Thursday, July 20, 2006

What is a Saros?













Theo said...

Eclipses tend to pass over broad swaths as the world rotates under them. E.g. the recent solar eclipse a year or two ago started in Mongolia and cut through Egypt, to set over the eastern Brazilian coast. Except for the problem that, in fact, even the penumbral eclipse is only visible on some comparatively narrow band along the eclipse's path, I could be convinced that the Babylonians would see two different eclipses 120° apart.

sigfpe said...

If you peek here (and some of the other maps at that web site) you can see some example pairs of eclipse paths separated by a saros (eg. Dec 4 2002 and Dec 14 2020 (Hey! This stuff really works!)). The paths generally don't intersect though maybe it's possibly that they might rarely. Even if they did intersect, the intersection would be very small and the chances of seeing enough such events to detect a pattern would be negligible.

If the Babylonians did know about the saros cycle then it would have to be because they had good communications with reliable astronomers over a wide area of the world. I'm familiar with valuable objects being traded over long distances at that time, but not astronomical information.

Maybe the saros could have been deduced based on general knowledge of the apparent motion of the Sun and Moon in the sky. I've no idea if Babylonian mathematics was up to the task and whether their models of the orbits would be good enough. Hmmm...having a heliocentric model of the solar system might not have been a handicap in this case. Still, it sounds like one hell of a challenge.

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