Q&A: General Astronomy and Space Science

Q:
Why can't we see the autumn constellations in the night sky in the spring?

A:
You can generally think of star constellations as fixed in position and not moving -- the stars are so far away that over a human lifetime they don't really change position, for the most part. The Earth, however, does move -- it completes one orbit around the Sun every year. At night, we see the stars that are in the direction opposite the Sun. That is, at night, you're looking out into the universe in a particular direction -- and this direction changes as the Earth orbits the Sun:

                      E
* 
                                *
A* *      <-E1      Sun      E2->       * B *
*                                       *
                      E

So in one season the Earth is at E1, and at night you're looking toward constellation A (constellation B is overhead when during the day). In another season, the Earth is at E2, and at night you're looking toward constellation B (constellation A is then overhead during the day).