FirstLight Astronomy Club

33°29.6'N / 117°06.8'W / 1190 ft.

Why the Night Sky Changes

I love this time of year as far as the skies are concerned. I can tell winter is approaching because the Pleiades are just peeking over the horizon late evening. In a few hours mighty Orion will heave his sparkling belt up and over the skyline. In the next months they both will rise earlier and earlier until the so-called winter constellations are all in place.

The summer constellations are, for all intents and purposes, gone. Sagittarius and Scorpius have seen their last until next spring. Cygnus the swan is flying farther westward every night. Alas!

But here's the question of the day: Why are there summer and winter - and even spring and autumn - constellations at all? Why don't we see the same old thing, night after night, all year long?

Simply? Because the Earth goes around the sun.

Imagine we are in a theatre-in-the-round, where the stage is in the middle, surrounded by the audience in all directions. Imagine the stage represents the sun. It is, therefore, a very, very well-lit stage.

The audience represent the starry hosts, the stars in our home galaxy, the Milky Way. Now imagine that you can walk around this stage. If you look towards the stage it is so bright that it is virtually impossible for you to see the audience behind the stage from your perspective. But if you face away from the stage - behold! - there is an entire audience before you.

That is like our night and day. Looking towards the stage is like daytime for us. Its brightness blinds us from everything around it. Facing away is like our night, we can see our star-studded audience fairly well. OK so far?

The reason we have seasonal constellations is because we walk around our stage.

If you are on the north side of the stage you can look north and see the various people sitting nicely in their seats. Walk a quarter of the way around, say to the west side of the stage, and there is a whole new group of people staring back. You can still, if you crane your neck, see the northern audience but they appear close to the stage now and are getting harder to see.

Go to the southern side and, again, there's an entirely new audience to look at. One more quarter walk around and you see the eastern audience. Before you know it you are back to the north, and to no surprise, there are your old northern friends sitting there, wondering why you are walking around.

Earth walks around a bright stage, as well. It just takes a year to complete the trip. And each month takes us slowly, but surely, to other sections of our starry audience.

We are about to walk to the part of the audience where Orion and family are sitting. I'm looking forward to seeing my old friends again. It's been a while.
Temecula Valley High School / Temecula, CA · Some images © Gemini Observatory/AURA Contact Me