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| Astronomy Essentials on Dec 20, 2010

Don’t miss 2010′s solstice lunar eclipse tonight!

Whether your calendar says December 20 at this moment – or December 21 – a total lunar eclipse might be visible from your part of the globe tonight.

It’s almost eclipse time! A total lunar eclipse will take place tonight for us in the Americas, islands of the Pacific, Greenland, northwestern Europe and northwestern Africa. If you are west of the International Date Line, your calendar says December 21 today, but this eclipse still might be visible for you. For more about specific times and time zones, click here.

A total lunar eclipse takes place whenever the moon passes right through the Earth’s dark umbral shadow. This can only happen at full moon, which is when the moon has swung directly opposite the sun in our sky, and when the fully illuminated hemisphere of the moon (day side) shines brightly in our night sky. There is a full moon every month. But – several times each year – the Earth, sun and moon line up exactly. Then the body of the moon may pass in front of the sun, creating a solar eclipse. Or – as on the night of December 20/21 – the Earth passes in front of the sun as seen from the moon. At such times, Earth’s shadow falls on the moon’s face.

As this eclipse day has approached, a number of publications have reported that the last time that the total lunar eclipse coincided with the (northern hemisphere’s) winter solstice was 456 years ago, in 1554. See the sample story here. EarthSky readers will know, however, that this date is incorrect. In 1554, both lunar eclipses were partial. What’s more, the lunar eclipse fell on Dec. 9, 1554, whereas the December solstice fell on Dec. 12, 1554 (Julian calendar). I’m not sure when the last solstice eclipse was. Anyone know? But it wasn’t 1554.

Tonight’s eclipse is special for falling close to the December solstice. It is the northernmost total lunar eclipse until December 21, 2485.

During the December 20/21 total lunar eclipse, the moon will be totally immersed in Earth’s shadow for 72 minutes. A partial eclipse lasting for nearly the same period of time will precede and follow the total eclipse. No special equipment needed to watch this solstice eclipse, although binoculars or a telescope will enhance the view. Just wrap up warmly and enjoy.

Jane Houston Jones at NASA’s Jet Propulsion Laboratory explains more about lunar eclipses.

Or read EarthSky Tonight’s post about the eclipse; Total lunar eclipse on December 20 or 21, depending on time zone