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Why Aren’t There Eclipses Every Month?

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Why Aren’t There Eclipses Every Month?

If solar and lunar eclipses occur when the Sun, Moon, and Earth form a straight line, why don’t we have a solar eclipse every time the Moon is in the new phase or a lunar eclipse every time the Moon is full—both of which happen every month?

And why do solar and lunar eclipses always seem to happen so close together? If you look back through the last several years of Farmers’ Almanac astronomy calendars, you’ll notice that solar and lunar eclipses always seem to fall within a couple of weeks of one another.

It’s About the Angle of the Orbit

It turns out that the answer to both of those questions is connected to the angle of the Moon’s orbit, which is inclined about 5° from the ecliptic, another name for the Earth’s orbit around the Sun. For eclipses to happen with every full or new Moon, the Moon and Earth would have to orbit at the exact same angle.

It takes the Moon a little less than a month (29.53 days) to orbit the Earth. In that time, it crosses the ecliptic plane twice. Each of these crossings is called a node. If the Moon is new or full at the time it crosses, a perfect alignment of Sun, Moon and Earth, called a syzygy (from the Greek word for “bound together”), occurs, resulting in an eclipse.

Most of the time, though, the Moon is either too low or too high for a syzygy to occur. If the Moon lies above or below the plane of the ecliptic when new or full, an eclipse won’t happen.

A Close Pairing

This is also why solar and lunar eclipses tend to fall within about two weeks of one another. If the Moon approaches close enough to the ecliptic during a full Moon for a lunar eclipse occur, it will once again approach the ecliptic about two weeks later, when it’s new, causing a solar eclipse, and vice versa.

The periods of time during which the Sun appears, from the Earth, to be close enough to the Moon to allow for an eclipse to occur is called an eclipse season. There are two roughly 34-day-long eclipse seasons each year, falling about 25 weeks apart.

At least two, and sometimes three, eclipses occur every eclipse season. If the first eclipse in the season falls at the very beginning of that season, there will be enough time for two more eclipses.

It’s even possible, though very rare, to have three eclipses in one calendar month. The last time that happened was during the year 2000, but it won’t happen again until 2206!

Because the space between eclipse seasons is about a week shy of half a year, eclipse seasons migrate backwards through our calendar, coming slightly earlier each year.

Most years have four or five eclipses, but it’s possible to have a year with six, or even seven. For seven eclipses to be possible, a single eclipse season must fall during December and carry over into the new calendar year. The last time we saw a year with seven eclipses was 1982. If you don’t remember that, you may still be around to see the next one in 2038.

Here’s a look at dates of eclipses past and future. By looking at several years at once, it’s easy to see the regularity of the pattern:

Date Event
September 13, 2015 Partial Solar Eclipse
September 27-28, 2015 Total Lunar Eclipse
March 8-9, 2016 Total Solar Eclipse
March 23, 2016 Penumbral Lunar Eclipse
September 1, 2016 Annular Solar Eclipse
September 16, 2016 Penumbral Lunar Eclipse
February 10-11, 2017 Penumbral Lunar Eclipse
February 26, 2017 Annular Solar Eclipse
August 7, 2017 Partial Lunar Eclipse
August 21, 2017 Total Solar Eclipse (This will be the first total solar eclipse visible from the mainland of North America since 1979).
January 31, 2018 Total Lunar Eclipse
February 15, 2018 Partial Solar Eclipse
July 13, 2018 Partial Solar Eclipse
July 27, 2018 Total Lunar Eclipse
August 11, 2018 Partial Solar Eclipse
January 5-6, 2019 Partial Solar Eclipse
January 21, 2019 Total Lunar Eclipse
July 2, 2019 Total Solar Eclipse
July 16-17, 2019 Partial Lunar Eclipse
December 26, 2019 Annular Solar Eclipse


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