On 3rd March 2007 there was a total eclipse of the Moon. In the picture above you can see the Moon imaged every 15 minutes from about 10 minutes before the start of the umbral phase until about 20 minutes after the end of that phase.
When the source of light is extended (like the Sun) then all shadows show two parts. The inner part, where the source is fully hidden, is called the “umbra”. Surrounding this shadow, from which the source appears partly hidden, is a lighter shadow called the “penumbra”. The penumbra of the Earth's shadow cast by the Sun is, at the distance of the Moon, a little larger than the Moon. So, as the Moon approaches the shadow, it first enters the penumbra, and a little later it enters the umbra. At the distance of the Moon, the Earth's umbral shadow is between two and three times the size of the Moon.
Look at my pictures. The first picture includes the outer penumbral boundary which theoretically passes between Mare Tranquilitatis and Mare Crisium - you cannot see it. The first picture does not include the penumbral-umbral boundary and yet the south-western region of the Moon is quite dark. Similarly, the last picture also includes the outer penumbral boundary passing roughly down the eastern edges of the Oceanus Procellarum, but again you cannot see it. The eastern limb is slightly darker than the rest despite the fact that the Moon had passed the edge of the umbra some 17 minutes earlier.
The Earth's atmosphere ruins the simple description of an eclipse given above. Light passing through the atmosphere is bent and, during a lunar eclipse, from the Moon the Earth would appear surrounded by a bright ring as light from the Sun is bent and scattered by the atmosphere. The result is that the eclipsed Moon is not invisibly dark and, because red light is bent more than blue light, it often appears a dark reddish hue. How dark depends on the state of the atmosphere at the time. On this occasion it was a rather dark eclipse and I could not show the eclipsed part of the Moon as well as the part still in sunlight.
The eclipse was a particularly dark one. so I had to use much longer exposures for the totally-eclipsed Moon than for the partial phases. The Moon passed through the northern part of the Earth's shadow so that the sequence of phases is not symmetrical.
If the Sun and the Earth were hard opaque spheres, the edges of the penumbra and the umbra would appear as sharp edges. In practice, however, the Sun is not a hard sphere, and the Earth is surrounded by an atmosphere. This makes the edges indistinct, and in any case the penumbra gradually darkens as one moves from its outer edge to the umbra; indeed there is really no difference across the penumbral-umbral boundary (or indeed across the outer penumbral boundary). To understand this, think of yourself on the Moon during an eclipse, and ignore the effect of the Earth's atmosphere. You watch the Sun approaching the Earth until it just touches the Earth. You are now at the edge of the penumbra. As the Sun slides behind the Earth, its light diminishes slowly and steadily as more and more of the Sun is hidden behind the Earth until eventually the last sliver disappears. You are now at the start of the umbra. You can see that the penumbra darkens evenly from nothing at its outer edge to completely dark at its inner edge. Home Back to Moon