It’s all coming together on Tuesday, December 21, 2010. We get not one but two events for those fond of astronomy. For the uninitiated, let’s look at a couple of explanations.
The winter solstice is the point in time when the Earth is the farthest away at its maximum tilt of 23 degrees, 26 minutes. For all of us, the days have been getting shorter and nights have been getting longer up to this point, the winter solstice. Afterwards, the days will begin to lengthen again and the nights will shorten. The exact date of the winter solstice may be December 21 or 22 depending on the shift in the calendar.
If you haven’t guessed, there is a summer solstice which is June 20 or 21. This is the longest day of the year for us. And don’t forget that down under, for those in the southern hemisphere everything is the opposite. Our longest day is their longest night.
Just to round things out, the middle point between the two solstices is called the equinox and we have two of them. There is the March equinox or the vernal equinox and then the September or autumnal equinox. According to the calendar, they may be March 20 or 21 and September 22 or 23.
Now how does all this work in respect to the sun? For an equinox, it’s simple. That is the point when the sun is directly over the equator. For our summer solstice, this is when the sun is the farthest north and directly over the Tropic of Cancer which is at latitude 23 degrees north 44 minutes. For our winter solstice, the sun is over the Tropic of Capricorn in the southern hemisphere at latitude 23 degrees south 44 minutes.
Years ago I doing a long car trip in June with an acquaintance, Mike. We were chit-chatting back and forth trying to wile away the time. At some point, realising what the date was, I wished Mike a "Happy Summer Solstice". He didn’t know what the word solstice meant so I explained it to him. For the next three days he kept chuckling away about my Happy Summer Solstice thinking it was quite amusing. Ah, fun with words.
Gee did you think you’d be covering all this astronomy stuff today? 🙂 So, December 21 is going to be our shortest day of the entire year. However added to this little treat is a lunar eclipse.
A lunar eclipse is when the moon passes behind the Earth and the Earth blocks the rays of the sun. For those in the known, you can get into the three parts of a shadow with the umbra, penumbra and antumbra but let’s try and follow the KISS principal and restrict ourselves to just "the shadow". NASA has a good diagram
of what’s happening which will explain everything to the reader. On Tuesday, this event starts at 2:41am and will last about 70 minutes or so.
So don’t forget to take advantage of this coincidence. Apparently the last time a lunar eclipse occurred precisely on the winter solstice was 1554 so that means the next time is 2466. I’m pretty sure none of us will be around for that next time. 🙂
to read more from William Belle
Wikipedia: Winter Solstice
Wikipedia: Lunar Eclipse
NASA:Total Lunar Eclipse