30

I am thinking of a partial lunar eclipse where the Sun has not set yet but all three celestial objects are at such an angle that the Earth can cast some of its shadow on the Moon. Is such a scenario possible? How common is it?

Aaron S
  • 441
  • 1
  • 3
  • 4
  • 3
    This happened here in Louisville, Ky many years ago. This was due to the Moon not being in the deepest part of the shadow yet, so the maximun eclipse came later. I imagine for every lunar eclipse there is a small area where this happens. – Greg Miller Jan 03 '23 at 00:37
  • 3
    Kiloyear (Ky) many years ago? – gerrit Jan 03 '23 at 08:03
  • 1
    Yes, it is possible: https://en.wikipedia.org/wiki/Lunar_eclipse#Selenelion – Nilay Ghosh Jan 03 '23 at 10:25
  • 4
    I have to assume you're asking if it can be observed in this state by someone who has not seen the sun set yet. Because if not, the answer is "of course"; half the planet has not had the sun set yet at any given point in time, whether or not a lunar eclipse is occurring. :-) – ShadowRanger Jan 03 '23 at 12:19
  • 2
    @gerrit "Ky" is short for Kentucky. US cities are typically identified with both city and state, but the state is often reduced to it's 2-letter abbreviation. City names are only unique within states, omitting the state results in a lot of duplicates. – Loren Pechtel Jan 03 '23 at 22:35
  • @gerrit, Sorry, I did mean Kentucky as Loren pointed out above. – Greg Miller Jan 04 '23 at 05:41

2 Answers2

44

Yes, barely.

The atmosphere bends light, especially at rise and set. When the sun appears to be on the horizon, it is actually about a degree below the horizon. This means that when the sun and moon are actually aligned they would both be visible.

You can simulate this in Stellarium (or another planetarium system that simulates atmospheric effects)

Set the time to a MJD of 61102.43689, (2026-03-03 10:29:07 UTC) the location to N 30° 38' 55.14", E 111° 52' 48.64" and use "Zero Horizon" (rather than the default picture of a field). There will always be some locations at which the moon is rising during an eclipse, so this is quite common. But actual locations on land where both horizons are visible and the air is clear enough to see the moon that low are rare.

This is strictly an atmospheric effect, it wouldn't be possible if the Earth had no atmosphere.

enter image description here enter image description here

This was observed by the French astronomer Antoine-François Payen, described in his 1666 treatise Selenelion ou apparition luni-solaire en l'isle de Gorgonne. The word "Selenelion" (a portmanteau of Greek selene + helion, ie "moon-sun") has been occasionally been used in English to describe this phenomenon.

James K
  • 120,702
  • 5
  • 298
  • 423
  • 4
    Is refraction necessary given that Earth shadow has much larger angular diameter than Moon? – Leos Ondra Jan 03 '23 at 15:43
  • 1
    Sunset is defined as the top of the Sun going below the horizon, and Earth's shadow is nearly 3x the diameter of the moon, so there's some wiggle room there for the geometric horizon. – Greg Miller Jan 04 '23 at 05:37
  • Can someone please convert the MJD to normal i.e Gregorian day and time? –  Jan 04 '23 at 12:36
  • (2026-03-03 10:29:07 UTC) https://heasarc.gsfc.nasa.gov/cgi-bin/Tools/xTime/xTime.pl?time_in_i=&time_in_c=&time_in_d=&time_in_j=&time_in_m=61102.43689&time_in_sf=&time_in_wf=&time_in_ii=&time_in_sl=&time_in_sni=&time_in_snu=&time_in_s=&time_in_h=&time_in_sz=&time_in_ss=&time_in_sn=×ys_in=u×ys_out=u&apply_clock_offset=yes – James K Jan 04 '23 at 12:47
  • 1
    @GregMiller: The Earth's shadow being large doesn't help. If you can see the Sun and the moon from a spot on the Earth at the same time, that means the moon must be just at the edge of the Earth's shadow. If not, the moon would be fully hidden from places on the Earth where you could see the Sun. The best case if when you're at the point on Earth that's tangent to the line from the Sun to the edge of the moon; with no atmosphere to bend light, being able to see both necessarily means that some light can go past the Earth straight to the moon, so it's not fully eclipsed. – Peter Cordes Jan 04 '23 at 13:32
13

It happens all the time, you just need to pick your place.

When there is a lunar eclipse, you need to travel to a location on the terminator. (The terminator is the edge of the illuminated hemisphere of the Earth.) Half of the terminator will experience sunrise, while the other half sees a sunset. Because of the Sun - Earth - Moon alignment and atmospheric refraction you are guaranteed to see the disc of the Moon too (EDIT -- provided terrain and weather allows).

FURTHER EDIT -- Apparently you need to be on a mountain top to catch a nice selenelion. See https://web.archive.org/web/20111220123836/http://www.skyandtelescope.com/community/skyblog/observingblog/97224024.html

g.kertesz
  • 231
  • 1
  • 4