The saros, the famous rhythm of similar eclipses a little over 18 years apart, is a fascinating subject with almost endless ramifications, explored over many pages of my Under-Standing of Eclipses.

For instance, in the section on the saros history of the great eclipse that’s coming in August, we see that it belongs to solar saros series 145; this began in the year 1639; its eclipses changed gradually from partial to annular and then to total; this eclipse is the 22nd member of the 77 in the series, and the 5th of the 44 total ones.

It may seem almost sacrilegious to complain that the saros has shortcomings.  The first – actually a long-coming – is what looks like the least significant thing about it: the fractional part of the number representing its period.  The period is basically 223 synodic months (cycles of the Moon), which works out at 18 years plus 10 or 11 or 12 days (depending on leap years) plus about a third of a day.  It’s that third of a day that spoils it!  Successive saros sisters may be similar, but they’re seen in far different parts of the Earth.

Relative to the situation on, say, 1999 August 11, when an eclipse that you may remember passed over Cornwall and Europe and Turkey, the spinning Earth has on 2017 August 21 made a third of a turn, and therefore the track passes over the U.S.A.

So a more convincing interval between similar eclipses is 3 saros jumps: 54 years plus a month (669 synodic months).  Here is the totality-track of this August’s eclipse, compared with its sisters a tri-saros before and a tri-saros after.  (And with one other eclipse.)

(The Moon’s shadow brushes from west to east.  The little ellipses are the footprint of the total shadow at 10-minute intervals.)

You can see another change that happens as a saros series progresses: the track shifts in latitude.  Series 145 began with partial eclipses near Earth’s north pole and is migrating southward.  As we are showing eclipses three saros-jumps apart, the differences in latitude are quite wide.

(The tracks slope southeastward geographically because the season is approaching the September equinox, when Earth’s north pole leans forward.  I tried rotating the picture counterclockwise – north-pole-forward – but I like it better as it is.)

The discovery of the saros periodicity in chronicles that mentioned eclipses probably did help ancient star-clerks to predict eclipses.  If you were in Alaska or Maine in 1963, you had the means of knowing that the sky there would darken again – though only partially – 54 years and a month later.

I wasn’t around to see the eclipse of July 1963.  I saw my first eclipse in July of 1972, from not far north of Maine, camping at Cap Chat on the shore of the St. Lawrence.  That was an eclipse of a different saros series, 126.  My mental arithmetic is pathetically slow, and when I thought of drawing this Tri-Saros picture I thought, Was that Canadian eclipse the precursor for this year’s?  No, of course not, but when I plotted the 1963 track it took me aback by crossing the Gulf of St. Lawrence.  So I realized that adding the 1972 track to the picture serves to show another debility of the sacred saros.  Some eclipses, even if not far apart in time, can be pretty similar without being saros sisters.


9 thoughts on “Tri-Saros”

  1. The Under-Standing of Eclipses has been very helpful to me. But, as often happens to me when I try to wrap my mind around a big astronomical concept, I vacillate from a sense of understanding the Saros, to vertigo, to incomprehension.

    I’m currently reading another wonderful book, _Sun, Moon, Earth: The History of Solar Eclipses from Omens of Doom to Einstein and Exoplanets_ by Tyler Nordgren. The prose is quite vivid, and weaves together the personal experience of viewing an eclipse, the role of eclipses (and transits and occultations) in human history, and the relevant science. The book is both highly informative and quite inspiring.

    I’ve seen partial solar eclipses and the 2012 annular eclipse. I’m looking forward to seeing my first total solar eclipse in Jackson Wyoming, with my astronomy club.

    1. Try to persuade your club to move a little north of Jackson to get right on the centerline. Shadow bands should be at maximum as a result.

  2. Old-timers have told me about the March 1970 eclipse, which passed across the north suburbs of Indianapolis… I was in kindergarten at the time and have no recollection of it (the first of the partial eclipses I’ve viewed was Feb. ’79). But that’s okay; saros series 139 is working its way to the north, so April 2024 clips the south side of Fort Wayne.

    If Saturn was termed the “Bringer of Old Age” ( to quote Holst) because of its 30-year passage around the sky, what does that make the tri-saros? In my case, it’s the bringer of eclipses.

  3. So Guy, will you head back to the states to watch the eclipse in August? Greenville SC seems pretty close to the centerline of the path of totality.

  4. My family and I traveled to Germany to see the August 1999 eclipse, but we were clouded out at our observing site west of Stuttgart, so I’m hoping we have better weather for this August’s eclipse! Another interesting illustration of the 54 year tri-saros would be the two eclipses sandwiched around the 2024 event. The first eclipse that I experienced was the 1970 total eclipse that went up the eastern seaboard of the U.S., although my parents stayed in Richmond, VA, so we only saw a deep partial phase. 2024 is the 54-year successor to that one, and quite a bit more to the west (“north” I guess in the way that eclipses move either north or south). Without looking up the 2078 eclipse on the web, my guess is that it will go up the U.S. west coast.

    1. Wish I could take time to run more eclipse plots, but have to leave in minutes on a rail journey, to be back on May 3.

    2. It was unfortunate that your parents stayed in Richmond – an hour’s drive east would have taken you to the Great Dismal Swamp, where I was fortunate to personally see a stunning eclipse. August 1999 was a different story – I was in the Belgian highlands with beautifully clear weather. But as the Moon began crossing the Sun, the cool-down was apparent, and the humidity in the air began condensing into clouds, which 15 minutes prior to scheduled totality, totally blanketed the sky. At totality, it became similar to a dark cloudy twilight Upon Lunar exit from the Sun, the clouds dissipated as the air warmed, and clear skies returned.

      So: Advice here is to avoid humid locales for watching this eclipse. I plan to be in Casper Wyoming, with a 500 mile (either way) ability to locate myself into decent sky conditions. If that doesn’t work, there is always 2024, on a line a few hundred miles west of the Great Dismal for a 1972 repeat performance!

      1. Tom,

        I don’t know why my Dad didn’t take us to see the 1970 eclipse, because he bought his first telescope in 1969, so I know he was interested in astronomy by that time. I was only 7 years old, so I probably didn’t badger him to take us to go see it LOL. My plan for this August was to go to South Carolina since it’s the closest to Virginia, but I’ll check the weather forecast a few days out and might head to Tennessee or Kentucky instead. I’ve read that western Tennessee and Kentucky have better prospects than SC at that time of year. Wyoming is too far for me and my daughter to this time. Good luck!

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