How many eclipses occur in a year?

Eclipses are among the most beautiful and fascinating astronomical events that can be observed from Earth. Since ancient times, people have been captivated by the unique occurence of an eclipse, sometimes appreciating their beauty, and other times looking at them with fear. Now we know that eclipses are just a natural phenomenon and we can precisely predict them centuries in advance. Since eclipses are regarded as rare, every time a place experiences one it is a big event, with many people traveling even to remote locations in order to see them. But just how rare are eclipses? How many eclipses happen all over the world in a year?

First of all, we need to understand what eclipses are and how they can happen. Two types of eclipses can be seen from Earth: solar eclipses and lunar eclipses. Solar eclipses occur when the Moon obscures the view of the Sun, while lunar eclipses happen when the Moon passes through Earth’s shadow. In both cases an almost perfect alignment between the Sun, the Earth, and the Moon is necessary. These eclipses can occur because, by pure coincidence, the Sun and the Moon have almost the same apparent size of around half a degree, when seen from the Earth’s surface. This happens because the Sun’s diameter is around 400 times the Moon’s diameter, but also the Sun’s distance from the Earth is approximately 400 times larger then the Moon’s distance from our planet.

Geometry of a total solar eclipse (left) and a lunar eclipse (right).

However, the apparent size of the two bodies can vary slightly depending on their position relative to the Earth. If the Moon and the Sun are perfectly aligned with the Earth, a total solar eclipse happens when the Moon appears large enough to entirely cover the Sun. Otherwise, annular solar eclipses occur when the Moon’s apparent size is smaller then the Sun’s, leaving a narrow bright ring around the obscured central area. If the two bodies are not perfectly aligned, partial eclipses can happen, with the Moon covering only part of the Sun. During a solar eclipse, the shadow casted by the Moon on the Earth is small, and it can be seen only in a narrow strip along the Earth’s surface. As the Moon’s shadow moves along the Earth’s surface, a solar eclipse only lasts for a few minutes in each place from which it is visible. The regions around the area in which a total solar eclipse can be seen experience a partial solar eclipse.

Total solar eclipse of July 2, 2019 from La Silla Observatory in Chile (left) (ESO/P. Horalek, CC BY 4.0), annular solar eclipse of June 21, 2020 from Taiwan (center) (KOKUYO, Wikimedia Commons, CC BY-SA 4.0), partial solar eclipse of June 10, 2021 from Canada (right) (Paulo O, Wikimedia Commons, CC BY 2.0).

In the case of lunar eclipses, total ones occur when the Moon falls entirely in the umbra, the portion of the Earth’s shadow in which sunlight is completely blocked. If the umbra covers only part of the Moon, then it’s a partial lunar eclipse. There are no annular lunar eclipses since the section of the shadow cone casted by the Earth is larger then the diameter of the Moon. However, there is also the penumbra, a region of the Earth’s shadow in which sunlight is only partially blocked. If the Moon falls inside the penumbra, a penumbral lunar eclipse happens. In this case, there is only a slight dimming of the brightness of the Moon, and it is usually hard to notice. Unlike solar eclipses, lunar eclipses can be seen from anywhere on Earth where it is night, and they can last for up to two hours.

Total lunar eclipse of July 27, 2018 from Italy (left) (Giuseppe Donatiello, Wikimedia Commons, CC0 1.0), partial lunar eclipse of August 7, 2017 from Kuwait (center) (Irvin calicut, Wikimedia Commons, CC BY-SA 4.0), penumbral lunar eclipse of January 10, 2020 from Austria (right) (H. Raab, Wikimedia Commons, CC BY-SA 4.0).

If the Moon’s orbit was perfectly circular and on the same plane as the ecliptic (the orbital plane of the Earth), every new moon would result in a solar eclipse, and every full moon would also feature a lunar eclipse. However, the lunar orbital plane is tilted by about 5 degrees with respect to the ecliptic, so eclipses can only happen when the Moon passes close to the nodes, the intersections of the two planes. This happens about every six months, so eclipses can occur only during these “eclipse seasons”, if the distance of the Sun and the Moon from the nodes is small enough.

Since the orbit of the Moon is not a circle, but it has a slight eccentricity, the position of the nodes changes, completing a full rotation in around 346.6 days. So, in a calendar year there could be three eclipse seasons, because if one happens in early January, a third one would be in December. This means that in a year there are usually two lunar eclipses, but there could be a maximum of three.

A solar eclipse can happen if the Sun is close enough to one of the nodes, at an angular distance of less then a value that can vary between 15 and 18 degrees. As the Sun moves by about 29 degrees in a month relative to the nodes, there is usually one solar eclipse for every eclipse season. Nevertheless, if the Sun is around 15 degrees before the node when an eclipse happens, the next one would occur about a month later, with the Sun being 14 degrees after the node. Thus, it is possible to have two solar eclipses in an eclipse season, meaning that three or four could happen in a year. If a year has three eclipse seasons, there could be even five solar eclipses in a year, with two for every eclipse season, and another one right at the end of the year. However, this is very rare, the last time it happened in 1935, and it will only happen again in 2206.

Counting both solar and lunar eclipses (including penumbral ones), there are a minimum of four eclipses every year, two solar and two lunar ones. The maximum number is seven, either four solar and three lunar eclipses or five solar and two lunar ones. There could also be four penumbral lunar eclipses and three solar eclipses. The last year with seven eclipses was 1982, and the next one will be 2038, when there will be three solar eclipses and four penumbral lunar eclipses. Excluding penumbral lunar eclipses, the next year with seven eclipses will be 2094.

Thus, contrary to what it may seem, solar eclipses generally happen all over the world more often then lunar ones. Nevertheless, since lunar eclipses last longer and can be seen from a large area, you are much more likely to see a lunar eclipse then a solar eclipse from a specific place. For any given location, a total solar eclipse can be seen only once every around 400 years. So, if you ever have the chance to experience one from where you live, without specifically traveling to see it, don’t miss it!