Actually, they're not. There's a group of stars and constellations arranged around the pole of the sky that's visible at any time of any dark, clear night, all year around. And any star or constellation in the rest of the sky is visible for roughly 11 out of every 12 months ... at SOME time of the night.
Constellations appear to change drastically from one season to the next, and even from one month to the next, only if you do your stargazing around the same time every night.
Why does the night sky change at various times of the year ? Here's how to think about it:
The Earth spins once a day. You spin along with the Earth, and your clock is built to follow the sun . "Noon" is the time when the sun is directly over your head, and "Midnight" is the time when the sun is directly beneath your feet.
Let's say that you go out and look at the stars tonight at midnight, when you're facing directly away from the sun.
In 6 months from now, when you and the Earth are halfway around on the other side of the sun, where are those same stars ? Now they're straight in the direction of the sun. So they're directly overhead at Noon, not at Midnight.
THAT's why stars and constellations appear to be in a different part of the sky, at the same time of night on different dates.
