How Often Do Solar Eclipses Occur?
There are between two and five solar eclipses every year, each visible only in a limited area.
Next Total Solar Eclipse: Mon, Apr 8, 2024 … See animation
How Many Eclipses in a Year?
Most calendar years have two solar eclipses. The maximum number of solar eclipses that can take place in the same year is five, but this is rare. According to NASA calculations, only about 25 years in the past 5,000 years have had five solar eclipses. The last time this happened was in 1935, and the next time will be in 2206.
Types of Solar Eclipses
There are three main types of solar eclipses:
In addition, there are hybrid solar eclipses, also called annular-total eclipses, when an annular eclipse changes into a total eclipse, or vice versa.
When Does a Solar Eclipse Occur?
Solar eclipses only occur around the New Moon because of the alignment of Earth, the Moon, and the Sun which happens at that time. But this does not mean that eclipses of the Sun happen every New Moon.
The New Moon also has to be near a lunar node. This can only happen during a period of time that occurs a little less than every six months, and lasts, on average, around 34.5 days. This period is called the eclipse season, and it is the only time that eclipses take place.
The lunar nodes are the two points where the plane of the Moon's orbital path around Earth meets Earth's orbital plane around the Sun, the ecliptic. The plane of the Moon's path around the Earth is inclined at an angle of approximately 5° to the ecliptic.
Solar Vs. Lunar Eclipses
Even though solar eclipses occur every year, they are considered a rare sight, much rarer than a lunar eclipse. This is because while a solar eclipse is only visible from a very narrow path on Earth, a lunar eclipse is visible from every location on the night side of the Earth while it lasts.
Total Eclipse in the US
On August 21, 2017, a total solar eclipse was visible in a narrow track spanning the United States. This was the first total solar eclipse visible from anywhere in mainland United States since the total solar eclipse in February 1979. The next total eclipse in the US is in April 2024.
On average, it takes about 375 years for a total solar eclipse to happen again at the same location. By comparison, a total lunar eclipse, also known as a Blood Moon, can be seen from any location approximately every 2.5 years.
How Often Do Solar and Lunar Eclipses Occur
Most years have four eclipses: the minimum number of eclipses in a year; 2 of these four eclipses are always solar eclipses. While rare, the maximum number of eclipses that can take place in a calendar year is seven.
There are two or three eclipses during every eclipse season. At least one of these is always a solar eclipse, sometimes two. The same is true for lunar eclipses. Which order they come in depends on how each eclipse season coincides with the lunar (synodic) month.
The lunar month is the period it takes the Moon to go through all the Moon Phases from a New Moon to the next, and it lasts, on average, 29.5 days. This is five days less than an eclipse season. Therefore, there will always be at least one New Moon, resulting in a solar eclipse, and at least one Full Moon, resulting in a lunar eclipse, during each eclipse season.
This is also why solar and lunar eclipses come in pairs—a solar eclipse always takes place either about two weeks before or after a lunar eclipse, and vice versa.
At most, there can be two New Moons and one Full Moon, or two Full Moons and one New Moon in the same eclipse season.
Similar Eclipses Every 18 Years
Solar eclipses occur in cycles, called eclipse cycles.
One of the most popularly studied eclipse cycles is the Saros cycle, which the ancient Babylonians used to predict lunar eclipses.
The Saros cycle is approximately 6,585.3 days, or around 18 years, 11 days, and 8 hours long, and it occurs due to a combination of three lunar cycles:
- The lunar (synodic) month: the time it takes from one New Moon to the next.
- The anomalistic month: the time it takes from Moon perigee to perigee, the point of the Moon's orbit closest to Earth.
- The draconic month: the time it takes from one lunar node to another. Also called the nodical month, it lasts, on average, 27.212220 days ( i.e. 27 days, 5 hours, 5 minutes, and 35.8 seconds).
Two solar eclipses separated by a Saros cycle have certain similarities: they occur at the same lunar node, with the Moon roughly at the same distance from Earth, and they take place around the same time of the year.
The path of totality for the eclipses also looks very similar on a map—with one important difference. The path of totality is shifted westwards, one-third of the way around the globe. This is due to the fraction 0.3 in the Saros period of 6,585.3 days. During this one third of a day, the Earth spins on its axis, meaning the eclipse takes place further to the west.