Can two lunar eclipses occur in the same month? (Advanced)

I had come across your page regarding solar and lunar eclipses occuring together on the same month. Could you tell me if it's possible for two LUNAR eclipses occuring on the same month? I have heard it could occur. If it's indeed possible, even theoretically, could you please give the year/month that this could happen or already happened?

The answer to your question is no: it is impossible for two umbral lunar eclipses to occur in one month, and marginally possible for two penumbral eclipses to occur (which you would have to look hard to find). In this list of lunar eclipse dates from 1950 to 2050, I couldn't find a single occurrence to two lunar eclipses (even penumbral ones) in one month.

Here is the reasoning behind the conclusions drawn above (this gets a bit technical, so stop here if you don't want the gory details):

A lunar eclipse occurs when the Earth, Moon and Sun are aligned such that the Earth's shadow blocks part of the full Moon. We don't get eclipses every time a full Moon occurs, because the Moon's orbit around the Earth lies not exactly in the ecliptic (the plane in which the Earth orbits the Sun), but is inclined by about 5 degrees with respect it. So, in the course of a month, the Moon moves in and out of the ecliptic on its orbit around the Earth. The Moon is half a degree across in the sky, and the Earth's umbral shadow (the "dark" part) is about one and a half degrees across; therefore, in order for a lunar eclipse to occur, the centres of the Moon and the Sun (which governs where the Earth's shadow lies) must be less than a degree apart, which means that the Moon must be less than a degree above or below the ecliptic.

In order for two lunar eclipses to occur in one month, the Sun and the Moon must be less than 1 degree apart twice in 30 days. The most that the Moon can move with respect to the ecliptic is therefore 2 degrees; in this case you would get a partial eclipse covering one part of the Moon at the start of the month, and a second partial eclipse covering the opposite end of the Moon at the end of the month. So the question you're asking becomes: how many days does it take the Moon to move 2 degrees with respect to the ecliptic, and still lie on the opposite side of the Earth from the Sun such that a lunar eclipse may take place?

The keys to answering this question are the inclination of the Moon's orbit with respect to the ecliptic (5 degrees), and the fact that the Earth (and hence the Moon) moves about 1 degree in its path around the Sun per day. By making a triangle with sides made up by the ecliptic, the Moon's orbit and the Moon-Sun distance, we can conclude that the Sun moves two degrees relative to the plane of the ecliptic in about 20 days. So, it takes *less* than a month for the the Moon to move 2 degrees with respect to the Sun, which means that if a lunar eclipse occurs on day 1, by day 30 the Moon will have moved too far above from the ecliptic for a lunar eclipse to occur. This is strictly true for an umbral eclipse, and marginally true for a penumbral one (if you run through the calculation again using 3 degrees instead of 2 everywhere, you find that the Moon moves 3 degrees in about a month, so it's too close to tell).

This page was last updated on July 18, 2015.

About the Author

Christopher Springob

Christopher Springob

Chris studies the large scale structure of the universe using the peculiar velocities of galaxies.  He got his PhD from Cornell in 2005, and is now a Research Assistant Professor at the University of Western Australia.

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