The moon is a better timekeeper than the sun. At least as far as eclipses go. A solar eclipse can only occur on a new moon. So the NEXT solar eclipse has to be an integral multiple of new moons away. The sun can only be eclipsed when the moon it at its orbital nodes. And the type of eclipse—be it total or annular—depends on the moon-earth distance. The moon appears about 11% larger at its nearest distance to us in its elliptical orbit—this is called lunar perigee. All of these events are on different time scales.

The moon moves from new moon to new moon (the synodic month) in about 29.5 days. The moon moves from node to node (the nodical  or draconic month) in about 27.2 days, and the moon moves from perigee to perigee (the anomalistic month) in about 27.6 days. It turns out that 223 synodic months, 242 nodical months, and 239 anomalistic months are all about 6,585 days. That’s about 18 solar years.

Ancient astronomers were aware of this eclipse cycle and the word that’s come down from antiquity to describe it is saros. The saros is a group of related eclipses, all about 18 years apart. Eclipses separated by a saros have similar characteristics. They have the same geometry: they occur on the same node (ascending or descending), at approximately the same time of year, and when the moon is at the same distance from the earth. The 6585 days is really more like 6585-1/3 days, so every third saros (54 years) the eclipse is roughly on the same place on the earth. That 1/3 day means each eclipse is shifted about 1/3 of the way around the globe, so it takes three cycles to get back to the same area.

The saros was first noticed for lunar eclipses because a lunar eclipse is visible over half the earth at the same time. Ancient peoples did not have the travel and communication capabilities that we have today so they may have predicted a solar eclipse but were unable to know if it occurred. It may have taken place thousands of miles away!

Eclipses happen every year so there are multiple saros series going on at the same time. A saros lasts over a thousand years and is comprised of dozens of eclipses. The upcoming total solar eclipse is part of the same series as the 11 July 1999 total solar eclipse that was seen across Europe and was possibly the most-viewed eclipse in all human history. Eclipses in a saros begin with the most fleeting of partial contacts, pass through the annular/hybrid phases, peak with total eclipses, and then fade back again. They reflect, in depth and degree of the eclipse event, all the motions that have to coincide and overlap for us to be able to experience an eclipse. People mostly live in a narrow temperate-equatorial band on the planet, but the shadow path of a solar eclipse can brush the huge polar regions or plunge miles of empty oceans into darkness.

Humans had to view, record, and study eclipses for generations in order to learn about patterns like the saros. Take a look at the path of the upcoming 21 August 2017 total solar eclipse:

Now take a look at the path of another eclipse in the same saros series, this one 54 years away on 23 September 2071:

I don’t think I’m going to make it to 2071. Adding 54 to my current age pushes me well past the century mark. But it is interesting to see that the third eclipse in the saros after this one is pretty close to the same part of the earth’s surface. (GE means Greatest Eclipse where totality is longest.) The 02 September 2035 event passes over China, South Korea, and Japan. The next one after that on 12 September 2053 sweeps across North Africa and the Arabian peninsula. Finally the third in the group returns to the Americas. It amazes me that astrologers and astronomers of yore were able to predict eclipses, or at least create eclipse-predicting algorithms, of such precision. Obviously they lived in sufficiently stable societies that could preserve the records and educate succeeding generations about the measuring and calculating methods needed.

Fortunately for those who would like to view a total solar eclipse we have all these overlapping saros series to choose from. We don’t have to wait 18, 36, or 54 years to see another one. In fact, there is a total eclipse in July of 2019 if you want to go to Chile or Argentina. Hmm, it’s winter then, maybe I can go skiing! Actually I’m trying not to think too far past the trip to see THIS eclipse. But there is another one I want to plan for, it’s in 2024 when I’ll be almost Medicare age:

Anyone up for a trip to Austin, Texas? I hear it’s a really fun town.