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The apparent movement of the stars during the year

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.

© Keith Gibbs 2020