around the Sun, so we think our sky is rotating through the months, at approximately 4 minutes a day.
We think of the Earth rotating around once in 24 hours, but in actual fact the earth completes a full rotation in 23 hours and 56 minutes. The Earth has in that time moved around the Sun in our orbit just under 1 degree and the earth continues to rotate approximately 4 minutes longer to face the Sun again. If this were not the case our midday in December would be sunrise in March, midnight in June and sunrise in September.
Once you understand how the sky rotates, it is easier to understand the more complex concepts of orbits, precession, stellar evolution and cosmology, which are all part of the modern science of astronomy.
The planets move in their orbit also, this means that the location of any planet changes in relation to the stars nearby. From year to year the easier planets to locate (Saturn, Mars and Jupiter) can be found in a different place to the stars behind them. It can be hard to understand the movements if you cannot picture the solar system on a plate with the planets as small marbles.
This
is the inner solar system as seen from above the plane (or plate). Gaspra and
Ida are non-planets or asteroids that exist in our solar system that are large
enough to track with very large telescopes.
All these bodies rotate around the sun at a different speed and distance changing their position to background stars as they move. Also the Earth catches up to and passes these objects causing an interesting effect called retrograde motion.
This picture demonstrates the movement of Mars in the night sky as the Earth passes Mars position.
It is like throwing a tennis ball from a moving car. If the car (Earth) is travelling at 20 kms per hour and you throw the ball (Mars) behind you at 10 kms per hour. The ball appears to be moving away from you and the surroundings (Sky). The ball is travelling towards you at 10 kms per hour to the ground and -10 kms per hour to you, but as it hits the ground the direction of the ball appears to chage from moving away from you to moving towards you. This is why the sky gives us this picture, the stars are 'fixed' but the Earth and the other planets move.