If you go out on a
clear night and look at the sky for a while you will see that the stars seem to move across the
sky during the night. This movement is not due to the stars themselves moving but to the
Earth spinning on its axis.
Now go out at the same time on different night during the
year and you will notice that the star patterns that you see in, say, June will be different from
those that you see at the same time and looking in the same direction from the same place
when looking in December. This movement is due to the movement of the Earth round the
Sun – taking one year to complete one orbit. Choose a bright star and observe at what time it
rises above the horizon on one night. You will find that you can see it about four minutes
earlier on the following night.
You can see an example of this movement in Figure 1.
The Earth is shown at four places in its orbit and the arrows represent a person looking at the
sky. The positions of the star patterns seen from those places will be completely different.
Note: Figure 1 shows real star
patterns but is not meant to show exactly what you would see at that angle to the horizon – only to represent the
different positions of the stars at different times of the year. The diagram is not to scale.
The two
pictures on the following page show how the sky would look when viewed from London
looking south at about 10.30 pm GMT on 15th June and then 15th December. You can see
that the positions of the star patterns (constellations) have completely changed. These two
pictures really do show what you would see at the times given.
If you look north
towards the pole star then you would see many of the same constellations through out the
year but their positions in the sky would change as the months pass by.
