Saturday's Lunar Eclipse

Certain lucky people will get treated to a total lunar eclipse on Saturday, December 10th. Unfortunately for many people living in North America, the Moon will set before the total phase of the eclipse begins. For those of us in the west, the totally eclipsed Moon will set in the west as the Sun rises in the east.

Okay, quick review. A total lunar eclipse occurs when the Moon passes through Earth's shadow. That means the Sun, Earth and Moon (in that order) are in a straight line. Lunar eclipses always happen at full Moon, but not every full Moon heralds the arrival of a lunar eclipse (the Moon's orbit is tilted so most months it passes either slightly above or below Earth's shadow). A lunar eclipse is visible to everyone on the side of Earth facing away from the Sun when it occurs (in other words, on Earth's night side).

The penumbral phase (when the Moon enters the very outer part of the shadow) starts at 11:33UT (EST you subtract five hours from UT, CST subtract six, MST subtract 7, and PST subtract 8). The outer part of the shadow is not very dark so you might not notice the Moon get slightly dimmed if you weren't paying attention. The real action stars when the Moon enters the umbra, the dark part of Earth's shadow at 12:45UT. You can watch the Moon get progressively more covered until totality begins at 14:06UT. Totality ends at 14:57UT, the umbral phase ends at 16:17UT and the eclipse is totally over at 17:30UT. Here is a chart from NASA that shows where the eclipse is visible.

In Tucson, sunrise on Saturday is at 7:15am, just nine minutes after totality begins at 7:06am! In reality, the Moon may set a little earlier since there are mountains to the west. I already have my spot to watch and photograph the eclipse picked out. It will be interesting trying to photograph the setting, eclipsed Moon as the sky gets progressively brighter toward sunrise. I suspect there will be lots of great photos of the eclipsed and partially eclipsed Moon setting over scenic areas (Tucson Mountains in my case).

One thing I encourage everyone to do is follow the eclipse on twitter. I did this for last December's total lunar eclipse and it was a lot of fun. There will probably be a twitter hashtag (search for eclipse and you will find it...it might be #eclipse, #lunareclipse or something similar). People will be posting their observations, comments and pictures in real time. You can respond and converse with people about what is happening even if you can't observe it yourself due to your location or local weather conditions. For last Decembers eclipse, there were lots of people observing and conversing, many of whom I knew personally. It was like watching the eclipse with friends even though I was alone with my camera outside my townhome.

Reprinted with permission from the Half-Astrophysicist Blog.