A page on NASA’s website is at the center of the attention of flat-Earth followers. It is a page created by Fred ‘Mr. Eclipse’ Espenak, explaining the Saros cycle. Here’s the first paragraph from the page:
The periodicity and recurrence of eclipses is governed by the Saros cycle, a period of approximately 6,585.3 days (18 years 11 days 8 hours). It was known to the Chaldeans as a period when lunar eclipses seem to repeat themselves, but the cycle is applicable to solar eclipses as well.
After they discovered that the said ‘Chaldeans’ was a civilization from 25 centuries ago, they got excited and jumped to the conclusion that NASA used ancient technology to predict eclipses, and thus eclipses can’t be predicted from the motion of the Sun, Earth and Moon themselves.
Like other similar cases, this tells us more about the flat-Earthers themselves than about NASA.
The Saros cycle is a period that can be used to predict the occurrence of eclipses. The Saros cycle has a period of 18 years, 11 days and 8 hours. These days, the Saros cycle is used mainly to classify eclipses into the Saros series.
The eclipses that belong to the same Saros cycle have similarities, and are identified by their Saros series number.
For example, the great American solar eclipse of August 21, 2017 is an eclipse from the Saros series 145. This eclipse is the 16th eclipse in the series from a total of 77 eclipses. The next eclipse in this series will occur on September 2, 2035. The Saros series 145 itself began on January 4, 1639 and will not end until the 77th eclipse on April 17, 3009.
A Saros series overlaps with several other Saros series. In a year, there can be two to five solar eclipses, which obviously belong to different Saros series. At any one time, there are about 40 Saros series in progress. After a Saros series ends, a new Saros series will begin and take their place.
The cycle of lunar eclipses was certainly known by the Chaldeans. But it was Edmund Halley in 1691 who first named the cycle ‘Saros’. And the numbering system used in Saros series was introduced by G. van den Bergh, in 1887.
So, if not using Saros cycle, how did they predict eclipses? Saros cycle certainly can still be used to roughly predict future eclipses. But, today we can also predict the occurrence of eclipses in sub second resolution. We can also predict the exact path and location where a solar eclipse will occur. Even the terrain of the Moon and Earth are accounted for. Phenomenon like Baily’s Bead and the shape of Corona can also be predicted in advance. Obviously, these facts can never be known from the eclipse cycle alone.
These days, we use the Saros cycle unlike how we use the unit of time ‘year’. One year is the time the Earth takes to travel once around the Sun. Each year is sequentially numbered to make it easier for us to reference a specific year. Let’s not forget that the system is a technology brought to us by the ancient Romans, a technology that we happily still use today.