10 Things to Remember While Watching the Total Solar Eclipse

The sun is 400 times larger than the moon — but it's also 400 times further away. So when the moon's orbit takes it across the ecliptic — the apparent path of the sun through the sky — it can fit across it exactly. It happens roughly once every 18 months. Since the moon is drifting away from Earth at 3.8cm every year, such perfect total solar eclipses will not always occur. So we're born lucky, in an age of totality, but don't feel too blessed: The moon will one day be too far away from Earth to totally eclipse the sun (but that will take 538 million years).

When you watch a total solar eclipse, you're watching our corner of the solar system in perfect alignment, right in front of your eyes. It's a celestial phenomenon called syzygy. It may be tricky to say, but it's not a difficult concept to understand.

“Syzygy is when the Earth, moon and sun line-up,” said NASA Ambassador Eddie Mahoney, the director of astronomy and host of “Tour of the Stars” at the Hyatt Regency Maui in Hawaii. “Although there's no measurable gravitational effect, there is certainly an effect — it's inspirational,” he said. That's because it's not just Earth, moon and sun lining-up — it's you, too.

You might think that the cessation of sunlight for just a couple of minutes wouldn't make any difference to the ambient temperature. But as the last slivers of sunlight cease, the temperature drop by a few degrees. It won't send you into a shivering mess — this eclipse happens at the height of summer, after all — but you will definitely notice it. It happens because a layer of the Earth's atmosphere called the ionosphere that's produced by solar radiation weakens under the moon's shadow, which will race across the USA at 1,462-2,955mph. The advantage? It can make stubborn clouds disappear in seconds, which just might save the day.

As many as 100 million people may watch the eclipse on August 21, making it an era-defining event. Some are predicting the rebirth of respect for science in the U.S., others that it's going to be more important than Woodstock.

One thing it's bound to do is fuel an interest in chasing solar eclipses around the world, which is so far something of a niche hobby. “Twelve million people across the U.S. will awaken that morning in the path of totality, and aside from a 1999 eclipse that went through Europe, this is the highest concentration of people that have ever been touched by totality,” said Aram Kaprielian, international president at TravelQuest, who's seen over 20 total solar eclipses. “Even if 1 percent of that 12 million finds a passion for eclipse chasing, that will create a much larger audience.”

It's fun to watch a Total Solar Eclipse in a crowd, and there will be some very, very busy events across the U.S. on August 21. Traffic could be terrible in some places. So wherever you find yourself, it will likely be busier elsewhere. GreatAmericanEclipse.com estimates that South Carolina will be the busiest place on August 21, with two million people expected to travel down Interstate 95, which stretches from New England to Florida. The tiny village of Santee is where I-95 hits the centerline.

Although you'll probably be too busy gawking at the solar corona during totality, it will be possible to gaze at planets and stars during the day on August 21. Most obvious during totality will be bright planet Venus on the right of the sun and moon, though Jupiter may also be visible on the far left. Mars and Mercury are also close to the eclipse, though they may be hard to see with the naked eye.

Totality on August 21 will last for 2 minutes 41 seconds at the Greatest Point of Duration. That occurs near Carbondale, Illinois, though most observers will get just over two minutes of totality. That's actually not very much; a total solar eclipse can last a maximum of 7 minutes and 30 seconds. The longest total solar eclipse of the 21st century has already happened, for 6 minutes and 39 seconds in Asia in 2009, but a mighty 6 minutes 23 seconds of Totality will occur on Aug. 2, 2027. That Great North African Eclipse will cross the Valley of the Kings and Valley of the Queens in in Luxor.

Virtually identically eclipses occur every Saros Cycle, exactly every 18 years 10 days 11 hours. So those that saw the total solar eclipse of 1999 in Europe will experience the same Moon-shadow on Aug. 21, 2017. And those that did are excited.

“It's a cool thing to know that you're participating in a really big cycle of the cosmos and that you're back to see the same member of that Saros Series,” said David Makepeace, The Eclipse Guy, who has seen 22 eclipses. “It doesn't affect the eclipse itself, but it gives you a sense of being connected to a something much larger,” he said, pressing the metaphysical rewards of appreciating an eclipse. “I've never looked up at the sky during an eclipse and seen science.”

All that 'once in a lifetime' talk was just that. It's actually the Great American Eclipses — plural. Another Total Solar Eclipse will sweep through the U.S. on April 8, 2024. While totality lasts over two minutes in 2017, it's over four minutes in seven years, but is in April, so the chances of clear skies are less. It has a completely different path, starting in Mexico and finishing in Canada after busting through Texas, Arkansas, Missouri, Illinois, Kentucky, Indiana, Ohio, New York, and Vermont. Given that the last total solar eclipse in the U.S. was in 1979, this is something of a celestial jackpot.

An eclipse over Argentina and the South Pacific – The Great South American Eclipse — is next. Amateur astronomers will love the fact that it goes over La Silla Observatory at the southern tip of the Atacama Desert. Astrologers and hippies will make their way to Cochiguaz, the so-called magnetic centre of Earth in Chile's Elqui Valley, to experience totality. And island collectors will watch it near the remote volcanic archipelago of the Pitcairn Islands in the South Pacific, officially a British Overseas Territory with a bizarre history.