The middle of the current eclipse season falls on June 1, 2021. This eclipse season features two eclipses: a total eclipse of the moon on May 26, and then one fortnight (approximately two weeks) later, an annular eclipse of the sun on June 10. Want to learn more about the cycle of eclipses? Keep reading …
An eclipse season features an approximate 35-day period during which it’s inevitable for at least two (and possibly three) eclipses to occur. Typically, there are two eclipses in one eclipse season, and two eclipse seasons in one calendar year. Eclipse seasons repeat in cycles of 173.3 days (somewhat shy of six calendar months). The middle of the last eclipse season was December 11, 2020, presenting a penumbral eclipse of the moon on November 30, 2020, and a total eclipse of the sun on December 14, 2020.
In 2021, the middle of the year’s second eclipse season will fall on November 23, 2021, to usher in an almost-total partial lunar eclipse on November 19, 2021, and and total solar eclipse on December 4, 2021.
An eclipse season most often presents only two eclipses. However, if the first eclipse falls early in the eclipse season, then it’s possible for a third eclipse to occur before the eclipse season ends.
The last time 3 eclipses happened in one eclipse season was June-July 2020:
The next time 3 eclipses will occur in one eclipse season will be June-July 2029
Here are some words you need to know to understand eclipse seasons: lunar nodes and ecliptic. The ecliptic is the plane of the Earth’s orbit around the sun. A lunar node is the point where, in its monthly orbit of Earth, the moon’s orbit intersects that plane. An eclipse season is when – from Earth’s perspective – the sun is close enough to a lunar node to allow an eclipse to take place. If the sun is close to a lunar node at full moon, we see a lunar eclipse. If the sun is close to a lunar node at new moon, we see a solar eclipse.
To put it another way, if the moon turns new or full in close concert with the moon’s crossing of one of its nodes, then an eclipse is not only possible – but inevitable.
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) happens in one eclipse season. A maximum of three eclipses is possible (either lunar/solar/lunar, or solar/lunar/solar), though the first eclipse of the eclipse season has to come quite early to allow for a third eclipse near the end.
A minimum of 2 lunar eclipses and 2 solar eclipses occur in one calendar year. Yet, depending on how the eclipse seasons and lunar phases align, it’s possible to have up to 5, 6 or 7 eclipses in one year.
For the maximum of seven eclipses to occur in one calendar year, the first eclipse must come in early January, in order to leave enough room for the seventh eclipse in late December. In one scenario, an eclipse season sporting two eclipses comes early in the year, and late in the year, with the eclipse season in middle of the year staging three eclipses.
Also, it’s remotely possible for a calendar year to sport two eclipse seasons with three eclipses each, and one eclipse from an eclipse season that straddles into the previous or following year. By way of example, we present the years 1934-35 and 1879-80.
Any lunar eclipse that comes early, or late, in an eclipse season finds the full moon missing the Earth’s umbra (inner dark shadow) completely, and passing through the penumbra (outer faint shadow) instead.
A solar eclipse can happen only at new moon. A lunar eclipse can happen only at full moon. Additionally – for an eclipse to occur – the new moon or full moon has to take place within an eclipse season. Otherwise, the new moon passes too far north, or south, of the sun for a solar eclipse to take place, and the full moon sweeps too far north, or south, of the Earth’s shadow for a lunar eclipse to take place.
Why do we have eclipse seasons?
There are many cycles in the heavens. An eclipse season is just one of these many celestial cycles.
Consider that if the moon orbited Earth on the same plane that the Earth orbits the sun, then we’d have a solar eclipse at every new moon, and a lunar eclipse at every full moon.
But – in reality – the moon’s orbit is inclined by 5 degrees to the ecliptic (Earth’s orbital plane), so most of the time the new moon or full moon swings too far north, or south, of the ecliptic for an eclipse to take place. For instance, in the year 2021, we will have 12 new moons and 12 full moons, but only 2 solar eclipses and 2 lunar eclipses.
Twice every month, as the moon circles Earth in its orbit, the moon crosses the ecliptic (Earth’s orbital plane) at points called nodes. If the moon is going from south to north, it’s called the moon’s ascending node, and if the moon is moving from north to south, it’s called the moon’s descending node.
Whenever the lunar nodes point directly at the sun, that momentous event marks the middle of the eclipse season. The alignment of the moon, sun and Earth is most exact when an eclipse happens at the middle of an eclipse season, and the least so when an eclipse occurs at the start, or the end, of an eclipse season. Any lunar eclipse happening early or late in the eclipse season presents a penumbral lunar eclipse, whereas any solar eclipse happening early or late in the eclipse season features a skimpy partial eclipse of the sun.
This year, 2021, the middle of the eclipse season falls on June 1, 2021, and then again on November 23, 2021.
Bottom line: Eclipse seasons are periods during which eclipses not only an take place, but must take place. A minimum of two eclipses (one solar and one lunar, in either order) happens in one eclipse season. A maximum of three eclipses is possible (either lunar/solar/lunar, or solar/lunar/solar).
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.