The Saros cycle, a fascinating phenomenon in astronomy, plays a crucial role in predicting eclipses. Because the orbit of the moon is not exactly parallel to the orbit of the Earth around the Sun, eclipses (both lunar and solar) do not occur every time the moon is between the Earth and the Sun. They only happen when the moon passes through “nodes”, where the moon is exactly between the Earth and Sun (solar) or when the Earth is exactly between the Moon and Sun (lunar). These passes occur in recurring patterns, resulting in similar eclipse paths and durations at predictable intervals.
Duration and Mechanism:
The Saros cycle lasts approximately 6,585.3 days, roughly equivalent to 18 years, 11 days, and 8 hours.
It’s based on the alignment of the three celestial bodies: the Sun, Earth, and Moon. After one Saros cycle, these bodies return to a similar geometry, allowing for an almost identical eclipse (solar or lunar) to occur.
Key Points:
The Saros cycle comprises 223 synodic months, the time it takes for the Moon to return to the same phase relative to the Sun and Earth as seen from Earth (29 days 12 hours 44 minutes 3 seconds).
It also coincides with 239 anomalistic months (the time it takes for the moon to travel precisely from perigee to perigee, approximately 27.554550 days) and 242 draconic months (the time it takes for the moon to return to the same node, or alignment with the ecliptic. This is 27 days 5 hours 5 minutes 35.8 seconds). This remarkable coincidence is what enables the cycle’s accuracy.
Each Saros cycle results in a series of eclipses, called a Saros series. These series can last for 1226 to 1550 years, containing between 69 and 87 eclipses.
Each eclipse within a series has a similar path across Earth, but due to the slight shift in the Moon’s node, the series gradually moves westward across the globe by approximately 1/3 around the planet (because of the 0.3 component of 6585.3 days).
Why is this important?
Those are pretty precise numbers quoted above. Understanding the Saros cycle allows scientists to predict eclipses with exceptional accuracy, thousands of years in advance. The solar eclipse of 2017 across the United States was from Saros 145 and is number 22 of 77 eclipses in the series. This eclipse pattern will repeat in 2035 with an eclipse over China and the Pacific Ocean. The eclipse of 2024, across North America is from Saros 139 and is number 30 of 71 eclipses in the series. This eclipse pattern will repeat in 2042 with a path across Southeast Asia and the Pacific Ocean.