While driving early this morning, my fiancee and I were looking at the last sliver of the moon phase before it becomes a new moon. My fiancee claims she could see a thin trace of light around the entire edge of the moon, making a complete circle. I told her that I think it’s an optical illusion, but that I’d check it out (actually today is the second time we’ve had that exact discussion).
I don’t see how there could be light coming from the whole edge of the moon at that time. Without an atmosphere, I don’t think light could bend enough around the moon such that you could see sunlight reflecting off the bottom and still see a faint line on the top edge.
Can anyone cite a resource supporting one theory or the other?
I think the corona is an optical illusion, but possibly not for the reason you think. As the moon comes very close to the New phase, it presents a lot more surface to be illuminated by reflected Earth light. This means that the entire “disk” of the moon is illuminated just enough to distinguish it from the black background of space. However, the thin crescent is bright enough to prevent seeing any actual moon features on the Earth-lit surface, leaving only the contrast between the extremely faint edge of the moon and the blackness of space to appear to our eyes as a ring.
(Maybe Bad Astronomer will happen by and correct me.)
Not to confuse matters, but there are two kinds of rings you can see around the moon. One is caused by ice crystals and one is caused by water droplets. Was this ring tight in against the moon’s disk? It sounds like it was.
tomndebb’s suggestion is probably correct, but it’s worth teasing out two different possible effects.
The very common effect that’s seen in the conditions you describe is “earthlight”. Sunlight reflected off the Earth reflects off the dark part of the Moon’s disk back to us. This is pretty easy to see close to a New Moon. But the classic appearance of earthlight is that you have an entire faint disk sitting inside the brighter crescent. Hence one of the folk names for it: “the New Moon in the arms of the Old”.
Much less common is a more subtle effect mentioned by Minneart in Light and Colour in the Open Air (Dover, p139), the classic text about this sort of thing.
(His reference for this is “Phil.Mag.,4, 427. Also J.B.A.A., 28, 29, 45.”) This sounds exactly like what your fiancee was seeing.
In general, it’s quite common to find subtle contrasts - here between the faint earthlit disc and the night’s sky - being perceived by the eye and brain as illumination along the boundary.
I saw this effect this morning myself; it was a classic Earthshine effect, with the ‘old moon in the new moon’s arms’;
but now you come to mention it, there did seem to be a slightly lighter edge to the dark side, opposite the cresent.
I didn’t realise it was a mysterious phenomenon, but I’ll watch out for it in future.
I have a vague recollection that the contract between the light and dark sides cause a number of strange phenomena; the dark side seems to ‘pop out’ of the crescent somehow, for example.
As it happens I was trying to see Jupiter and Mercury, which were to the right and below the Moon respectively; all in all, a beautiful sight.
Thanks, and it does make a bit of sense. I’m still thinking about an experiment of covering the bright sliver from my view and trying to see if I can see the faint edge on the other side. I figgure if it’s a true image, that should make it clearer.
Just to clarify, here: Are you asking about the whole surface of the Moon being visible, or just a thin ring around the edge? The former is a very real phenomenon, due to reflected Earthlight, as mentioned above, but I’ve never noticed the latter.