The middle of the current eclipse season falls on December 11, 2020. It features two eclipses: a penumbral eclipse of the moon on November 29-30, and a total eclipse of the sun on December 14. Want to learn more about the cycle of eclipses? Keep reading …
An eclipse season is a 35-day period during which it’s inevitable for at least two (and possibly three) eclipses to occur. There are four to seven eclipses every calendar year. Typically, there are two eclipse seasons. They recur in cycles of 173.3 days (somewhat shy of six calendar months). Each eclipse season has two or three eclipses. In some years, it’s possible to have a third eclipse season straddling into the previous or following year; that’s how we get to seven eclipses in some years.
The first eclipse of 2020’s second eclipse season came with the June full moon on June 5, 2020. The eclipse season of June and July 2020 featured three eclipses: a lunar eclipse on June 5, a solar eclipse on June 21, and the third eclipse of that eclipse season, a lunar eclipse, on July 5. That eclipse season – June and July 2020 – will be the last eclipse season with three eclipses until the year 2029.
We’ll also end 2020 with an eclipse season: a lunar eclipse on November 30 and a solar eclipse on December 14.
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.
If there are seven eclipses in one calendar year, there are a few possibilities. The first one belongs to an eclipse season that started in the previous year, and/or the seventh eclipse belongs to an eclipse season that ends in the following year. It’s rare for seven eclipses to occur in one calendar year, however. It last happened in the year 1982, and will next occur in the year 2038.
Any lunar eclipse that comes early, or late, in an eclipse season finds the full moon missing the Earth’s umbra (inner dark shadow), 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 2020, we will have 12 new moons and 13 full moons, but only 2 solar eclipses and 4 lunar eclipses (all of the lunar eclipses in 2020, unfortunately, are faint and hard-to-see penumbral 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, 2020, the middles of the eclipse seasons fall on June 20, 2020, and December 11, 2020.
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.