Solar Eclipse

Click this image for an expanded view of NASA's map of total solar eclipses, 2001-2025.

April 8 is the first new moon following the spring solstice, which means that solar eclipse season is once again upon us. NASA, as always, has all your eclipse info. U.S. residents are mostly out of luck on this one, although those of you in Texas will get 20-30% eclipsing goodness. If you happened to be in Venezuela or out in the middle of the south Pacific, you'd be treated to a rare hybrid solar eclipse, which begins and ends as an annular eclipse but becomes total in the middle.

Also from this NASA site, I found this map, showing the paths of all solar eclipses predicted between 2001 and 2025. I'll have to be sure to head back to San Antonio for spring break in 2024; looks like the total eclipse path passes right over my home town April 8 of that year. In the meanwhile, I need to come up with a good excuse to be in Turkey next spring. (Incidentally, next year's Africa-Mideast eclipse is the Saros precursor to the 2024 North American one.)

In principle, you can look directly at a total solar eclipse, and it's supposed to be a very beautiful sight. Some care is required, though, because it may seem plenty dark while there's still a small sliver of the Sun's photosphere exposed. Look then and you can console yourself with the knowledge that you're only irreversibly grilling a small part of your retina! Even during totality, though, I'd suggest you wear UV-resistant glasses if you've got them, since the solar corona is much brighter in the ultraviolet portion of the spectrum than in the visible.

You can only look directly at a partial solar eclipse through a solar filter. There's cheap ones to be had, and I trust them about as far as I'd trust a used bargin-basement bike helmet. Aluminized mylar is apparently okay if you don't feel like shelling out for a real filter or some welder's goggles, provided you get the good stuff, but traditional things like smoked glass are next to useless.

For a partial eclipse, I've always been preferential to projection, anyway. As a kid, I'd turn my little potbelly Cassegrain on it's side and point the smallest eyepiece I had at the sun. The barrel of the telescope would then project a big image of the sun onto a wall. I'm not certain that this was necessarily the best way to treat my telescope, but at least it wasn't dangerous to me. In theory you could do with with binoculars, too, except that their ocular lenses are generally fairly large, so I'd be concerned about overheating and cracking something.

The easiest thing though, especially for those of you who don't have a telescope lying around, is just to make a pinhole camera. Fun fact about ray optics: if you have two surfaces, and put a hole in one surface that is much smaller than the distance between the two, light passing through the hole will automatically form an in-focus image on the other side. Practical application here: put a small hole in a piece of cardboard, and you can project the sun onto the ground. Magnification (or the lack thereof) is the key here, as the resulting image will be 9/1000 times the distance from the hole to the image, or just under a centimeter across per meter separation (half an inch per yard).

In fact, nature does this quite nicely on its own. In most parts of the world (and everywhere that this eclipse will be visible, I think) trees will have put on leaves by now. Look underneath; during a partial eclipse the spots of sunlight shining through the leaves will take on a distinct crescent shape. That's because the gaps that the sun does manage to shine through make rather bad, but all-natural, pinhole cameras.


In about five years, I suppose there will be a lot of kids born around St. Louis who are going to grow up thinking total solar eclipses are about as common as leap years. Spoiled brats, the lot of 'em. Probably a couple of millenialist cults, too.

On the other hand, maybe we'll get a whole generation of astronomers and artists out of the deal. That'd be okay by me.

Here we go; more neat stuff from NASA's eclipse page. I thought I recalled at least two partial solar eclipses when I was a kid in San Antonio, plus another one my senior year of high school, for which I dragged my chemistry class outside. Sure enough, these maps show three eclipse paths that path close enough to Texas that a partial eclipse would have been visible there, from 1991 to 1998.

Okay, that's enough commenting on my own post.

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This page contains a single entry by Milligan published on April 4, 2005 7:47 PM.

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