The Half-Astrophysicist Blog

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!

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December 9, 2011 - Posted by | lunar eclipse, Moon, Observing

7 Comments »

  1. I had the good fortune to witness and photgraph a selenelion in September 1996. I was honeymooning in Maui. A lunar eclipse was set to occur, moonrise at that location. We drove to the top of Mt Haleakala (10,000 ft elevation) to get above the marine layer. To the west, we saw the sun setting through the clouds, looking like the Japanese flag. To the east the moon was rising, already well immersed in the shadow. A once in a lifetime view ………. Now i have to dig thru the boxes of photos to find the pics.

    Comment by Matt Kucharski | December 9, 2011 | Reply

  2. Not once a month. In 2011 there are 2 total eclipses and four partial. There will be 1 total and 1 partial eclipse in 2012.

    Comment by Gally Leo | December 9, 2011 | Reply

  3. About how long will the selenelion last tomarrow?

    Comment by Kamren | December 9, 2011 | Reply

  4. Karmen, selenelion lasts only a couple of minutes…as the Sun comes up, the Moon goes down so they are only in the sky together a brief time.

    Gally, I think you misunderstood my query. I know lunar eclipses don’t occur every month. However, there is a full Moon every month and my point is that every month you have an opportunity to see the full Moon and the Sun in the sky at the same time due to atmospheric refraction regardless of whether or not the Moon is eclipsed at the time.

    Would love to see those pictures, Matt. A mountain would be a great place to get those types of images!

    Comment by halfastro | December 10, 2011 | Reply

  5. […] Seeing the “Impossible” During Saturday's Lunar Eclipse: The … Uncategorized Comments are closed. […]

    Pingback by Manatee moon | Youtoobelong | December 10, 2011 | Reply

  6. The moon appears full for a half day or so (depending on how careful you are, I guess) on either side of its moment of astronomical fullness. That half-day represents about seven degrees in the sky (14 moon-widths, quite a bit) or about half an hour’s apparent motion. Thus even without atmospheric refraction you would expect to see an apparently full moon in the sky opposite the sun, at sunrise just before it was truly full, or at sunset just after. The moon, earth, and sun are not perfectly in line, but they are close enough that the moon looks round. Depending in how picky you were, you might see each of these over about a half of the earth’s surface each month, all other things being favorable.
    Having this happen during a lunar eclipse is rarer (because eclipses are rarer than full moons, and the eclipse is much shorter than the period of apparent fullness) and shows that refraction really is at work – the moon, earth, and sun are then exactly in line. So that’s why the eclipse is special.
    Getting the photograph would be tricky, as the sun and moon would be in opposite directions! You could use a big silvered ball or a flat mirror.

    Comment by Robert Dawson | December 12, 2011 | Reply

    • Good points, Robert. Being off from full Moon by a few hours can make it easier to see the full Moon in the sky at the same time as the Sun (of course,if you are off in the wrong direction, it can make this site impossible!)

      But remember, there will always be some locations on Earth where full Moon occurs at EXACTLY sunrise or sunset! At those locations, you don’t get any help except for atmospheric refraction.

      Comment by halfastro | December 13, 2011 | Reply


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