Saturday, November 5, 2011

The Shadow of the Earth

Everyone is an astronomer.

When we wake up in the morning, we look out the window and see if the Sun (the nearest star) is up yet. We talk about the days getting shorter as winter approaches, and we notice star formations like the Big Dipper when we're out walking in the evening.

It's human nature to be observers, and we notice a lot more than we sometimes realize. One of the things we notice in the sky is change, especially changes in position and shape. The Moon, of course, exhibits the most change each night we see her in the sky. Sometimes, she's a crescent shape, hanging low in the western sky after sunset. A few weeks later, the Moon is full, high in the sky near midnight.

Yes, you knew those locations (low in the west for a crescent Moon, high at night for a full Moon) even though you may not have thought of them before now. Again, it's all about observation. Mankind has looked at the Moon as long as there have been people on the planet. Four hundred years ago, Galileo Galilei turned one of his handmade, 30-power telescopes toward the Moon and drew this:

It's pretty much the same view I saw of the Moon last night:

Both Galileo and I saw the same phenomenon you've noticed with the Moon moving across the sky each night: the Moon has phases of light and dark, and the phases change over the month. 
Why does the Moon have a crescent shape sometimes, and is full other times? I thought this was a question with an obvious answer, and I thought everyone knew that answer. I was wrong about the latter bit. Last year, I took an Astronomy course whose first homework was to ask people to answer the question of why the Moon has phases. The answers were surprising, and to me, saddening.

The most common answer I received from the dozen people I asked "Why is the Moon a crescent shape sometimes and full other times?" was "Because the shadow of the Earth falls on the Moon differently each night." I was amazed. Wasn't the way the phases of the Moon worked taught in elementary schools since, like... ever?

I'm sure they've taught why the Moon has phases since at least 1904. I found a book about how to teach science topics from that year and they had a picture in the book on how to explain Moon phases: 

It's not difficult to understand. The *only* source of light in our Solar System is the Sun. Other stars are too far away to light things up, so every bit of light you see in the Solar System is either from the Sun, or light from the Sun bouncing off something else.

If you look at the nice graphic, half the Earth is in sunshine at any given moment, and half of the Earth is in darkness. It's the same for the Moon - - half of the Moon's surface is always lit by the Sun, and the other half is dark.

Now, since the Moon circles the Earth every 28 days or so, we on Earth get to see the sunlit parts and the dark parts every month. It's got NOTHING - zip, zilch, nada - to do with any shadow from the Earth falling on the Moon - - it's just that we look at the Moon from different angles as it's getting lit up by sunshine on half of its surface. 

The graphic also explains why we only see a new crescent Moon only just after sunset, and not in the middle of the night. The Earth is spinning counterclockwise in that graphic as we move around the Sun every year. If the Moon is in the position of a crescent between the new Moon and the 1st quarter Moon, we're only going to be able to see it just after sunset (about where the 'E' in Earth is if we're standing on the planet). By midnight, the Moon will have passed below the local horizon and we wouldn't be able to see it anymore. Conversely, a full Moon is best viewed in the middle of the night, when it's high in the sky over where it says "NIGHT" on the Earth graphic. If we saw a full Moon at sunset, we'd only see it *rising* in the East, directly opposite the Sun setting in the West. Imagine that you're standing at the "R" in Earth looking at the full Moon and you'll see what I mean.
Yes, the Moon sometimes falls into the shadow of the Earth - - we call that a lunar eclipse, but it only occurs one night every two years or so. It's not a Moon phase. 

Next time, we'll talk about the basics of the Map of the Moon. See ya then!

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