The Great American Solar Eclipse arrives Monday, August 21st. We’ve got the answers to 10 of your most burning questions about this grand celestial event!
Q: How does a total eclipse of the Sun take place?
A: A solar eclipse occurs when the Moon passes between the Sun and Earth, and lines up precisely so that it covers the disc of the Sun. Actually, the Moon passes between between the Sun and Earth every month in what is known as the “new Moon” phase. But because the Moon’s orbit is tilted slightly relative to the Earth’s orbit, it more often goes unnoticed each month either above or below the Sun at each new Moon. Thus most of the time when we have a new Moon there is no eclipse. But when the line-up is just right, the Sun is “eclipsed” by the Moon.
Total solar eclipses are a fortuitous accident of nature. The Sun’s 864,000-mile diameter is fully 400 times larger than that of our puny Moon, which is 2,160 miles in diameter. But the Moon also happens to be about 400 times closer to the Earth than the Sun (the ratio varies a bit, as both orbits are elliptical), and as a result, when the orbital planes intersect and the distances align favorably, the new Moon can appear to completely blot out the disk of the Sun.
Q: Are total eclipses rare?
A: Contrary to popular belief, total solar eclipses are not particularly rare. Astronomers predict sixty-eight to take place during the present century—one about 17.6 months. On such occasions, the Moon casts its dark, slender cone of shadow (called the umbra) upon the Earth’s surface (see illustration above). The track traced by the umbra can run for many thousands of miles, but it’s also very narrow, at most about 167 miles wide. It has been calculated that on average, a total eclipse of the Sun is visible from the same spot on Earth only once in about 375 years.
Q: Where have some of the most recent total eclipses been visible?
A: In recent years, assiduous eclipse chasers had to travel to such remote locations such as Novosibirsk, Siberia (2008), Easter Island, Chile (2010), the Norwegian archipelago of Svalbard (2015), and most recently, Palembang, South Sumatra (2016).
Q: When was the last time a total eclipse of the Sun was visible from the United States?
A: August 21 will mark the first time in this century and the first time since 1979 that a total solar eclipse will cross the contiguous (48) United States (Alaska had its turn in 1990; Hawaii in 1991). And for the very first time ever, the shadow track – better known as the “path of totality” – will sweep only over the United States and no other country, leading some to refer this upcoming event as “The Great American Eclipse.”
Q: How many people will witness the total phase of the eclipse?
A: Nobody knows! This total eclipse has a potential audience of some 12 million people who luckily live within the totality path. However, the number of people who are within just a one day’s drive of the totality zone probably number around 220 million. And some are predicting traffic jams of “historic proportions” on many interstate highways leading into the path of the total eclipse, such as Interstate 5 out of Portland, Oregon, and Interstate 85 out of Atlanta, Georgia.
Q: How many states will be in the total eclipse path?
A: Initially, the shadow will travel along nothing but the North Pacific Ocean for 28 minutes. Finally it will make landfall along the coast of Oregon at the community of Lincoln Beach. Traversing the United States, the total eclipse will be visible within a path of darkness stretching from Oregon, then on through Idaho, a small slice of Montana, Wyoming, Nebraska, Kansas, a snippet of Iowa, Missouri, Illinois, Kentucky, Tennessee, Georgia, North Carolina and finally South Carolina. Fourteen states in all. The path will average 67 miles in width, but while moving through western Kentucky, about 12 miles northwest of the town of Hopkinsville (pop. 3,000), the shadow’s size will widen to a maximum of 71 miles.
Q: How long will the total phase last?
A: Because the Moon’s shadow is moving at such a tremendous speed, totality cannot last very long in any one place. The duration of the total phase is always longest along the center of the shadow’s path. As one moves way from the center, the duration of totality time decreases, becoming zero along the path’s edges. Right at the Oregon coastline totality lasts 1 minute 59 seconds as the shadow will be traveling at more than three times the speed of sound (2,400 mph).
Heading southeast along the center line, the totality time slowly lengthens, reaching a maximum of 2 minutes 40.2 seconds at a spot in southern Illinois about a dozen miles southeast of the city of Carbondale. The shadow which at this point has slowed to 1,450 mph, then begins to increase in speed and the duration of totality will subsequently begin to diminish. Indeed, when it arrives at the South Carolina coastline, the duration of totality will have dropped to 2 minutes 34 seconds. The shadow then exits out to sea, finally leaving the Earth 75 minutes later at local sunset in the North Atlantic Ocean, 390 miles to the southwest of the island nation of Cape Verde Are there any big cities that will see the total eclipse? Notable cities that find themselves inside of the totality path include Idaho Falls, ID, Casper, WY, Grand Island, NE, Lincoln, NE, Columbia, MO; Nashville, TN, Columbia, SC and Charleston, SC. The metropolitan areas of Kansas City and St. Louis find themselves right on the edge of the totality path.
Q: If I don’t travel into the totality path, will I still see the eclipse?
