Welcome to week 3 of the Solar Eclipse Blog! Are you intrigued, like I am, by the patterns found in nature? This week Ken Brandt from the Robeson Planetarium and Science Center has brought us some fascinating facts about natural event patterns and the science of predicting eclipses.

I open with this excerpt from a twitter post by Neil DeGrasse Tyson: “Odd. No one is in denial of America’s Aug 21 total solar eclipse. Like Climate Change, methods & tools of science predict it.” The eclipse Dr. Tyson was writing about was “the great American eclipse” of 2017. Everything about the eclipse and the predictions that went into it went exactly according to planned expectations. There were no unpleasant surprises, except poor weather and crazy traffic post-eclipse, in some cases.  I find it interesting that in some areas of science, some concepts are accepted completely, while others are fraught with strife and contradiction. If you want an example of “unsettled science”, bring up “the anthropocene” at the local pub, and see what happens.

Annular Solar Eclipse of 2005 Oct 03 (Carrascosa del Campo, SPAIN)
Nikon D100 and Vixen 80mm f/8 Fluorite Refractor
Exposure: f/16 @ 1/250 second (ISO200)
Photo ©2005 by Fred Espenak

Natural events do occur in patterns, and humans are good at sleuthing out those relationships if they exist. One such relationship is called a Saros. Civilized people were predicting eclipses prior to systemized writing! And eclipses were predicted worldwide. 

So how does an eclipse get predicted? The Moon orbits Earth every month, and its orbital plane is tilted 5 degrees relative to Earth’s orbit around the Sun. Imagine the Moon’s shadow, always draped behind it relative to the Sun. Most months, when the New Moon crosses Earth’s orbital plane, its shadow passes above or below the Earth in space.  No eclipses are seen in this relatively normal configuration. But every once in a while, the Moon’s orbital plane crosses Earths, and an eclipse is possible. Astronomers refer to these points in the Earth-Moon orbit as nodes.  

2006 Total Solar Eclipse
This sequence encompasses the entire eclipse from start to finish.
Fred Espenak, 2006

Effective eclipse prediction requires knowledge of the Saros cycles, which are sets of eclipse alignments based on repeating patterns in the Moon-Earth orbital system. From NASA’s eclipse FAQ’s: “Eclipses occur in patterns. The Saros Series is a period of 223 lunar months that has been used to predict eclipses for thousands of years. In a Saros Series, exactly 9 years, 5.5 days after any lunar eclipse, a solar eclipse will occur, and vice versa. Approximately 6585.3211 days, or 18 years, 11 days, and 8 hours after one eclipse, the Sun, Earth, and Moon return to about the same relative geometry, and a nearly identical eclipse occurs. These similar eclipses are part of the same Saros Cycle, and the time between the two eclipses is called a saros.

Total Solar Eclipse of 1991 July 11 (San Jose del Cabo, Baja, MEXICO)
Nikon FE
Photo ©1991 by Fred Espenak

The master of eclipse prediction, mapping, and modeling, recently retired from NASA’s Goddard Space Flight Center, is Fred Espenak. Most of the eclipse information and diagrams are from work Dr. Espenak and his team at Goddard compiled while at NASA. It’s part of a larger canon of work, the Five Millennium Catalog of Solar Eclipses. According to Dr. Espenak, “…The eclipse belongs to Saros 134 and is number 44 of 71 eclipses in the series. …” Stay tuned in February of 2024, and I’ll tell you where Dr. Espanak will be going to view the total eclipse. He’s been chasing eclipses for many years, and will know where there’s likely to be good weather on April 8th.

The science of eclipse prediction is very well established. It’s one way we know that the Earth-Sun-Moon system is extremely stable, as the Saros cycle pattern has remained almost unchanged for millenia.


Ken Brandt directs the Robeson Planetarium and Science Center in Lumberton, NC.  He is a volunteer in NASA’s Solar System Ambassador Program. He is also a member of the 3rd cohort of NC Space Grant Ambassadors, and an Ambassador for the Mars Society.