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.

Moving Forward, Forever Falling

This week sees the anniversaries of some rocket-related events - the first successful flight of a German A-4 missile, the launch of the first satellite, Sputnik, and the birth of the father of modern rocketry, Robert Goddard. So what better time to talk about the fundamental, but rarely understood phenomenon of sending something into orbit. 

How do we do it? How on earth do those things stay up there? Are they suddenly weightless? Is there no gravity up there to yank them down to the ground? Put on your thinking caps!

You may have seen a space launch, either from Vandenberg up north or a Shuttle launch, or one of the Apollo missions from days gone by. Did you notice that when they were launched they didn't head straight up? They all started rolling over at an angle right away. If you could follow them long enough you'd notice that they eventually fly a path parallel with the surface of the earth. That's the secret. Let me explain.

Isaac Newton, in his attempt to explain what being in orbit was all about, used an example of a cannon firing a cannonball from a mountaintop. Let me use a football instead, since it is the season. When we throw a football straight out in front of us, gravity causes it to take a curved path to the ground. If we throw it with more force, the football goes farther in its curved path until it eventually hits the ground. So far, no problem.

But what if we could throw that sucker out with a lot of force, I mean a lot. It would go really far, wouldn't it? Actually farther than you think. Since the surface of the earth is curved, your football gets to travel a bit farther down the way than expected. Throw it harder and it may travel part way around the earth before it hits the ground. Throw it harder still and it may never hit the curved surface of our planet. It will just continue going round and round the planet. That is being in orbit.

Is there still gravity? Yes! Gravity is bending its path, causing it to fall, but that curved planet isn't allowing the poor football to hit the ground. 

Get something to travel fast enough - over 17000 mph near Earth's surface - but also parallel to that surface, and the object "falls" forever with the planet continuously curving away from the falling object. (Of course, we are assuming air resistance is playing no role in slowing it down.)

Those satellites you see sometimes in the evening, the Shuttles, the Space Station, the Moon itself, are all falling to Earth. But their forward motion prevents them from crashing into it. 

Did this all give your head a good spin? Good! Until next time, clear skies!
Temecula Valley High School / Temecula, CA · Some images © Gemini Observatory/AURA Contact Me