Friday, December 9, 2011

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!

Reprinted with permission from the Half-Astrophysicist Blog.


kkdither said...

Ah, that we have unanswered questions is what makes living a day longer worthwhile.

OrbsCorbs said...

As far as I can follow, hale, you're right.

Wouldn't you want as flat a landscape as possible to observe it?

hale-bopp said...

Yeah, flat horizons are key, orbs. On my blog, someone said they saw this from a mountaintop in Hawaii. A good mountain peak would work well too!