A Winter Sun & Copper Moon
Tuesday, December 21, 2010 is a day unlike any other we'll see in our lifetime because two astronomical events conspired to occur together - the winter solstice and a total lunar eclipse. You may recall from our earlier blog post back in June "Summer is here…officially," that a solstice is when sun stands still in declination, that is the apparent movement of the sun's path north or south comes to a stop before reversing direction. In astronomical terms, the winter solstice means the Earth's axial tilt puts us (in the Northern Hemisphere) as far away from the sun as we're going to get this year while our friends in the south are enjoying the balmy weather of mid-summer.
Well, solstices certainly aren't a rarity since we have two every year - winter and summer. But last night, we also experienced a total lunar eclipse. There are at least two lunar eclipses each year, though they're usually partial. The total lunar eclipse is a much less common event. What's a lunar eclipse? Well, it's when the shadow of the earth "covers" the moon. This can only happen when the sun, earth and moon are perfectly aligned or nearly so with the earth in the middle. Unlike a solar eclipse which can only be viewed in a relatively small area of the world, a lunar eclipse can be viewed from anywhere on the "nightside" of earth. Now while our little planet isn't able to make the moon disappear in its shadow, the effect can still be quite dramatic, as the moon often turns a lovely shade of copper as it did last night.
What makes today so special is the fact that these two events occurred on the same day. A total lunar eclipse and the winter solstice falling on the same day hasn't happened since 1638 and as far as historians and astronomers know, that's the only day this has happened in the last 2,000 years.