2017 Solar Eclipse. Image Credit: Kyle Denny
Throughout history, solar eclipses have been viewed with great significance and wonder. Many ancient cultures associated eclipses with supernatural events or ominous signs, while others saw them as a chance to observe celestial phenomena and make astronomical discoveries. Over time, meticulous observers noticed that eclipses appeared to come in cycles and that one could predict exactly when and where an eclipse would happen.
Eclipse Seasons
Diagram explaining a solar eclipse. Image Credit: NASA
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But how often do eclipses occur? On average, there are at least two solar eclipses per year. However, not all of these are total solar eclipses, and many are, in fact, partial solar eclipses. For example, while the upcoming solar eclipse in the United States is the first total solar eclipse visible since the 2021 total solar eclipse, other eclipses such as the hybrid solar eclipse in April 2023, the annular solar eclipse in October 2023, as well as two partials in 2022 all were visible. Looking through the historical record, it is fair to say that a total solar eclipse is visible somewhere on the planet every 18 months or so, while some sort of solar eclipse can be expected every six months.
The reason we know this is because eclipses fall in seasons. Eclipse “seasons” can be defined as a roughly 35-day period during which at least two, perhaps even three, eclipses will take place. These seasons repeat in cycles of about 173 days, just short of six months. For example, the April 8th, 2024 eclipse will be preceded by a penumbral lunar eclipse on the morning of March 25th. The reason we have seasons instead of monthly occurrences is because the Moon’s inclination relative to the ecliptic (the path the Earth and planets follow around the Sun) is tilted about 5 degrees, causing the tilt of the Moon’s orbit to be aligned with the ecliptic only every 6 months.
Saros Cycle
Image of the Saros Cycle: Courtesy of Fred Espenak, NASA GSFC
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Eclipses can also be predicted centuries in advance. To understand this, we need to look at something called the “Saros cycle”. The Saros cycle has a period of about 6,585 days, or roughly 18 years and 11 days. The timing is significant as after which the Earth, Sun, and the Moon all return to roughly the same relative positions and the cycle of lunar and solar eclipses begin to repeat themselves.
So, for example, the solar eclipse of June 30, 1973 was followed by one 18 years later, on July 11, 1991. Likewise, the famous solar eclipse of August 21, 2017, will have a “twin” eclipse on September 2, 2035 - 18 years later as part of Saros cycle 145. It was recognized by ancient civilizations such as the Chaldeans as a pattern where lunar eclipses seemed to repeat themselves. Other civilizations noticed the same patterns and used them to predict the timing of solar eclipses as well.
Solar eclipses occur regularly and predictably due to the alignment of the Earth, Moon, and Sun. Eclipses happen in seasons, typically every six months, during which multiple eclipses can occur. This is due to the inclination of the Moon's orbit relative to the Earth's orbit around the Sun. The Saros cycle, lasting about 18 years and 11 days, allows for the prediction of eclipses centuries in advance by observing the repetitive patterns of lunar and solar eclipses. These cycles have been recognized by ancient civilizations and continue to be studied and utilized in modern astronomy for predicting future eclipses!
