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.
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| 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:
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