Eclipses of the new moon and full moon don’t occur every month, because the moon’s orbital plane is inclined by about 5 degrees to the plane of the ecliptic (Earth’s orbital plane). But the moon’s orbital path does intersect the Earth’s orbital plane at two points called nodes. When a line between the two lunar nodes points directly at the sun, we are at the midpoint of an eclipse season. Each eclipse season lasts approximately 35 days. The middle of an eclipse season recurs in cycles of about 173.3 days, or nearly 10 days shy of every six calendar months. The middle of the eclipse season (when the line of nodes pointed directly the sun) took place on December 30, 2019, and then on June 20, 2020. Looking ahead, the middle of the eclipse season will next fall on December 11, 2020.
During the 35-day eclipse season, any new moon or full moon will undergo an eclipse. Given that the lunar month (period of time between successive new moons or successive full moons) is about 29.5 days long, a minimum of two eclipses (one solar and one lunar, in either order), or a maximum of three eclipses (either lunar/solar/lunar, or solar/lunar/solar) can take place in one eclipse season.
Most often, there are only two eclipses in one eclipse season. For three eclipses to occur, the first one has to come quite early in the eclipse season to allow for a third eclipse near the end. For the first time since the year 2018, we’ll have three eclipses in one eclipse season in 2020. We won’t have three eclipses in one eclipse season again until the year 2029.
The year 2020:
If an eclipse happens at or near the mid-point of the eclipse season, as the upcoming solar eclipse on June 21st does, then we’ll have a central eclipse. If it’s a solar eclipse, the central eclipse presents either a total or annular eclipse of the sun; or if it’s a lunar eclipse, the central eclipse features a total eclipse of the moon. If the eclipse falls near the beginning or the end of the eclipse season, it’s either a penumbral eclipse of the moon or small partial eclipse of the sun.
Because the lunar eclipses occur so early and so late in the June/July 2020 eclipse season, the lunar eclipses on June 5, 2020, and July 5, 2020, will be extremely faint and hard-to-see penumbral lunar eclipses. See the illustration of these eclipses below.
On the other hand, the solar eclipse on June 21, 2020, which takes place almost dead center in the eclipse season, will present a central eclipse, exhibiting an annular eclipse of the sun. See diagram above.
Thirty-eight eclipse seasons (19 eclipse years) are almost exactly commensurate to 223 lunar months, a period of 18 years and 11 1/3 days (four intervening leap years) or 18 years and 10 1/3 days (5 intervening leap years). Therefore, the eclipses coming up in June/July 2038 display similar geometries to those in June/July 2020. This 223-lunar-month period of time is known as the Saros.
The year 2020:
The year 2038:
Interestingly, the Sar or Half Saros, representing a period of 111.5 lunar months (9 years and 5 2/3 days), gives us alternating eclipses (solar/lunar/solar) of similar character. Contrast the years 2020 and 2038 listed above with the years 2029 and 2047 listed below.
The year 2029:
The year 2047:
The eclipse master Fred Espenak tells us a Saros series can last anywhere from 1,226 to 1,550 years and is made up of 69 to 87 eclipses. A Saros series, whether it be solar or lunar, always starts off with skimpy eclipses and ends with skimpy eclipses. The middle of a Saros series brings about the closest alignment of the three celestial bodies – Earth, sun and moon – whereby they line up almost perfectly in space.
In any eclipse season where there are three eclipses, the first and third eclipses are meager productions whereas the middle eclipse is a highly visible central eclipse. And in any Saros series, the early and late eclipses are also paltry at best, whereas the middle part of a Saros series presents central eclipses.
Here’s something that may surprise you: Any eclipse happening early in an eclipse season always occurs late in a Saros series – and vice versa. For example, let’s look at the upcoming three-eclipse season in June/July 2020:
The year 2020:
The first eclipse of the eclipse season on June 5, 2020, belongs to Lunar Saros 111 and presents the 67th of 71 eclipses in this Saros series. Yet, the third and final eclipse of the eclipse season on July 5, 2020, belongs to Lunar Saros 149, and features the third of 71 eclipses in this particular Saros series.
Unsurprisingly, perhaps, the second (or middle) eclipse of the eclipse season on June 21, 2020, is the 36th of 70 eclipses in Solar Saros 137.
Bottom line: The middle of the eclipse season falls on June 20, 2020, and this eclipse season produces three eclipses (lunar/solar/lunar), though only the second of these three eclipses – the annular “ring of fire” eclipse on June 21, 2020 – will produce any real theatrics on the great stage of the sky.
Bruce McClure has served as lead writer for EarthSky's popular Tonight pages since 2004. He's a sundial aficionado, whose love for the heavens has taken him to Lake Titicaca in Bolivia and sailing in the North Atlantic, where he earned his celestial navigation certificate through the School of Ocean Sailing and Navigation. He also writes and hosts public astronomy programs and planetarium programs in and around his home in upstate New York.