What is an eclipse season?

There are many cycles in the heavens. An eclipse season describes one of the most fascinating, and we’re in the midst of an eclipse season right now. Two eclipses – one lunar and one solar – will be forthcoming on May 26 and June 10, 2021

A totally eclipsed sun with a bright light emerging on one side: the diamond ring effect.

View larger. | EarthSky community member Beverley Sinclair captured this beautiful view of a total solar eclipse outside Charleston, South Carolina, on August 21, 2017, and wrote: “The skies were very cloudy leading up to totality but, miraculously, slowly cleared as totality approached. This photo shows the diamond ring and Bailey’s beads.” Thank you, Beverley!

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 …

EarthSky’s lunar calendar shows the moon phase for every day in 2021. Order yours before they’re gone! Makes a great gift.

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:

June 5, 2020: Penumbral lunar eclipse
June 21, 2020: Annular solar eclipse
July 5, 2020: Penumbral lunar eclipse

Read more: Middle of eclipse season June 20, 2020

The next time 3 eclipses will occur in one eclipse season will be June-July 2029

June 12, 2029: Partial solar eclipse
June 26, 2029: Total lunar eclipse
July 11, 2029: Partial solar eclipse

Read more: How often are there 3 eclipses in a month?

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.

Diagram of orbit of Earth with moon in four positions with moon's orbit at slight angle to Earth's.

Lunar nodes are where the moon’s orbit cuts through the ecliptic, or Earth-sun plane. When these nodes point directly at the sun, it marks the midpoint of an approximate 35-day eclipse season. In the year 2021, the middle of the eclipse season occurs on June 1, 2021, and November 23, 2021. Image via Go Science Go.

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.

It’s quite rare for seven eclipses to occur in one calendar year, however. Seven eclipses last happened in the year 1982, and will next occur in the year 2038.

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.

Three images, narrow rings around sun, two on sides with gaps.

View at EarthSky Community Photos. | Progression into and out of the annular eclipse on December 26, 2019, from Tumon Bay, Guam. Eliot Herman reported: “It was a beautiful day in Guam to observe the eclipse mostly clear blue skies with a little marine haze on the coast. These images were captured with a Questar telescope and a Nikon D850 camera using a Baader solar filter.” Thank you, Eliot!

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.

Read more: What’s a penumbral eclipse of the moon?

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.

Graphic of Earth, moon, and sun showing moon's shadow blocking the sun.

Eclipses are all about alignments. In a solar eclipse, the sun, moon and Earth line up, with the moon in the middle. Image via NASA.

Graphic of Earth, moon, and sun with Earth shading the moon.

In a lunar eclipse, the sun, Earth and moon line up, with the Earth in the middle. Image via NASA.

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.

Four columns of numbers and dates, one column for each phase of the moon.

In the year 2020, there are 12 new moons and 13 full moons. A = annular solar eclipse, T = total solar eclipse, and n = penumbral lunar elcipse. Moon phases via Astropixels.

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.

Read more: Node passages of the Moon: 2001 to 2100

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.

Previous to that, the middles of the last three eclipse seasons occurred on December 11, 2020, June 20, 2020, and December 30, 2019.

Middle of eclipse season: December 30, 2019
First eclipse (solar): December 26, 2019
Second eclipse (lunar):January 10, 2020

Middle of eclipse season: June 20, 2020
First eclipse (lunar): June 5, 2020
Second eclipse (solar): June 21, 2020
Third eclipse (lunar): July 5, 2020

Middle of eclipse season: December 11, 2020
First eclipse (lunar): November 30, 2020
Second eclipse (solar): December 14, 2020

Line drawing of sphere with oblique view of orbits.

The plane of the moon’s orbit is inclined at 5 degrees to the plane of Earth’s orbit around the sun (the ecliptic). In this diagram, however, the ecliptic is portrayed as the sun’s apparent annual path in front of the constellations of the zodiac. The moon’s orbit intersects the ecliptic at two points called nodes (labeled here as N1 and N2). It’s the middle of the eclipse season whenever this line of nodes points directly at the sun. In the above diagram, the line of nodes does not point at the sun.

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