(images via Wikipedia)
For the past two weeks, as we've followed the moon nearly halfway around the sky, I've talked about the relative positions of the earth and the moon and the sun using a simple system clock-based system to point to everything. As we've been doing it, we're looking down from above the north pole of the earth, which you can draw as a clock face.
The sun is off in the distance in the 6 o'clock position. Almost two weeks ago, when we first saw the little sliver of a moon and the remainder of the disk full of earthshine, the moon was around the 5 o'clock position. When it flew past Jupiter almost a week ago it had moved up to 3 o'clock, and now, as it is coming around to full, it is nearly to 12 o'clock.
Often when you present this way of looking at positions of the sun and the earth and the moon to kids for the first time (and it work even better if you use a lamp for the sun, your head for the earth, and a Styrofoam ball for the moon), you will be asked "but wait, how come we can ever see the full moon?"
It's true that in this simple view every time the moon moves to the 12 o'clock position it should fall into the earth's shadow and disappear. We should never see a full moon. The only reason we do is because the orbit of the moon is tilted by just a little bit, so most of the time when the moon reaches the 12 o'clock position it is a little above or a little below the long shadow of the earth.
That's the same reason, of course, that we don't get eclipses of the sun every time the moon is at the 6 o'clock position. It's too bad, really, because if eclipses of the sun actually happened every month maybe I would finally get a chance to see one. Which I still haven't. Which makes me bitter.
Every once in a while, though, the sun, earth, and moon do line up just right, and at the precise moment when the moon should be completely full, it instead really does disappear into a lunar eclipse.
One of those times when this happens is tonight. Starting at 9:30pm here in southern California (where, incidentally, we received record rainfall yesterday and the probability of clear skies for tonight hovers in the low zeros), or 5:30am GMT, the moon first touches the earth's shadow. I like to think of it from the perspective of someone on the moon watching. First the limb of the earth touches the sun, then slowly the earth slides across, making the lunar landscape darker and darker, until finally the entire sun is extinguished and it is almost as dark as a frigid lunar nighttime.
The full eclipse starts at 11:41pm PST for me, when I will be looking up at the clouds in wonder, or 7:41am GMT, and lasts a little longer than an hour before the moon slowly reappears.
If you're lucky enough to have clear skies and be awake for that hour of total eclipse, one of the most interesting things that you will notice is that the moon doesn't actually disappear, but instead can turn a dim spooky red. What is going on? Let's think about standing on the surface of the moon again. From the moon's perspective, the earth is new – totally unilluminated – and though you could see some of the brighter lights of cities and fishing fleets it is not enough to light your landscape. But with the sun directly behind the earth, all parts of the earth's atmosphere would glow with a 360 degree ring of sunset, so all around you would not be night, but an eerie twilight.
It's a sight no one has ever seen. No pictures exist anywhere of the view from the moon of the earth, with its full atmosphere aglow, yet I still know that it must be one of the most wondrous sights in the solar system. I wish I could be up there on the moon tonight staring back towards home, but I'll just have to imagine – as you should too – that moment, standing on the edge of some crater, looking out across the desolate lunar landscape.
You're looking up at the earth but you can't see too much yet other than the glare from the quickly shrinking sliver of sun. Suddenly the entire sun is covered, your eyes adjust and you look up to see the red glow all the way around, the tiny outer cover of earth that makes it home.
You watch for an hour as the glow changes with the clouds and with the slow rotation of the earth until suddenly a little sliver of sun appears from the western side of the earth and you're quickly blinded again as lunar day time slowly reappears across your landscape, lighting your footprints in the dust showing you the way to begin the long walk home.