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:
|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|