You probably know that a total solar eclipse is when the moon blocks out the sun's disc entirely, for some people on earth, for a short while. Basically, the moon casts its shadow on the earth, and whoever is in that shadow sees an eclipse. This only happens rarely, because the line-up of sun-moon-earth isn't usually exact—the moon's shadow misses the earth, usually, and there is no eclipse.
But we know a lot about exactly where the sun, moon, and earth are located, and how they move, so we can predict where a solar eclipse will happen years in advance.
For example, we know that there will be a total solar eclipse for people living along earth's equator on November 13 this year, and on March 20, 2015, March 9, 2016, August 21, 2017, July 2, 2019, and December 14, 2020....plus more...including December 5, 2048, and August 28, 2994!!! Check out NASA's eclipse website.
|Samuel Williams made a mistake in math...|
Let's get back to 1780...
On this date in 1780, British soldiers were fighting the American army in the Revolutionary War. Harvard professor Samuel Williams had predicted the exact time and location of a total solar eclipse, but that location was behind enemy lines! So Williams asked British military to allow his expedition to set up observational equipment at Penobscot Bay in Maine. The eclipse, he explained, would take place between 11:11 in the morning and 1:50 in the afternoon.
|This is what you see when the moon|
only MOSTLY covers the sun.
The British generals graciously allowed the American astronomical expedition safe passage, and the Americans set up at Islesborough, on Penobscot Bay. However, it turned out that the Williams's computations weren't exact enough—and, it turned out, the scientists were in the wrong spot to experience the total eclipse! Instead of covering the sun totally, the moon only covered MOST of the sun.
|The sun's corona|
That's a bummer, because it is only when the sun is totally covered by the moon that we can see the corona—the thin outer atmosphere of the sun that is millions of degrees in temperature and very bright, but which is normally invisible because the much brighter, much hotter sun drowns it out.
So the good news was that people put science ahead of warfare.
The bad news is that someone didn't check his math carefully enough, and about half of the hoped-for science wasn't completed!
We don't necessarily need to say, “The moral to the story is, always check your math,” because these days we use calculators and computers to do computations!
Also on this date: