Astronomy Essentials

June 1 is the middle of an eclipse season

A totally eclipsed sun with a bright light emerging on one side: the diamond ring effect.
View larger. | Every eclipse falls within an eclipse season. Beverley Sinclair captured this beautiful view of a total solar eclipse outside Charleston, South Carolina, on August 21, 2017, and wrote: “This photo shows the diamond ring and Baily’s beads.” Thank you, Beverley!

We’re midway through an eclipse season

We had a total eclipse of the moon on May 26, 2021 (photos here). Coming up, we’ll have an annular eclipse of the sun on June 10. Thus we’re now midway through an eclipse season.

What’s an eclipse season? It’s an approximate 35-day period during which it’s inevitable for at least two (and possibly three) eclipses to take place. Typically, there are two eclipses in one eclipse season, and two eclipse seasons in one calendar year, which translates tofour eclipses per year. Eclipse seasons repeat in cycles of 173.3 days (somewhat shy of six calendar months).

Why don’t you see that many eclipses then? For lunar eclipses, the moon has to be above your horizon in order for you to see a lunar eclipse. In other words, it has to be night, or close to night, and that might not always be the case during the eclipse. Solar eclipses are even harder to catch. A total solar eclipse can be seen only from a narrow track along Earth’s surface. The accompanying partial solar eclipse can be seen only in areas adjacent to that track.

2020 and 2021 eclipse seasons

The middle of the last eclipse season was December 11, 2020. It presented 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 approximately six months from now, on November 23, 2021. That late 2021 eclipse season will feature an almost-total partial lunar eclipse on November 19, 2021, and a total solar eclipse on December 4, 2021.

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 a scenario where the moon orbited Earth on the same plane as 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). 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 two solar eclipses and two lunar eclipses.

Four columns of numbers and dates, one column for each phase of the moon.
In the year 2021, there are 12 new moons and 12 full moons. A = annular solar eclipse and T = total solar eclipse; t = total lunar eclipse and p = partial lunar eclipse. List of moon phases via AstroPixels.com.

Lunar nodes point at the sun

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.

Diagram 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. Image via Wikimedia Commons.

Two or 3 eclipses in one eclipse season?

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 three 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 three 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?

Eclipse terminology

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 Earth's orbit with the moon in 4 positions and 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.

Minimum of 4 eclipses in one year

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 two lunar eclipses and two solar eclipses occur in one calendar year. Yet, depending on how the eclipse seasons and lunar phases align, it’s possible to also have five, six or seven eclipses in one year.

For the maximum of seven eclipses to occur in one calendar year, the first eclipse must come in early January. That leaves 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. The middle eclipse season stages 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.

Maximum of 7 eclipses in one year

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 of eclipsed sun showing as narrow rings.
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!
Diagram 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.
Diagram 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.

Bottom line: Eclipse seasons are periods during which eclipses not only can 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). In 2021, the middle of an eclipse season falls on June 1, 2021, and then again on November 23, 2021.

Read more: Annular solar eclipse on June 10, 2021

Posted 
June 1, 2021
 in 
Astronomy Essentials

Like what you read?
Subscribe and receive daily news delivered to your inbox.

Your email address will only be used for EarthSky content. Privacy Policy
Thank you! Your submission has been received!
Oops! Something went wrong while submitting the form.

More from 

Bruce McClure

View All