It was kind of hard to miss the lunar eclipse this week, although I didn't see it in person (Sod's law means that on every relatively major astronomical event clouds cover where I am). From what I understand it lasted about 100 minutes. I work that out as being about 9 minutes short of 1 degree of the Moon's orbit?

How did it last so long? Surely 100 minutes is more than enough time for the Moon to move out of Earth shadow, or for Earth's shadow to "overtake" the Moon?


1 Answer 1


Since the Earth is much bigger than the Moon, (by about a factor of 4 in radius) its shadow at the distance of the lunar orbit is much larger than the Moon itself. So it is possible for the moon to spend considerable time in the Earth's shadow depending on the geometry of the eclipse.

Since the Moon's orbit is inclined relative to the Earth's orbit around the Sun, it doesn't always pass through the center of the Earth's shadow (and most of the time misses the shadow completely which is why there isn't a lunar eclipse every month). If the geometry of the eclise is such that the moon only passes through the edge of the Earth's shadow, the eclipse will be fairly short. If, however, as it was in the case of the June 2011 eclipse, the moon passes through the center of the shadow, the eclipse can last for a considerable time.

The mean orbital speed of the Moon is about 1 km/s (it's just a tad over that actually). At the distance of the lunar orbit, the Earth's shadow is about 9214 km in diameter. Totality begins once the trailing edge of the moon enters the Earth's shadow. Since the Moon is about 3475 km in diameter, once it has entered the Earth's shadow, it still has another 5739 km to go to get out. Traveling at 1 km/s, that's another 95 minutes before the eclipse ends at the longest point.

However, the Moon isn't always at its mean distance or traveling at its mean orbital speed. If it is near apogee (the point in it's orbit furthest from the Earth), it will be travelling somewhat slower. The size of the Earth's shadow doesn't vary considerably over the range of lunar distances so it is still nearly the same size in this case. However, since the moon is moving slower, it takes longer to traverse through the shadow. The maximum theorical length of a lunar eclipse in this configuration is just over 100 minutes.

There is actually another (very) minor effect that adds to the length of any lunar eclipse. The Moon is chasing the Earth's shadow across the sky. And during the time of the lunar eclipse, the Earth is still orbiting the Sun. Since the Earth moves approximately 1 degree in its orbit around the Sun every day, and a lunar eclipse can last on the order of an hour, the Earth will have moved about 1/24 th of a degree during the eclipse. This shifts the shadow of the Earth slightly forward requiring the Moon to move just a little farther to get out of the shadow. It's not a very big effect but it is there.

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    $\begingroup$ one more small point: if lunar apogee had occurred closer to solar pericenter (early Jan) when the earth is moving fastest, the eclipse would have lasted slightly longer $\endgroup$
    – Jeremy
    Commented Jun 17, 2011 at 13:58
  • $\begingroup$ Hang on! If the Earth were closer to the Sun, the Sun would be a larger apparent size, meaning the Earth's shadow would be narrower. This effect would act to shorten the eclipse. Would that trump the greater speed? $\endgroup$
    – Andrew
    Commented Jun 24, 2011 at 19:31
  • $\begingroup$ If we were closer to the sun the shadow would be slightly smaller, the trade off is left as an exercise for the reader :) $\endgroup$
    – dagorym
    Commented Jun 24, 2011 at 20:32

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