May-June 2021: A special pair of eclipses

Written by Graham Jones and reprinted with permission from timeanddate.com. Additional edits by EarthSky.

Solar and lunar eclipses can occur only during a short period of time known as an eclipse season. No two eclipse seasons are the same, and some are particularly noteworthy. The upcoming eclipse season – May-June 2021 – will produce a pair of full eclipses: a total lunar eclipse on May 26, 2021, and an annular solar eclipse on June 10, 2021. This kind of pairing isn’t rare. It happens, on average, once every eight years or so. Still, this upcoming pair of eclipses is unusual in that we’ll have an especially short lunar eclipse and an especially long solar eclipse. You can watch both these eclipses live on timeanddate.com.

When two full eclipses are squeezed into a single fortnight, the alignment of the Earth, moon, and sun for each event is only just good enough to produce a totality or an annularity.

A short lunar eclipse … In the total lunar eclipse on May 26, 2021, the totality, when the moon is fully submerged in Earth’s shadow, will last a little over 14 minutes. That duration will make it the 10th-shortest totality for any lunar eclipse for the approximately 1,000-year span between the years 1600 and 2599. In terms of the total worldwide duration of the eclipse, which includes partial and penumbral phases, the May 26 event is even more exceptional: it is the second-shortest of all 693 total lunar eclipses in the same 1,000-year span.

… and a long solar eclipse. Within the same period, the June 10 annular solar eclipse is also notable for its length. Although it is not particularly long when compared to solar eclipses in general, it stands out as having the fifth-longest worldwide duration (including partial phases) of any total or annular solar eclipse that is paired with a total lunar eclipse in the same eclipse season.

Between the years 1600 and 2599, there are 2,108 eclipse seasons. Of these, only 126 seasons contain a pair of full eclipses: one total or annular solar eclipse, plus one total lunar eclipse.

At timeanddate.com, we looked at the worldwide duration of each eclipse in these pairs. This is the length of time between the first and last moments the eclipse is visible from somewhere in the world, including partial and penumbral phases.

Generally speaking, the better the alignment of the Earth, moon, and sun, the longer the worldwide duration of the eclipse. (There are several ways to measure how closely the three bodies are aligned, such as the magnitude of the eclipse, or a more technical number referred to as gamma. However, for fun, we looked at the total length of the eclipse in seconds.)

How the May-June 2021 eclipses compare to others. The worldwide duration of the annular solar eclipse on June 10, 2021, is 17,939 seconds, or just under five hours. For a full solar eclipse that occurs in the same season as a total lunar eclipse, this turns out to be unusually long: it is the fifth-longest solar eclipse in our 1,000-year sample of 126 pairs of full eclipses.

This means that the worldwide duration of the total lunar eclipse on May 26, 2021 is likely to be short … and indeed it is. Its total length of 18,127 seconds, or just over five hours, might not seem that quick. But for a lunar eclipse that includes penumbral, partial, and total phases, it is exceptionally fast.

Across all 2,108 eclipse seasons in the period 1600 to 2599, there are 695 total lunar eclipses. Only one of these has a shorter worldwide duration than the May 2021 eclipse. It is literally a few seconds shorter – four seconds, to be precise – and it will occur 345 years from now (on May 25, 2366).

If we consider only totality – in other words, if we ignore the partial and the penumbral phases – the May 2021 eclipse has a duration of 858 seconds (about 14 minutes). By this criteria, it is the 10th-shortest of all 695 total lunar eclipses in our sample period of one millennium.

Eclipses need a new moon or a full moon. The basic requirement for a solar eclipse is a new moon, which occurs when the moon passes between the Earth and the sun. A lunar eclipse requires a full moon, when the moon is on the opposite side of Earth from the sun.

The moon takes around 29.5 days to go through all its phases, and new moons and full moons are separated by about two weeks.

Eclipses don’t happen every month. The moon’s orbit around Earth is tilted by about 5.1 degrees to Earth’s orbit around the sun. This is why eclipses don’t happen every month: at new moon and at full moon, more often than not, the moon is too high or too low to align precisely with the Earth and the sun.

Eclipses come in seasons. About every six months, Earth comes to a sweet spot in its orbit where a perfect – or almost perfect – three-way alignment of the Earth, moon, and sun can occur. Each sweet spot lasts for about 34.5 days: an eclipse season.

If there is a new moon near the middle of an eclipse season, it will form a straight line with the Earth and the sun. The result will be a total solar eclipse, or an annular solar eclipse, if the moon is too far from Earth to cover the sun completely. In a similar way, a full moon near the middle of an eclipse season will produce a total lunar eclipse.

Read more: What is a hybrid solar eclipse?

Read more: What is an eclipse season?

On the other hand, if a new moon or a full moon comes near the beginning or end of an eclipse season, the three-way alignment will be not quite perfect. In this case, the result will be either a partial solar eclipse, a partial lunar eclipse, or, if the moon passes through only the faint outer part of Earth’s shadow, a penumbral lunar eclipse.

Eclipses guaranteed. Since a lunar month (the time taken for the moon to go from new to full to new again) is only 29.5 days, each eclipse season is guaranteed to produce – somewhere in the world – one solar eclipse and one lunar eclipse. There can also be a second solar or lunar eclipse, if the first one occurs in the first few days of the season.

Eclipses in balance. In most cases, the solar and lunar eclipses within each season balance each other: if one is a full eclipse, the other tends to be partial. To put it another way, if the alignment of the Earth, moon, and sun is perfect for one eclipse, the other eclipse will likely be near the beginning or end of the season, when the alignment is less perfect.

For example, the “Great American” total solar eclipse of August 21, 2017, was preceded by a small partial lunar eclipse 14 days earlier. And the total lunar eclipse of July 27, 2018 – the longest of the 21st century – was sandwiched between two small partial solar eclipses (on July 13 and August 11).

Two full eclipses in one season. Occasionally, however, both the new moon and the full moon fall close enough to the midway point of a season to produce a pair of full eclipses. It is possible to fit in two full eclipses like this, but it’s a squeeze: in each case, the alignment of the Earth, moon, and sun is only just good enough to produce a full eclipse.

Again, there is a balance between the two eclipses. The better the alignment is for one, the worse it will be for the other. This is what makes the forthcoming eclipse season especially interesting.

Note on the accuracy of eclipse predictions. All eclipse predictions contain a small margin of error. One of the uncertainties in lunar eclipse calculations, for example, is that the atmosphere gives Earth’s shadow a fuzzy edge.

This can lead to borderline cases such as October 17, 2024. We at timeanddate.com classify this as an “almost” lunar eclipse, while some sources list it as a penumbral eclipse.

Bottom line: Solar and lunar eclipses can only occur during a short period of time known as an eclipse season. The upcoming May-June 2021 eclipse season is an unusual one. Here’s how to understand it in the context of 1,000 years of eclipses.

Read more: Total eclipse of year’s closest supermoon on May 26, 2021

Read more: Annular solar eclipse on June 10, 2021

Read more: When is the next total solar eclipse in North America?