Saros (astronomy)

In astronomy, the saros is a special period of time that helps us predict when eclipses of the Sun and Moon will happen. This period lasts exactly 223 synodic months, which is about 18 years, 11 days, and 8 hours. During this time, the Sun, Earth, and Moon line up in almost the same way as they did before, meaning a similar eclipse will occur.

Because of this pattern, each eclipse belongs to a group called a saros series. Eclipses in the same series happen one saros apart. For example, if there is a solar eclipse or a lunar eclipse, after one saros period, the Moon will be at the same point in its path around Earth, making another eclipse possible.

This natural cycle comes from the way the Moon’s phases, its path around Earth, and its closest and farthest points all line up just right. By studying the saros, scientists can figure out when future eclipses will happen and what they will look like.

History

The earliest known record of the saros comes from Chaldean astronomers in ancient times. Later, famous thinkers like Hipparchus, Pliny, and Ptolemy also knew about it.

The term "saros" was given to this eclipse cycle by Edmond Halley in 1686. He took the name from a book called the Suda, which described it as a measure used by the Chaldeans. The saros period of 223 lunar months is even shown on the Antikythera Mechanism, an ancient Greek device made around 150–100 BCE.

Description

The saros is a special period of time that helps us predict when eclipses will happen. It lasts for 6,585 days, which is about 18 years and 11 days. During this time, the Moon, Earth, and Sun line up in almost the same way as they did before, meaning a very similar eclipse will occur.

Eclipses can only happen when the Moon is between the Earth and the Sun (for a solar eclipse) or when the Earth is between the Sun and the Moon (for a lunar eclipse). This lining up happens when the Moon is at a special point in its orbit called a node. Because the saros matches up with the Moon's phases and its orbit, eclipses that happen one saros apart look almost the same.

Saros series

A saros series is a way to group together similar eclipses that happen over many years. Each series begins with a small, partial eclipse and, over time, the path of the Moon shifts slightly with each eclipse. This means that the eclipses in a series can change from happening in one part of the world to another.

For example, lunar saros series 131 started in the year 1427 with a partial eclipse. Over time, it produced total eclipses, with the most recent one happening in 1950. These total eclipses will continue until the year 2078, after which the eclipses will become partial again before the series ends in 2707. During this time, where an eclipse is best seen changes with each occurrence, moving across different parts of the Earth.

Relationship between lunar and solar saros (sar)

After a solar or lunar eclipse, wait about 9 years and 5 and a half days — this is called a sar. During this time, the type of eclipse switches: if the Sun’s shadow partly covers Earth during a solar eclipse, half a sar later the Earth’s shadow will partly cover the Moon during a lunar eclipse. This pattern helps us understand how solar and lunar eclipses are connected.

The sar includes 111 and a half synodic months, with the extra half month (called a fortnight) allowing the eclipse type to change. You can see this pattern clearly in this chart, where each row shows what happens one sar apart.