The total solar eclipse that will traverse a large chunk of the continental US on Monday, along with parts of Mexico and Canada, will be one of the most spectacular celestial events in recent memory. Here’s what you need to know:
What is a total solar eclipse?
The phenomenon occurs when the moon moves in front of the sun, and blocks its face completely, causing the bright sky to darken to twilight in just seconds. The track of the moon’s shadow is called the path of totality, which for Monday’s eclipse will be about 115 miles wide and sweep across 15 states in the US in a north-east direction, from Texas to Maine, for roughly an hour, beginning at 1.30pm CT and ending at about 3.35pm ET.
How rare is it?
The most recent total solar eclipse in the US was in 2017, but an interval of only seven years is unusual. The previous one before that took place in 1979, and the next one visible in the contiguous US will not be until August 2044. Only 16 other total solar eclipses have occurred over at least a part of the lower 48 states in the last 155 years.
Didn’t we just have a solar eclipse?
Last October’s stunning “ring of fire” show was what’s called an annular solar eclipse, which occurs when the moon moves directly in front of the sun, but is at or near its farthest point from Earth, so is not large enough to entirely cover the sun’s face. You can find Nasa’s guide to the various types of eclipses here.
Why is this one special?
An estimated 31.6 million people, almost three times as many as in 2017, live in the path of totality – which is longer, and also wider than almost every other total eclipse before it, because the moon is closer to Earth and casts a broader shadow. The time of totality in any given location is also longer. Seven years ago, the longest duration anywhere was two minutes and 42 seconds. On Monday, it will be four minutes and 28 seconds in Torreón, Mexico, while almost every place along the path can expect between three and a half to four minutes of totality.
How can I watch it?
If you live anywhere in the path of totality, or can travel to it, congratulations, you have a front-row seat. Just remember you need eclipse glasses (more on that in a minute) to look up at all times other than the few minutes of full eclipse. But everyone in North America outside the path should be able to see at least a partial eclipse of varying percentages, and Nasa will be livestreaming the entire event here. Here’s the caveat: longer-range weather forecasts call for rain and clouds in many areas of the southern and central US on Monday, so the best viewing could be in the east.
And here is Nasa’s Eclipse Explorer, where you can enter any US address or zip code to find out what will be visible from that location.
What can I expect to see?
You can find Nasa’s guide to the stages of a total solar eclipse here. First contact is when the moon’s outer edge first appears to touch the sun, creating the beginnings of a partial eclipse and a crescent sun reducing in size until totality (second contact). In the moments before totality, look for (in order) shadow bands, Baily’s Beads and a diamond ring, three of the most memorable stages of a total eclipse.
Shadow bands, says Nasa, are rapidly moving, long, dark bands separated by white spaces that can be seen on the sides of buildings or the ground just before totality; Baily’s Beads are short-lived light rays from the sun streaming through the valleys along the moon’s horizon, and give way to the diamond ring effect, the final, single spot of bright light immediately before the end of any direct sunlight.
The sun’s corona, the outermost part of its atmosphere, can only be seen during the total part of the eclipse, then, following totality, the same effects come in reverse. For most locations, the partial eclipse phase lasts from 70-80 minutes, according to Nasa.
How do I watch safely?
To look at the eclipse directly, you must have special eclipse glasses. Regular sunglasses, frosted or darkened glass, or any other method lacking the necessary eye protection just won’t cut it. Nasa’s must-read eclipse safety guide on the subject says: “Viewing any part of the bright sun through a camera lens, binoculars, or telescope without a special-purpose solar filter secured over the front of the optics will instantly cause severe eye injury.” Don’t do what Donald Trump did in 2017.
Many stores, businesses, schools and libraries are still offering glasses free or at low cost (Google is your friend here), but you need to make sure they are safety rated. The American Astronomical Society warned last month that unsafe fakes and counterfeits were circulating, and has published a handy list of suppliers of safe solar viewers and filters (certified ISO 12312-2 compliant).
Experts advise against taking photographs of the eclipse with a mobile phone (unless it’s protected by a solar filter) because it could damage the camera. Nasa has thoughts here.
What if I don’t have eclipse glasses?
You can make a box pinhole projector following the steps in this video, courtesy of Nasa’s Goddard space flight center. An even simpler method is using anything with a small hole, such as a kitchen colander, to project an image on to the ground. And remember that many places across the entire country, such as science centers and zoos, will be hosting special eclipse events so you can watch the action safely.