Levels of predictability

The recent solar eclipse demonstrates much more than astronomy.  (Image: one page of Guy Ottwell’s The Under-Standing of Eclipses, available here)

Last month’s solar eclipse, passing west-to-east across the width of the US, was certainly an impressive event.  It’s possible that more people saw totality than in any previous eclipse (though here at Five Colors we only saw partial phases).

The success of those millions was due to the fact that they could plan for the event far in advance.  Eclipses have been predictable for thousands of years, with increasing precision.  Nowadays the track of the Moon’s shadow can be forecast to an accuracy of yards far into the future, so that you can plan exactly where best to stand during, say, the solar eclipse of June 3, 2114.

But not everyone managed to see the spectacle of an eclipsed Sun.  Even a few experienced eclipse-chasers, armed with last-minute weather forecasts and flexible transportation, were skunked by clouds.  It might seem strange that we can work out what the heavens are doing in minute detail, while ordinary Earthly events are beyond us; but weather is a scientifically hard problem.

There was something even harder to predict for this event: the traffic jams afterward.  Some observers were able to retire from their chosen spots quickly, while others spent hours in their cars trying to get home.  There were warnings that something of the sort could happen, but no one knew exactly where and how bad.

Why this enormous variation in predictability?  In part, because astronomy (at this level) is so simple.  Scientists need only consider gravity, leaving other forces out, and objects that are tiny compared to the distances between them.  The resulting formulae may look intimidating, especially if one is not on friendly terms with sines and tangents, but all is under control.  Looked at another way, the events themselves (quite apart from the calculations involved) are simple.  There are patterns in eclipses, so that even if you have no scientific theory as a guide, you can figure out when the next  one is.  The Babylonians had this worked out well before the Greeks invented their different approach to mathematics. The image above is an illustration of the main patterns, found in Guy Ottewell’s wonderful The Under-Standing of Eclipses.

In contrast, figuring out where and when clouds will form is much harder.  You must take into account heating and cooling, evaporation and winds, not only at the location you’re interested in but everywhere.  What happens in the sky over your particular head depends on the weather patterns over the whole continent as well as the small parcel of air between you and the Sun.  It’s the multitude of things that are equally important that makes weather so complicated.

And then, you add people.  You can inform and advise them, direct and try to control them, but they will make their own decisions.  And if they have cars, there will be traffic jams.  Even when events are routine and in themselves predictable, like football games and even the working day, there are traffic jams; a solar eclipse adds extra uncertainty about what people will actually do.

Astronomers can tell you just what will happen in their subject on the third of June 2114.  We don’t know what the weather will be like that day.  Indeed, we cannot even say what the world’s climate will be like then, not only because weather is hard to predict but because it depends on what people do between now and then.  And it’s anyones guess what the traffic will be like.