View Full Version : Lunar Eclipse Moon apppears as a sphere

2007-Jun-30, 09:20 PM
I have a question about a known but seldom discussed lunar eclipse phenomenon that I personally experienced a couple of years ago in Fullerton, California.

As you know, the full moon appears as a flat disk to the human eye. It has absolutely no limb darkening like Jupiter or other planets. The reason why was not answered until the Apollo moon landings.

The surface of the moon is covered with very fine particles of moondust that was formed by the pulverizing effect of eons of meteoroid bombardment. This is an effect that was coined as "gardening" by lunar geologists. The moondust is as fine as talcum powder, but is not smooth in texture. The particles are all angular with faceted surfaces. So the particles reflect light directly back to the viewer.

As a result no matter what the angle of incidence of the curved surface of the moon, light is reflected straight back to your eye in a linear fashion, so equal amounts of photons strike your retina from not only the center of the moon's face, but also the edges of the moons surface at the limb. So the moon appears to be as flat as a dinner plate pasted into the sky.

However, this effect is nullified during a total lunar eclipse. When the eclipse occurs, all of the sunlight illuminating the moon is from a scattered source as the sunlight is refracted through the earth's atmosphere. So the light striking the moon is coming from more than one angle. The light reflecting back to your eyes from the moon is not only red colored, but the light level falls off as you look towards the limb of the moon's disk. This is the only time that you can see a lunar limb darkening effect.

With this limb darkening in view, and especially when the moon's face is framed by clouds or terrestrial objects like trees or buildings, the eclipsed moon magically and spookily appears to you as a three-dimensional sphere hanging in the sky. The first time I saw this I was absolutely stunned.

Now my question is, I learned about this effect by reading an article about it somewhere in a book, magazine or online. I cannot remember who wrote it and where I read it.
Can somebody point me to the article or references to this effect?

Matthew Ota
16 inch LX200classic prototype (at Mt. Wilson Observatory)
10 inch LX250GPS (at home)

2007-Jun-30, 09:28 PM
What are you doing..? Mathew.
You have posted this twice under dif,. header..?
scroll down to see replies...

2007-Jun-30, 09:38 PM
I'm only seeing this one thread...

And yes, the OP is bang on, he's talking about the Lommel-Seeliger photometric function which a lot of planetary surfaces have :). (the moon's a convenient one because it doesn't have an atmosphere with it's own photometric function to further scatter any light too).

I've not heard of the lunar eclipse effect but it makes sense from a physical point of view... next time I see one I'll have to make a point of looking for that :).

grant hutchison
2007-Jun-30, 10:28 PM
The original thread, with replies, is here (http://www.bautforum.com/showthread.php?t=61337).

Grant Hutchison