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Lunar eclipses

Updated | December 21, 12:30 p.m. ET: Last night’s eclipse lasted about three hours and 28 minutes and was a spectacular sight from New York City, where the sky was the clearest in recent memory. We’re sorry for our friends in cloudy California who missed it, but not for our East Coast friends who were too lazy to get out of bed. Scroll down for a play-by-play from NASA.

The lunar eclipse on the winter solstice, Dec. 21, as seen from Arlington, Va. Photo: NASA

Mark your calendars for the final lunar eclipse of the decade, which will be visible in much of the Western Hemisphere on December 21. The moon will begin its descent into darkness at 1:33 a.m. ET. The spectacular total eclipse will begin at 2:41 a.m. and last a little more than an hour. If you happen to be watching it at the wee hours of the morning, you might want to know a little more about what caused this particular eclipse, and why now.

1. A thousand years later

This eclipse is part of a series that began almost 1,000 years ago. Babylonian astronomers were the first to recognize that eclipses occurring at the same node (where the moon’s orbit crosses the ecliptic, or the path the sun appears to trace across the sky) are members of the same family, and repeat every 18.031 years. These repeating cycles are part of a longer series called a Saros series, which last roughly 13 centuries. NASA keeps track of 204 lunar Saros series spanning 5,000 years.

2. One cycle in a long series

The upcoming eclipse is Saros series 125. This series began way back on July 17, 1163, and has 72 cycles. During the first few cycles of the series, only a small part of the moon is obscured. Each succeeding cycle carves away a little bit more. Close to the middle of the series, the moon is completely covered by Earth’s shadow, causing a total eclipse.

This particular eclipse, occurring just past the middle of the series, is cycle number 48. This Saros series has 26 total lunar eclipses, with the last taking place on March 19, 2155. After that, ever smaller partial eclipses will result. When the final sliver of the moon is eclipsed, in September 9, 2443, this Saros cycle will be complete.

3. Getting the right alignment

Eclipses occur when a combination of the sun, moon and Earth are in a straight line. A lunar eclipse happens when the moon moves through the shadow of the Earth at night, obscuring moonlight. A solar eclipse occurs when the moon is between the sun and Earth, blocking sunlight in the middle of the day.

A lunar eclipse in 2000, as seen from Florida. Photo: NASA

4. The red moon

While solar eclipses can only be safely viewed with protective eyewear, lunar eclipses are safe to look at. Another bonus of a lunar eclipse is its surprising red color. Indirect sunlight passing through the Earth’s atmosphere casts a glow on the moon. The exact color depends on the amount of dust and clouds in the atmosphere.

5. A pair of eclipses

Eclipses occur in pairs: a lunar eclipse is followed two weeks later by a solar eclipse. On rare occasions, another lunar eclipse will follow the solar eclipse. The corresponding partial solar eclipse will take place on January 4, 2011 (Saros series 151) and will be visible in Europe, North Africa and Western Asia.

For more than three hours, the moon was obscured by the Earth's shadow. Chart: NASA