What causes different stars to appear in the sky at different times of the year?
There are two major motions affecting the Earth: its rotation around its axis, and its rotation around the Sun (which we call 'revolution'). While the rotation of the Earth on its axis causes the nightly movement of the stars across the sky, the revolution is responsible for the fact that we can see different parts of the sky at different parts of the year.
Take a look at the image above. On a given day (meaning on a given position on the orbit), you will only be able to see the stars that are in the opposite direction to the Sun. All the stars that are 'behind' the Sun won't be visible during that day, because they are above the horizon during the day (and we can see stars only during night)! Now if you wait 6 months, the Earth will be at the opposite on its orbit, and you will now be able to see those stars that you couldn't see 6 months earlier because they were blocked by the Sun. This is why over the course of one year, we end up being able to see all the stars that are possible to observe from our latitude on Earth.
This page was last updated June 28, 2015.