Wednesday, August 23, 2017

The Great American Eclipse (and a bit about the web).

Most of this post is probably better suited to the other blog, but Field Notes has been a bit quiet for the past, um, years, so why not?

You may be aware that there was recently a total eclipse of the sun in the United states.   If you've never seen a total eclipse, I highly recommend it.   If you happen to be near the path of totality when one occurs in your area, don't be lulled into thinking that seeing a 95% eclipse or whatever is 95% as good as seeing a total eclipse.  The difference is, literally, night and day.  During totality, the sun is not so much covered as replaced by a black hole surrounded by the corona, about as bright as the full moon.  And then, before you know it, there's an impossibly bright spot at the very edge and you have to look away.  Seconds later the light is already a thousand times brighter and it feels like day again -- a weirdly dim, clearly-lit day, to be sure, but definitely day.  Miss totality and that last part, the strange light, is about all you get.  So if you get the chance ...

There are two main concerns in getting a good view of an eclipse: traffic and weather.  Some people are able to bypass traffic by booking a train, or flying into the zone, or even being in the air during totality, but most of us will end up taking to the road.  If enough people decide to do this, things can get hairy.  A rule of thumb in traffic engineering is that one lane of highway can handle about 2000 cars an hour.  If your main route into the zone is a four-lane highway, that is, two lanes each way, that's about 4000 cars an hour.  If it's an hour before totality and you have 5000 cars between you and the zone, your chances are not looking good.

It would be nice to have some idea of what to expect, something like "If you live here, you should try to go here (assuming good weather).  Leave at this time and expect the trip to take this long."  But the problem is, nobody really had a good idea how many people would be trying to go where, when.  I'm not a total umbraphile, but I'm sure I paid a lot more attention to this eclipse than most of the population.  My personal attitude of "Yeah, gotta try to see that" was probably not typical.  Typical attitudes were probably more like  "Sounds kinda cool ... but I've got work on Monday.  Maybe I should take a look on my lunch break."  So not everyone is going to hop on the freeway or book a hotel months in advance.

Furthermore, a certain number of people who were thinking about it will hear reports of possible gridlock and think the better of it, or try to find an alternate route, or whatever.  In this internet-connected age people will be telling each other where they are and how conditions are, and watching real-time traffic, or at least trying to.

This sort of uncertainty makes projections a bit difficult, and there's not really any relevant historical data.  The last time an eclipse went all the way across the continental US, in 1918, there was no interstate system, much less an internet (though telegraphs were very much in use).  Even in 1979, the last time a total eclipse was visible in the contiguous US, the picture was still considerably different from today, if only because there were only 70% as many people in the country.

None of this stopped people from trying to project.  One such effort calculated the "drivesheds", analogous to watersheds, to show which locations on the centerline of the eclipse were closest, by road, to the highest number of people.  The top three were Santee, SC, where I-95 meets the centerline,  Idaho Falls, ID, where I-15 meets the centerline, and Sabetha, KS, where US 75 meets the centerline.

The first sounds pretty likely.  I-95 gets a lot of traffic to begin with, and it runs from the Canadian border in Maine through Boston, New York, Philadelphia and Washington,  DC on its way to Jacksonville and Miami.  The second seems plausible.  It's the closest point for Phoenix, Salt Lake City and, perhaps surprisingly, San Diego and from the looks of it most of LA.

The third ... well, it seems to assume that anyone in Texas wanting to see the eclipse is going to head up I-35 toward the Kansas City metro, a chunk of which is directly in the path, then veer off onto two-lane local highways to get to the (technically) closest point on the centerline.

I don't think traffic works that way.

In the event, traffic was bad in a few places in the days leading up to the eclipse, but not too bad.  It looks like people trickled in from here and there over a period of days, and relatively few people headed into the zone of totality on the day, or even at all.  There are 25 million people in the driveshed for Salem, Oregon.  Officials in Oregon planned on one million.  It's not clear that there were even that many.

Coming home, on the other hand, was a different matter.  There was no particular schedule for getting to the zone of totality, but the show ended at a very precise time, and suddenly there was no reason to stick around.  Hours-long delays were common across the country.  This seems obvious in hindsight, but in the run-up to the event the main concern was "will I be able to make it in time?" Most people were probably not really concerned about getting home at any particular time.

As far as I can tell, Sabetha, Kansas saw the same thing in miniature [Toward the bottom of the page it mentions that Sabetha's Sixth Street Park had a crowd of ... about 200 people --D.H.].

So much for traffic.  The eternal concern for eclipse chasers everywhere, whether on the I-95 corridor or Mongolia, is the weather.  Personally, I've seen two total eclipses (this one and the Great European Eclipse of 1999), and in both cases sheer dumb luck brought a break in the clouds in time to see the corona and the diamond ring.

I followed weather forecasts closely in the days leading up to the eclipse.  Unlike 18 years ago, there were a number easily available online.  Weather forecasting has advanced significantly.  Live radar is available on any number of sites, as are recent satellite images.  If there's a hole in the cloud cover, it doesn't seem like it should be too hard to find.

If you've got an internet connection.

I gave up trying to get online and our party ended up just picking a spot.  My guess is that if we'd had to make an emergency call it would have had priority and it would have been connected.  Data, not so much.  It's almost as though there were more people in town than usual and they were all trying to get on the web at the same time.

We thought about trying to move toward what looked like a clearer spot, but in the time it took to try to figure it out the clouds shifted and a beautiful blue gap opened up with about 30 minutes to go.  Then closed again as the encroaching moon shut the light down by bit.  Then the sky abruptly darkened more.  The eclipsed sun was up there somewhere.  The clouds above were dark, but the horizon was dusk (or dawn, if you prefer) in all directions.  It looked like some of the low clouds were moving away from where the sun must have been, but it was hard to tell.

And then the clouds shifted and there was the silvery ring of the corona a minute or more into totality.  A little washed out, like the full moon through high clouds, but there.  Then, after what seemed like no time at all, an incredibly bright pinpoint of light widening into a slim maybe-crescent.  Time to look away.

A mile down the road, people saw nothing but clouds.  On the way back, through what had been overcast and rain, the sun was out.  Would live radar really have helped?  Who knows.

The two eclipses I've seen happened to be part of the same Saros series, a set of eclipses at 18-year intervals with nearly identical geometry.  The latitude was similar.  The duration was similar.  As it happens, the weather was similar.  In theory, technology would have played a much larger role in this one than the last one, but in practice, not really.

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