Saturday, October 23, 2021

The Great American Eclipse


 

 

For the first time in 99 years, on August 21, the shadow of the Moon will sweep across the entire width of the United States. We have had brushes with total eclipses more recently, the lower Eastern Seaboard in 1970 and the Northwest in 1979 for example, but 1918 was the last year the whole country had a shot at witnessing it. Of course, the main mode of long distance transportation then was by train, so not many people outside the path had a chance to view it. Today it’s different; about two thirds of the population of the United States lives within a day’s drive of the path. That 70-mile wide strip of land between Oregon and South Carolina is going to be very crowded.

Here in Marco Island you won’t be totally out of the action (pun intended). The eclipse at maximum (2:52 PM) will have the Sun over 80% covered, and the Sun will be seen as a thin crescent. Which is a fascinating sight to see all by itself.

Important disclaimer: direct viewing of the Sun is dangerous! Remember as a kid burning a piece of paper or an ant with a magnifying glass? That is what the lens inside your eye is going to do to your retina if you look at the Sun! When the Sun is a thin crescent on August 21st some people may get the idea they can get away with it. Wrong. The next morning there will be a crescent-shaped burn on your retina. Just be smart and don’t do it. Telescopes and binoculars are out of the question also for obvious reasons unless they are fitted with professional full aperture filters, not eyepiece filters – those are horribly dangerous.

All that being said, there are safe ways to view the progress of the eclipse. First, follow its progress on TV. No joke, you always get the best view and it’s perfectly safe. It rather lacks the flavor of the event though. Secondly, indirect viewing like pinhole cameras and telescopic projection. The NASA website listed below contains information on how to do this. If young children are involved, indirect viewing is the way to go. Third, direct viewing with commercially purchased devices. These devices include #14 welding glass (no less than #14) and specially made eclipse glasses (ISO rating 12312-2 only). This is for adults only, children have a habit of snatching away protection for a moment constantly to get a view of the unfiltered Sun. Sunglasses cannot be used instead of the above recommended equipment.

Important to note: counterfeit eclipse glasses are flooding the market from China. Fake ISO logos, standards and manufacturers are printed on them, making it difficult for the ordinary person to tell the difference. Approved retail vendors can be found on eclipse.aas.org/resources/solar-filters

Just what is a Solar Eclipse anyway? No mystery about it, the Moon gets in front of the Sun and casts it’s shadow upon the Earth. As readers of my previous articles know, the Sun is always found on a line called the ecliptic. This line is important for determining eclipses, now you know how it gets its name. The Moon’s orbit is tilted to the ecliptic by about 5 degrees, which isn’t much, but it gives the Moon a clear miss of the Sun most of the time. About once every 18 months or so the Moon crosses the ecliptic at the same moment that the Sun occupies the spot. The Sun is 300 times larger than the Moon, but it is also 300 times further away; this makes the two nearly the same apparent size. Most of the time the Moon is just barely larger. A small shadow falls on the Earth (called the umbra) varying from 0 to about 100 miles wide. When a person stands inside the umbra the eclipse is said to be total. This will not be happening on Marco Island unfortunately.

The Moon orbits a great speed and the Sun remains covered for only a few minutes at best. The maximum time is just over seven minutes; for this particular eclipse it will be two minutes and 40 seconds at best. The umbra sweeps from west to east, drifting north or south according to the complicated geometry of the particular eclipse. An eclipse pathway can be many thousands of miles long, but very narrow. A given spot on Earth only sees a total eclipse once every 364 years on average and it might be much longer. The last total eclipse Marco Island saw was in 1752; the next will be several more centuries into the future. (One will come very close in 2045.) On the other hand, Carbondale, Illinois will get another one in just seven more years!

The lesson here is, if you want to see a total eclipse, you are going to have to travel and when one comes close – go for it! This one though might be a problem, hotel rooms have been booked for many months.

The Moon also throws an incomplete shadow upon the Earth where the Sun is only partially covered (the penumbra). This is what we will be experiencing on Marco Island. The penumbra is far larger than the umbra and is in fact a couple of thousand miles wide. Unlike a total eclipse, non-travelers can experience several penumbra eclipses in their lifetime.

Wherever you choose to view the eclipse, may you have clear skies and stay safe!

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