About the Stars

When we look at the night sky, it seems we can see billions of stars.  They glitter in mysterious combinations across the sky and seem to be constantly changing.  In fact, however, we can see only about two thousand stars at a time and then only in the best conditions.  And the combinations are not all that mysterious.  They are called constellations, and they never change–not in relationship to each other or to the earth.

How is it then that we can see different stars at different times of the night and of the year? The complicated explanation requires thinking about geometry and minutes and imaginary lines on the earth.  But it is actually fairly simple to understand without these complications.

First, imagine that you are standing on the earth looking out at the night sky.  As you stand
there, the earth turns toward day so that the stars rise and set just like the sun and
moon.  Some of the constellations will be not be visible first thing in the evening
because they haven’t risen.  Some will set well before dawn.

Now also remember that the earth is rotating around the sun.  The stars that are visible to you when you are on one side of the sun will not be visible when you are on the other side.  Orion, for example, is a constellation we generally can’t see in May because we are facing it during the daytime and therefore it is blotted out by the sun.

(Photo by NASA)