More on Eclipses.

It is really hard to contain my excitement for the coming eclipse this August 21st. The intensity and expectation are rising, and the talks about the eclipse are popping up everywhere. It is great to have a scientific subject being predominant on the media, at least for a few days.
Additional concerns have been raised since it is expected to have a massive exodus toward the eclipse central line early Monday, as always TV news manage to put a sense of concern to everything, but fortunately, these are fewer reports than those talking about the wonderful spectacle that we’re about to have.
And to continue with the hype, here’s an addendum to my previous article about Eclipses. As I consider, there’s one big question I didn’t address in it.

So, Who was first?

Who was the first person to predict eclipses with accuracy? There’s a passage in the “Histories of Herodotus” stating that Thales of Miletus predicted an eclipse back on May 28th 585 BC, in which, Herodotus says, it stopped a battle among the Medes and the Lydians. Although this can’t be confirmed both due the lack of evidence and due to the limited knowledge about the planetary movement that Greeks had at that time, at least for what is known historically.
The recognition of being the first person to predict eclipses belongs to Edmund Halley. Yes, as usual, before being a comet, Halley was a man from whom the comet took its name. Edmund Halley was an English astronomer and mathematician (among other trades), who was the first to compute with accuracy the orbit of the now known Halley comet, which visits the solar system vicinity every 74 to 79 years.
And Halley was also capable of predict the occurrence of a total solar eclipse that crossed over southern England, among other regions, in May 3rd, 1715, with such accuracy that his prediction had only a slight difference of four minutes; eclipse that was also known as the “Halley Eclipse”.
But Halley didn’t achieve this just by himself, he had a bit of help from no other than Sr. Isaac Newton, one of Halley’s closest friends. And was in fact Halley who motivated and financially supported Mr. Newton to write his greatest work “Philosophiæ Naturalis Principia Mathematica”, in which Newton presented his three laws of motion.
And it was with the help of Newton’s laws of motion that Halley was able to accurately predict the movement of celestial bodies and make such predictions as the occurrence of eclipses or associate the registers of previously seen comets and deduct that it was actually the same comet coming at regular intervals. This is the power of Science!

The Saros cycles.

But coming back to eclipses, Halley, following the Moon’s motion also discovered the now known as Saros cycle, or more precisely re-discovered it, as this cycle was first deduced by Babylonians and Greeks to predict moon eclipses, (Since they have larger duration than solar eclipses). The Saros cycle is the conjunction of three distinctive cycles the Moon has, each one slightly different, these are recognized as different Moon’s orbital periods or Months.

• Synodic Month: Happening from one new Moon to the next new Moon, with a period of 29.530589 days.
• Anomalistic Month (perigee to perigee, due to the elliptic orbit of the Moon), with a period of 27.554550 days.
• Draconic Month (node to node, related to the different orbital plane the Moon has in relation to the Ecliptic, or the plane of all solar planets), with a period of 27.212221 days.

Moon’s precession – wikipedia.org

Well, the really interesting point with these cycles is that, as you might have noticed, they have slightly different periods; but all three converge in a big cycle that last 6,585.3 days, or roughly 18 years and 11 days; in this period, the months’ convergence is almost perfect, with a marginal difference of about two hours, and in this convergence the saros cycle starts again, as the Moon and Earth are positioned at the same relative geometry they had at the beginning of the previous cycle. The result is that in each cycle we have similar eclipse paths, although with changes in latitude or longitude (basically the position on earth where they occur), this due to the effects of the Moon’s precession while it moves around the Earth. There are additional factors affecting this phenomenon related to, let’s say, the long term gravitational relationship that the Earth and the Moon have. But this will be a subject for another article.

Eclipse in different Saros Cycles – eclipse.gsfc.nasa.gov

Eclipses between Saros cycles have quite similar geometry, occurring approximately at the same time of the year, with the moon at a similar distance from Earth, but due to the mentioned precession differences, these eclipses occur at slightly different positions on earth. Yet these cycles are good enough to predict the periodicity and the change or variation these eclipses will have between cycles.
Halley, using a Greek historical reference, was the one who named these cycles as “Saros”, and with this knowledge it was possible to predict eclipses with great accuracy, since knowing where an eclipse occurred is the key to be able to predict the next, which shall occur in the next saros cycle; in a slightly different location on earth but with similar duration and trajectory.
If you want to find out about coming eclipses in your region, there are different resources and eclipse calculators available on the web, but definitely my favorite is NASA’s Goddard Space Flight Center Eclipse website; in it, you can find the calendar of coming Lunar (Moon), and Solar (sun), eclipses.

 A matter of inspiration.

Finally, I want to share a TED conference presentation from David Baron, precisely talking about the excitement of seeing a total solar eclipse. In it he says, and I fully agree, a total solar eclipse is something you should experience at least once in your lifetime.

So, for the moment, prepare yourself and enjoy the show (with the necessary safety and precautions); either partial or total, this coming eclipse will be memorable.

Regards, Alex-ScienceKindle.