The Durations Of Lunar Eclipses

How long do lunar eclipses last? How long does the Moon take to cross the umbra and the penumbra along paths like those shown in Figure A-13? The sums are quite easy to do once one knows the speed of the Moon in its orbit. (One might imagine that it is more complicated because the Earth's conical shadow is not staying still, moving along as the planet orbits the Sun, but remember that the Moon is moving with us.) A few taps on the pocket calculator show that the Moon's speed in its geocentric orbit is around 2,300 miles per hour, although variable between perigee

and apogee. The diameter of the umbra is about 5,700 miles, so the Moon takes close to two and a half hours to traverse a central line through that shadow.

At least, that is what you get if you are considering just the center of the lunar disk. In reality, that is not what one observes. The Moon is large, and observers note when the edges of its apparent disk touch the extremes of the umbra and penumbra. As shown in Figure 2.5 it is conventional to define several distinct contact points. The first is P1, when the leading edge of the lunar limb touches the periphery of the penumbra. U1 is similarly defined for the initiation of entry to the umbra, and U2 is when the Moon is completely immersed therein. Exit from the umbra is U3, and then U4 is when the trailing part of the Moon escapes the umbra, the final exit from the penumbra being P4. (One could similarly define junctures P2 and P3 but they are of limited utility.)

The phase of totality for a lunar eclipse is between U2 and U3. This may last for an hour and a half, but it can be much less if the Moon crosses the umbra far off-center. Under such circumstances certainly most of the Moon is within the umbra for about an hour, but true totality is only briefly achieved. Unlike with total solar eclipses, the distinction is not important. The entire eclipse may be considered to last throughout the interval, with some part of the Moon within the penumbra, meaning from P1 to P4. This lasts for up to five and a half hours.

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