In last session, we saw that the stars, like the Sun, appear to move from east to west overnight. The sky we see at 9 pm is not the same as the sky we see at 4 am. Does the night sky change over the year? Do we see the same stars in January and in June? From your observation, you might know that the night sky looks different at different times of the year. Let us see why that happens.

Procedure:

1. Form a group of six. One of you will become the Earth and another will become the Sun. The remaining four will become the stars and stand at different locations around the Sun-Earth system as shown in Figure 6.

2. The Earth should face the Sun. Let us see what the person on the nose, Nitu, would see.

3. The Earth should make one rotation slowly. Nitu would first see the Sun. At midnight she would see Star A at the zenith, Star B rising on the eastern horizon and Star D setting on the western horizon.

4. The Earth should start revolving around the Sun and stop when it completes a quarter of the revolution. Now, the Earth will make another rotation and observe the stars and record which stars are visible for Nitu. At midnight Nitu would see Star B at the zenith, Star A setting on the  western horizon and Star C rising on the eastern horizon.

5. The Earth should now move around the Sun by another 90 degrees. Now, at midnight, Nitu would see Star C at zenith, Star D rising on the eastern horizon and Star B setting on the western horizon. She will not see Star A at all, since it is behind the Sun.

6. Carry out  another rotation when the Earth completes three quarters of its revolution. Now, at midnight Nitu would see star A again, rising on the eastern horizon.

Thus all the stars are not seen during the whole year. We see different stars at different times of the year.

Figure 6: Why do we see a different night sky at different times of the year