Seeing the “Impossible” During Saturday’s Lunar Eclipse: The Selenelion

I just posted a blog last night about tomorrow’s lunar eclipse. It seems that on facebook and twitter, a lot of people are passing around a story about seeing an impossible site dubbed the selenelion. I am not quite sure this site is so impossible or even so rare and would love comments from other hardcore astronomers/atmospheric physics experts if my thinking is fuzzy or spot on here.

Okay, as I discussed yesterday, a lunar eclipse occurs when the Sun, Moon and Earth are in a perfectly straight line. Therefore, the full, eclipsed Moon should set just as the Sun rises and you don’t see them at the same time…assuming Earth has no atmosphere and you have nice flat eastern and western horizons with no buildings/trees/hills etc. But this phenomena happens every month at full Moon!  Even if you argue that it’s rare because the Moon will be eclipsed, well, every lunar eclipse happens at sunset somewhere in the world so a selenelion happens during every lunar eclipse (of course, the world is 70% ocean so you could argue that many of them occur over water where no one can see them, but it is still not a rare phenomena).

The best I can tell this is “rare” because it is happening over a heavily populated portion of the United States.

But Earth does have an atmosphere. Due to refraction of light by Earth’s atmosphere, we see the Sun rise a few minutes earlier than it should and the Moon set a few minutes later than it should. Therefore, you can see both the Sun and the full Moon in the sky at the same time even though it should be impossible if they are 180 degrees apart. This is something I have known for many years (and has been known for a long time) so no new discovery here.

So my question is, why is this such a rare event? This happens EVERY MONTH at full Moon. Even if you are at a spot on the world where the sun rises (or sets…this can happen at sunset with the full Moon rising in the east) .  Yeah, this is a cool phenomena and I am trying to figure out where I can go to attempt to see it here (lots of mountains in Tucson) because I am fortunate enough to be in the right place at the right time, but it is something that happens during lunar eclipse and even every full Moon!

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.