The Myth of the Rare Lunar Eclipse
by Philip Chien
The December 21, 2010 lunar eclipse was a pretty site for astronomers, but unfortunately its significance was totally overshadowed by the hype that it was the first lunar eclipse on the winter solstice in hundreds of year and an incredibly rare event.
It's certainly true that the lunar eclipse took place on the Northern Hemisphere's winter solstice (and Southern Hemisphere's summer solstice) and that is an incredibly rare event. It last occurred in 1638 and won't happen again until 2094. (The pedantic will note that for viewers on the West Coast the eclipse started on the evening of December 20th - the day before the solstice.)
The winter solstice is the shortest day of the year. Contrary to popular myth it is not when the Earth is furthest from the sun, that actually occurs in early July during the heat of summer for the Northern Hemisphere.
Having a lunar eclipse on the winter solstice isn't an unusual event (there's a difference between rare and unusual). It's just as rare for a lunar eclipse to take place on the summer solstice; St. Patrick's Day, Arbor Day, or any other day of the year!
It's similar to predicting when a Friday the 13th coincides with a full moon (of course that happens much more often) or when the first day of a new century is on a Sunday. Take any two astronomical phenomena which occur on different cycles and they will occasionally coincide. The winter solstice occurs on the same day each year without fail, but the precise moment occurs at slightly different times because of leap year corrections. Lunar eclipses occur on a more irregular, but predictable intervals. In elementary school math we learned about prime factors and least common denominators and it's a similar principle.
Lunar eclipses are a function of the Moon's orbit around the Earth. By definition a lunar eclipse occurs when the Earth is directly between the Sun and the moon. If the Moon's orbit was directly over the Earth's equator a lunar eclipse would happen every month, but since it's inclined by 5 degrees it only happens every now and then. You can look up information on "Saros cycles" if you're interested in more details on how the moon's orbit repeats over time. (It's rocket science - but not difficult rocket science.)
Of course pseudo scientists have placed extreme significance when events coincide and doomsayers claim that events like this prophesize the end of the world. But in science there's nothing extraordinary or special about a lunar eclipse coinciding with the same day as a solstice.
Lunar eclipses are pretty cool - you're literally watching the combined effect of sunsets and sunrises simultaneously in a band around the Earth as the Sun's light is refracted by the Earth's atmosphere. That's what makes the moon red and if a lunar eclipse happens within a couple of months after a large volcano explosion the red color is especially spectacular. If you happen to be near the terminator (dawn or dusk) when a lunar eclipse occurs you would see the Sun and Moon on opposite sides of the sky with the Sun just rising or setting. If you happen to be an astronaut on the moon looking towards the Earth you would see the dark side of the Earth surrounded by a ring of red light.
Links
Saros Cycles
All Lunar Eclipses from 2001-2020.
A NASA site with a fairly comprehensive almanac of previous and future eclipses.
About the author
Philip Chien is a semi-professional astronomer.
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