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THE LAST ECLIPSE... EVER by Derrick Rohl, Sudekum Planetarium Manager

A Total Solar Eclipse is considered by many to be the greatest sight in nature. Nashville will be lucky enough this August to experience a total eclipse of the Sun – something this area hasn't seen since 1478 (long before Music City ever came to be). A Total Solar Eclipse occurs when the Moon completely blocks the Sun's surface from view. From within the Moon's shadow, you will be able to see the Sun's corona and beautiful prominences, which are typically hidden from view due to the Sun's bright light illuminating Earth's atmosphere. 

The moon's shadow is small compared to the size of the Earth, so you'll need to be in just the right location in the 2017 path of totality (the gold-shaded region of the map) to see a total solar eclipse.

Anywhere outside that gold region is outside the path of totality – viewers there will only see a Partial Solar Eclipse – a crescent of the Sun's light remains unblocked by the Moon. Don't look directly at a partial solar eclipse! Get some eclipse glasses or use a safe viewing technique such as pinhole projection.

Totality is a remarkable occurrence because the Moon is so much smaller than the Sun. Lucky for us, it's also a lot closer than the Sun, so when we look at it in the sky, it appears to be up to 4.6% larger than the Sun, depending on how close its orbit brings it to Earth.

Here's where things get fun: the Moon is moving away from the Earth – not very quickly, but still moving about 4 centimeters further away each year. Remember that it only appears 4.6% larger than the Sun right now – when it moves just 4.6% further away from Earth, it will reach a limit where there will never be another Total Solar Eclipse.

I'm going into full-on nerd mode for a moment, feel free to try this math for yourself. A few approximate numbers we can use:

  • The average Perigee (minimum distance from Earth to the Moon) is 362,600km.

  • The Moon moves away from Earth about 3.8 cm/year.

  • We need to find out how long it takes the moon to get 4.6% further away.

Ready, set... GO!

There are a lot of estimates there, but it gives us a rough estimate of 438 million years!

This is just an over-simplified "distance = velocity x time" approximation, but it's a more complex issue. The force of gravity between the Earth and the Moon decreases with the inverse square of the distance between them, so as the Moon gets further away, the force from Earth's tidal bulges (which cause this displacement) will get weaker, and it will slow down in its departure from Earth. This slowing means it will actually be a much longer time before Earth sees its last Total Solar Eclipse.

Hundreds of millions of years – maybe even into the billions... that's a long way into the future. But anyone who has seen a Total Solar Eclipse will tell you it's an indescribable experience.

We're lucky to have such an incredible sight coming into view later this year, and I'm excited at the interest this will generate for future generations to pursue careers in Science, Technology, Engineering, and Mathematics. We have countless years ahead to continue this studying, as long as we take good enough care of our planet that it remains a livable climate for humans. Don't get me wrong, our planet itself is tough – it'll be here for billions of years to come; I just want future generations to be there, too. We live in an incredible universe, and there's so much more for us to learn.

- Derrick Rohl, Sudekum Planetarium Manager

Thanks to this post from Cornell University's Department of Astronomy for inspiring some of the math described above!

Posted by Molly Hornbuckle at 3:30 PM
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