Once in a blue moon
2009 had almost run out when I saw a yahoo post about how we're going to get a blue moon on New Year's Eve. A blue moon is just what they call it when we get a second full moon in a single month. I'm sure many of you saw the same article and so it wouldn't be all that interesting to repeat the news here, now would it? But what would be interesting is to discuss how, in fact, we aren't seeing a true full moon. And, to make matters even more interesting, I'm going to assert that you've NEVER seen a full moon – not even once. Not a true one, at least.
So exactly how can I make this outlandish claim? Well. Let's start with the following pic.
I've used clever color coding here to indicate the Moon, the Earth and the Sun. Can you guess which is which? That's right! The big yellow disk is the Sun, the gray/white disk is the Moon and the blue disk is good old mother Earth. The scale is all wrong, of course, because if it weren't it'd be impossible to see anything on the pic (astronomical distances being what they are).
Okay. Notice how the half of the Moon that's facing the Sun is all bright white – bathed by the Sun's life-giving effulgence. The Earth isn't quiiiite in line and so, although we'd see something close to a perfectly illuminated moon face, it wouldn't be 100%. However, once the moon moves a bit, we WILL be in line. Something like this:
Ah ha! Now imagine somebody standing at the red dot on the Earth. If he looked up he'd see an absolutely perfect circle of Moon, wouldn't he? A true full moon.
Soooo, what was I talking about when I said that nobody's ever seen one? Well, I left out one tiny little thing. Can you guess?
You see, we have a name for when the Earth is placed precisely between the Moon and Sun. It's called a lunar eclipse. Because the shadow of the Earth is the same size as the Moon, the light from the Sun is cut off. Something like this:
How inconvenient. The only time we could possibly see a true full moon, we're kept from doing so by the Earth itself. The only way to actually see one is to take a quick trip up in a spaceship and get Earth out of the picture. Once you've done that it would be no problem to see an honest to goodness full moon. But, until then, you'll have to be satisfied with "almost" full.
- And that's today's word from the bird
