That’s right, the absolute best eclipse of our lifetimes is going to occur tomorrow! First off, total Solar Eclipses are rare enough, and spectacular when they happen.
After all, a total solar eclipse allowed us to first test Einstein’s General Relativity! A total Solar Eclipse happens when the Moon passes directly in between the Earth and the Sun, casting a shadow on the Earth.
This only happens rarely, for three reasons. First off, the Moon’s orbit is not in the same plane as the Earth and the Sun. The Moon’s orbit is inclined by about 5 degrees to the plane that the Earth orbits the Sun. For comparison, the size that the Moon appears to take up in the sky (its angular diameter) is only half-a-degree.
This means that more than 90 percent of the time, the Moon’s shadow isn’t even in the same plane as the Earth. But tomorrow, we’ll get lucky, and the Moon will be in just the right part of its orbit to cast a shadow in Earth’s plane.
The second thing we need is for the Moon’s shadow to fall in the Earth’s plane during the New Moon. Why the New Moon? Because this is when the Moon passes as close as possible to syzygy (i.e., perfect alignment) between the Earth and the Sun.
We only get about 12 or 13 of these a year. And tomorrow, July 22, is, in fact, the New Moon. So we’ve got two things in place: the Moon’s shadow falling in Earth’s plane during the New Moon. When this happens, we can see one of three things:
- A partial eclipse, if the completely shadowed region (the umbra) lands off the Earth, but the penumbra lands on it. On the other hand, if the umbra does land on the Earth, we’ll see:
- An annular eclipse, if the angular diameter of the Moon is smaller than the angular diameter of the Sun (i.e., if the Moon is close to apogee), or
- A total solar eclipse, if the angular diameter of the Moon is larger than the angular diameter of the Sun (i.e., if the Moon is close to perigee).
Well, I went and looked up the Moon’s position, and the Moon makes its closest approach of the year to the Earth at 8:15 PM (Universal Time) tonight, coming to within 357,464 km of the Earth (under 228,400 miles)!
Not only will the Moon’s umbra fall on the Earth, it falls extremely closely to the equator of the Earth. Meaning that, during the New Moon, when the Moon is closest to the Earth (and appears largest in the sky), the Earth, Moon, and Sun will be almost perfectly aligned. The result? A total solar eclipse that lasts for over six minutes, the longest Solar Eclipse that will happen in all of our lifetimes. These things come together very, very infrequently, less than once per century. And it’s happening tomorrow! Where can you see it from?
Anywhere on this path. The official NASA eclipse page is here, but I’ve done a little digging and found where you can see the total eclipse! If you live in India,
Or in China,
Or in Japan,
you’re in luck. In fact, if you live almost anywhere in the Eastern Hemisphere, you should be able to see a partial eclipse, and you should also be able to tell that the Moon looks larger than the Sun!
(Hey, Jim Fry, if you read this and you’re in Shanghai right now, don’t miss this! And take some pictures for me!!!)
But if you get the total eclipse, you’ll find that you can see stars during the day. I have never been so jealous of people who live in Asia as I am at this moment. If anyone who reads this takes pictures and lives in an area where you can see the eclipse, feel free to send them to me! If I get any good ones, I will happily post them here! Enjoy the show; there’s not going to be another one like it as long as you live!