<BLOCKQUOTE id=quote><font size=2 face="Verdana, Arial, Helvetica" id=quote>quote:<hr height=1 noshade id=quote>[Jeremy]: I saw a progressive darkening that seemed to move around slowly on the surface and which brightened and darkened a couple of times. Only towards the end did it settle down and get uniformly darker. Have I smoked too much good weed or is this for real? I was particularly surprised by the rolling movement of the shadow.<hr height=1 noshade id=quote></BLOCKQUOTE id=quote></font id=quote><font face="Verdana, Arial, Helvetica" size=2 id=quote>
For an idea of lunar eclipse geometry, see [url]
sunearth.gsfc.nasa.gov/eclipse/LEplot/LE...001/LE2005Oct17P.gif
[/url]. It shows the Earth's shadow and the Moon for a different eclipse, but also a partial one, which I gather yours was based on your description.
The Earth's shadow has two parts: the umbra, which is the deep shadow into which no sunlight penetrates; and the penumbra, in which an observer would see the Sun as partially eclipsed, but would still receive some direct sunlight. The penumbra surrounds the umbra. Both progress slowly against the star background as Earth orbits the Sun, but might be considered as effectively fixed-in-place during a lunar eclipse.
The Moon does almost all the moving during a lunar eclipse as it orbits the Earth monthly. It moves its own diameter in roughly an hour. So the Moon first penetrates the penumbra, which produces a slight darkening, but not enough to notice unless you are watching for it.
As the Moon finally penetrates into the umbra, the really dark part of Earth's shadow begins to cross the Moon's surface. If the Moon passed right over the middle of Earth's shadow, the shadow would seem to progress from one side of the Moon's surface to the other as viewed from Earth. But in partial eclipses, the Moon just dips part way into the shadow, which never covers the whole surface. You can see from the picture at the above link that the part of the Moon's surface in shadow will appear to "roll" across the surface as the Moon moves straight across the circular edge of the umbra.
The actual darkness of the umbra does change greatly from one eclipse to another because of conditions in Earth's atmosphere. Most of the time, the atmosphere scatters some sunlight into the dark part of Earth's shadow, which then looks orangish on the Moon. But when we have had a recent volcanic eruption and Earth's atmosphere has lots of ash in it, scattered light is eliminated and the Moon has been known to disappear from view to the naked eye during a total eclipse.
However, the darkness of the umbra is unlikely to change much during a single eclipse. The brightness of the various parts of the Moon then depends somewhat on local viewing conditions -- how much moisture, cirrus cloud, and dust is in the air through which you are looking.
But the strongest effect is the contrast effect. The Full Moon (lunar eclipses can only occur when the Moon is Full) is very bright. The human eye cannot easily look at a very bright light source and still see detail in a dark area nearby. So when the eclipse is near its beginning or end, the umbra looks totally dark. But as the eclipse progresses, uneclipsed moonlight get dimmer and more detail can then be seen in the shadowed part. It was there all along, but the eye cannot see it when the Moon is too bright. It's a little like trying to see bright stars in the daytime -- possible, but only if one eliminates all direct and most scattered sunlight from entering the eye. -|Tom|-
