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We've got a good reason to keep the kids up late tonight, and to stay up ourselves: a once-in-a-lifetime event. Actually, it's rarer than that . . .
You'll remember that a clock and a calendar are each a model of the Solar System. The hour hand follows the Sun around the Earth, once for a day and one more time overnight. The pages of a calendar are based loosely on the phases of the Moon; when we run out of moons and pages, we’ve followed the Earth around the Sun. We get out another calendar and call it the New Year.
Over the course of a year the Sun appears to move up and down in the sky. If we were patient enough to chart it out, we’d see it trace out a lopsided figure-eight in the sky, a figure called an analemma. We call the north and south ends of the analemma (the top and bottom of the 8) the Solstices—and the moments when the sun crosses its own path are the Equinoxes. We mark the four seasons by those dates.
Of course, we pay attention to many other things. Our months were adjusted by the Romans to suit themselves, but long ago we marked them strictly from the Moon’s darkest moment to the next dark of the moon. We still call the instant afterward the New Moon. Light returns to the night, and the Moon again challenges the stars for the sky.
The year is hardly our longest time-cycle. We count the centuries, too. And yet the stars don’t care; the sky is a calendar unto itself. Every so often the Sun, Earth, and Moon align; the Earth shadows the Moon, or the Moon casts its tiny shadow on the Earth, and we call it an eclipse.
Tonight, and tomorrow morning, these cycles will all come together: the Winter Solstice, which begins winter in the Northern Hemisphere, begins during a night of a Full Moon. At 1:33 a.m. Eastern Time (10:30 p.m. Pacific Time, still technically the 20th), the Moon will creep into the shadow of the Earth, and a lunar eclipse will occur. This will be the first night with a lunar eclipse on the same morning that the Solstice begins since the 21st of December 1638; before that, you go back a long way to find it (the year ends in “B.C.”—that long). Our friends at NASA advise:
If you're planning to dash out for only one quick look - it is December, after all - choose this moment: 03:17 am EST (17 minutes past midnight PST). That's when the Moon will be in deepest shadow, displaying the most fantastic shades of coppery red.
Humans make a lot of these coincidences, just as we give a meaning to birthdays and anniversaries. So watch the sky tonight, and when the Moon turns from silver-white to red, remember the moment.
Perhaps the stars don’t know, or care; we do.