A: Definitely! (So long as the weather where you are is clear that day). Surrounding the dark umbra is the penumbra or partial shadow, also conical but much larger (nearly 6,000 miles) in diameter. The penumbra is simply the half shadow that lies outside every deep shadow whether it’s cast by the Moon or a house. Wherever the penumbra falls, a partial eclipse will occur. All of North America will be inside the penumbra, causing a rather large partial eclipse for much of the U.S. and portions of southwest Canada. Hawaii will also see a small partial eclipse that will coincide with sunrise.
Q: So what can I expect to see if I’m in the zone of total eclipse?
A: It starts quietly, about 80 minutes before the total phase, with first contact: a small dent appears on the Sun’s right hand limb, growing slowly larger each minute.
The Moon will continue to advance steadily, reducing the disc of the sun down to a crescent. In those last few minutes before totality, events pile in on each other: an eerie twilight begins to descend, the distant landscape becomes enveloped in a strange grayish pallor; the air temperature may suddenly dip several degrees or more. As totality draws nearer and the sky grows still darker, you might even begin to feel a little nervous, for here is a situation that neither you nor any of the people who might be around you has any control over. Only now can you begin to understand why eclipses both fascinated and terrified ancient people.
If you spread out a large white sheet on the ground, you may see shadow bands rippling, flickering, and scurrying about. These stripes of light and shade are believed to be caused by the last of the sun’s rays being distorted by our turbulent atmosphere, just as a star’s light is disturbed making it appear to twinkle. As the sun narrows to a thin filament, it suddenly disintegrates into irregular dots and points of light called “Baily’s Beads,” an effect caused by the last rays of sunlight streaming through the rugged mountain valleys on the lunar limb.
Then the Moon’s dark shadow then comes rushing in. Those watching for its approach should look to the west-northwest sky, where clouds will darken dramatically as if some great storm were brewing. At totality’s onset, the shadow suddenly engulfs you with the darkness of a sky similar to about a half hour after sunset. You’ll likely be hearing oohs and ahhs, gasps, shouts and screams from people all around you (you may even be shouting too!) as the Moon completely covers the Sun.
The Sun appears as a jet-black disc rimmed for several seconds by the vivid pastel-pink extension of the Sun’s atmospheric envelope: the chromosphere. If you use binoculars, you may see in several places around the black disc, tiny flames of pink or magenta. Called prominences, these are hot clouds of hydrogen gas pushing up from the sun’s surface for tens or even hundreds of thousands of miles into space.
The most spectacular view, however, is the pearly-white corona, which haloes the dark disc of the Sun and extends out into space for millions of miles. It can only be seen during totality and differs in size, tints, and patterns from one eclipse to another. Streaming outward, ragged at the edge, streaks running through it; sometimes it has a soft continuous look and at other times long rays may be seen shooting out in three or four directions.
Once you are able to tear yourself away from the Sun, other celestial objects can be sighted. The most obvious will be the brilliant planet Venus shining like a dazzling white jewel and which will be positioned well to the right of the Sun. And at an even greater distance to the Sun’s left, though not shining quite as bright, will be Jupiter. A few stars may be visible here and there, and if you have binoculars, you might notice a bluish one that will be plainly visible just to the left of the darkened Sun. That will be Regulus, in the constellation Leo and one of the 21 brightest stars in the sky. The combination of darkness and starlight at midday always helps to create a lasting memory of a total eclipse.
As for the overall sky illumination, it will be unlike any dusk or dawn you’ve ever experienced. A weird saffron tint will form a bright border around the horizon, while clouds in the area may take on striking hues of sienna or salmon.
Just before the end of totality, the chromosphere will again reappear, followed suddenly by a brilliant solitaire of steely-white light set upon a thin, luminous ring – the inner corona. The streamers vanish; the gem grows; the stars and planets fade away; the sky fills with light as the great “diamond ring” in the sky soon becomes too dazzling to look at.
All the phenomena seen prior to totality now reappear in reverse order as the Moon moves off the Sun’s disc. But . . . be careful! Whenever an eclipse of the Sun is due to occur there are usually dire warnings broadcast over the airwaves and in newspapers telling people that there is no safe way to view an eclipse and the best thing to do is watch it on television. Television, however, is a poor substitute for the real thing and should you choose to ignore the eclipse entirely, you’ll only be cheating yourself out of seeing the grandest celestial spectacle visible from Earth.
This is not to say, however, that you should not take precautions. Staring at the Sun with unprotected eyes or inadequate filters during the partial stages can cause severe retinal damage or blindness.
Q: So after this year, when is the next solar eclipse?
A: After 2017, the next total solar eclipse for the United States occurs on April 8, 2024, whose path stretches from central Texas to northern New England. The duration of totality will average just under 4 minutes (4 minutes 27 seconds in Texas). Interestingly, the totality path will again encompass Carbondale, Illinois – their second total eclipse in less than 7 years!
Good luck and clear skies!
Joe Rao is an astronomer and contributes regularly to the Farmers' Almanac.