This extract first appeared in BBC Sky at Night magazine and republished in BBC Science Focus – for the latest science news, innovations and discoveries delivered straight to your door subscribe here.
The date is etched in the brains of eclipse enthusiasts: 21 August 2017. On that Monday, for the first time in nearly 40 years, the path of a total solar eclipse cuts right across the US. For about two and a half minutes, the Moon will completely cover the face of the Sun, turning day into night.
For thousands of years, solar eclipses were seen as shocking and fearful events; our ancestors would witness them and wonder if the world was coming to an end. Today, eclipses no longer take us by surprise: astronomers can calculate when an eclipse will occur hundreds of years in advance. Knowing the physics behind an eclipse, however, doesn’t diminish the spectacle. A total solar eclipse is, quite simply, a spellbinding event, one of the most captivating phenomena the natural world has to offer.
People that have never seen a total eclipse might question what all the fuss is about, says astronomer, author and photographer Alan Dyer. “They think it just gets dark, the same way it does every night. No! A total eclipse is unlike anything you’ve experienced,” says Dyer, who’s seen 15 total eclipses over the past 40 years. “You see, hear and feel a total solar eclipse. Experience one and you’ll be hooked.”
There’s another bonus: an eclipse can be enjoyed without any expensive astronomical equipment; you don’t need to go out and purchase a telescope or even a pair of binoculars. A word of caution is in order, though.
During the partial phases of the eclipse, when some portion of the Sun’s disc remains visible, it’s not safe to view directly without eclipse glasses or equipment fitted with a certified solar filter. But when the Moon is completely covering the Sun – during the total phase of the eclipse – you can gawk at it safely. You can even use binoculars or take photographs with a telephoto lens (again, that’s only during totality).
It’s been a long wait for the Moon to cast its shadow on US soil again. The last time it happened was back in 1991, when it landed in Hawaii but didn’t reach the mainland. Prior to that it was 1979, when observers in the contiguous 48 states last saw a total eclipse, and even then it was only visible from the northwestern corner of the country. The situation for eclipse observers will be very different in August. The path of totality – the narrow zone within which the total eclipse will be visible – will be just 110km (68 miles) wide, but will stretch from coast to coast, running from Oregon to South Carolina.
During a solar eclipse, the Moon’s shadow (think of it as a long, narrow cone that points away from the Sun) makes contact with Earth’s surface. Since Earth rotates east to west, the Moon’s shadow travels along in the opposite direction, running from west to east. After making landfall on Oregon’s Pacific coast, the shadow continues east through the Rockies and on into the nation’s heartland. It continues its eastward rush, crossing the Appalachian mountains and finally zipping across the Carolinas and out over the Atlantic, near the historic city of Charleston.
Note that simply being within the path of totality isn’t enough: you’ll want to be near the middle of the path, known as the centre line. Most locations near the centre line will experience about two and a half minutes of totality. People living just south of Carbondale, Illinois, can brag that they’ll get the longest duration of totality, with a little over two minutes and 40 seconds. That duration drops sharply as you move away from the centre line. Meanwhile, anyone viewing from north or south of the path of totality will experience a partial eclipse – far less dramatic than totality.
As the moment of totality approaches, the entire landscape can appear altered. In the half an hour or so before the Sun disappears, the quality of the light changes, shadows get sharper and the temperature drops. Dogs bark and roosters crow in confusion. This is the moment to make sure the batteries in your cameras are fully charged.
Because the eclipse path cuts right through the US, a record number of people are expected to witness the spectacle. More than 10 million Americans live within the path of totality; nearly 30 million live within 100km (62 miles) of the path. Some have called on the federal government to declare Monday 21 August a national holiday.
Location, location, location
With the eclipse’s path running some 4,500km (2,796 miles) across America, where should you go to watch it? The weather, of course, is a big issue. Roughly speaking, the weather prospects improve from east to west; once you’re west of the Mississippi, you’ve got a better than 50/50 chance of having a clear sky on 21 August, based on many years of climate data. Of course, what the local forecast says the day before the eclipse is more important than historical weather data! Some of the driest spots, with the highest chances of clear skies, include the valleys of central Oregon and central Idaho; some locations have a roughly three-in-four chance of cloudless weather. And of course, there’s the scenery. No doubt, many visitors will be drawn to places like Grand Teton National Park, in northwest Wyoming, right inside the path of totality. Nearby Yellowstone is just outside the path, but many people will likely drop by for a visit before or afterward.
Another big unknown, apart from the weather, is the size of the crowds. “My guess is that they’ll come by the thousands, from all over the US and other parts of the world,” says Randy Holst, president of the Boise Astronomical Society in Idaho. Congestion is a real concern: most of the highways in the Northwest, especially those in the mountains, are two-lane, winding roads. And as Holst and others point out, this part of the country is famous for its natural beauty and is often packed with tourists in August, even when there isn’t an eclipse. Not surprisingly, many hotels and campsites are already booked up – but remember, this is an eclipse that you can, at least in theory, drive to. If your hotel is 80km (50 miles) outside the path of totality, you may still be okay – as long as you don’t end up stuck in traffic!
Farther east, the population density is greater, which means that millions of Americans will be able to see the eclipse from their backyards. “Every day the momentum is building,” says Don Ficken, who heads the Eclipse Task Force for the greater St Louis area in Missouri. “This is a historic event.” In Columbia, Missouri, 50,000 people are expected to gather at a public event at the city’s football stadium; meanwhile, the airport in St Joseph, in the northwest of the state, will host up to 60,000 people at an eclipse-viewing event.
But what if you miss this particular eclipse? The next total solar eclipse you could go and witness will happen on 2 July 2019 – the path of totality passes through Chile and Argentina. The next one visible from the US comes on 8 April 2024.
Why wait until then, though? As solar eclipses go, this one is fairly accessible and the weather prospects in many locations are reasonably good. As Jay Anderson, a meteorologist and avid eclipse chaser puts it, “You only go around once. So do it while you can.”
For a first hand account of the eclipse follow Sky at Night Magazine‘s Elizabeth Pearson as she travels across the USA chasing the eclipse.