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Bruce BettsJuly 17, 2017

Your Guide to the Great American Eclipse of 2017

The Moon will totally eclipse the Sun for the first time as seen from the continental United States in more than 40 years on August 21, 2017. A partial solar eclipse will be visible throughout North America and in northern South America and Western Europe. Solar eclipses are spectacular events, but they require some knowledge and planning to safely observe them. Here I provide information about eclipses in general, and specific information about the spectacular eclipse of August 2017. 

What are Eclipses?

The basic concept of eclipses is straightforward, but the details and variations from one eclipse to another get somewhat complicated due to the nature of the geometries involved. Eclipses occur when one celestial body passes in front of a second celestial body as seen from a third celestial body, in other words when the three bodies line up (which is given the fun name syzygy). A lunar eclipse as seen from the Earth occurs when the Moon passes into the shadow of the Earth, in other words when the Earth blocks some or all of the Sun as seen from the Moon. Lunar eclipses, which occur during full Moon, can be seen from the entire night side of the Earth and they last for several hours.

As seen from the Earth, a solar eclipse occurs when the Moon passes in front of the Sun. In a total solar eclipse the Moon completely blocks out the Sun and those who experience a total solar eclipse are completely in the Moon’s shadow. Unlike a lunar eclipse, a total solar eclipse is only visible from a limited area: the line traced by the Moon’s shadow as it moves across the Earth. Typically this line has a width of a few tens of kilometers. Thus one must either be very lucky, or one must travel to see a total solar eclipse. A partial solar eclipse, which occurs over a much broader area occurs when the Sun is only partially blocked by the Moon.

Why don’t we have a solar eclipse every time the Moon is new, so between the Sun and the Earth, or a lunar eclipse every time the Moon is full, so the other side of the Earth from the Sun? The reason is because the Moon’s orbit is tilted 5° compared to the plane of the Earth’s orbit. Thus the Moon only can cause these eclipses when those two planes approximately intersect. This occurs twice a year, and there are at least two solar eclipses of one kind or another each year. However, because the area affected is limited, particularly for total eclipses, people often travel great distances to witness them.

Even though there are at least two solar eclipses per year, there are not necessarily two total solar eclipses per year. That is because both the Moon’s orbit and the Earth’s orbit are somewhat elliptical, noncircular. Thus, the distances between the Earth the Moon and the Sun vary from eclipse to eclipse causing the angular diameters of the Moon and the Sun to vary from eclipse to eclipse. So, in addition to total eclipse when the Moon’s angular diameter is at least as large as the Sun’s angular diameter, there are also eclipses where even though the Moon is centered relative to the Sun, it does not appear large enough to cover the entire Sun. This is referred to as an annular eclipse because of the annulus or circle of Sun that surrounds the Moon as seen from Earth. Because the Earth is round there is a third permutation called a hybrid eclipse where from some locations on Earth the eclipse appears total, and in other locations that are slightly farther from the Moon, it appears as annular. The result of all of this geometry is that a total solar eclipse for any given location is very rare.

Eclipses in History

In history, eclipses have been taken as everything from portents of doom to portents of peace. Some ancient empires believed them to be signs of bad fortune for their rulers. Others interpreted them differently. The ancient Greek historian Herodotus wrote that an eclipse that occurred during a battle between the Medes and the Lydians caused both sides to put down their weapons and declare peace.

Eclipses have also been used by modern historians to help with chronologies of ancient civilizations, for example, a solar eclipse recorded in an Assyrian text was correlated with a solar eclipse that would’ve occurred in 763 BC. Chinese records of eclipses begin at a similar time, and indicate their ability to predict eclipses.  

Eclipses in Science

Solar eclipses have enabled scientific studies and discoveries since at least the mid-19th century. During a total solar eclipse, the Sun’s upper atmosphere, known as the corona, becomes visible. Eclipses enabled the first study of the corona, which is normally not visible because it is overshadowed by the brightness of the photosphere, the upper “surface” of the Sun from which we see light emanating.

Amazingly, the first evidence of the element helium came from observations made during an 1868 total solar eclipse. Spectra, created by the breaking up of the Sun’s light into its wavelength components, indicated an unknown element, which was eventually determined to be helium. Because of the method of discovery, it is named after the Greek word for Sun which is helios.

A 1919 solar eclipse was used to test Einstein’s theory of general relativity. By taking pictures of stars during the eclipse, scientists were able to show that their light was indeed bent by the gravity of the Sun as predicted by Einstein.

The August 21, 2017 Eclipse

The path that totality (where a total eclipse can be observed) for the August 21 eclipse passes from Lincoln Beach, Oregon across the United States to Charleston, South Carolina. This will be the first eclipse to cross both the US Pacific Coast and Atlantic Coast since 1918. The path of totality is exclusively within the United States making it the 1st such eclipse since the founding of the United States – the last equivalent was in 1257.

Totality will begin over land at 1716 UT on the coast of Oregon. It will move off of the continent a little over an hour and a half later at the coast of South Carolina. The path of totality is approximately 100 km wide. Anyone within this corridor will see a total eclipse, weather permitting. But those anywhere in North America as well as the northern part of South America or western Europe can witness a partial solar eclipse. Timing of the eclipse, whether partial or total will depend on your location, but will fall during these few hours on August 21. 

For those along the path of totality, the total eclipse will last approximately 2 minutes with the maximum duration being around 2 minutes and 40 seconds in southern Illinois. The partial eclipse on the other hand, from just about anywhere in North America, will last for 2 to 3 hours.

Solar Eclipse Safety

Solar eclipses are spectacular events, but one must follow appropriate procedures to observe them safely. Looking directly at the Sun can cause permanent eye damage without proper filters. The exception is during the brief period of complete totality, the 2 minutes or so in which the Moon is completely blocking the Sun except for the corona. But during all other times, one needs to use appropriate safety glasses or filters. Sunglasses are definitely not sufficient. One can easily order eclipse glasses online prior to the event. They range from simple cardboard frames to sturdier versions. One can use Welders glass but only if it is #14 or darker otherwise it is not safe. It is very dangerous to look at the Sun through an unfiltered telescope, binoculars, camera or other optical device. And once again the filters used with optical devices must be the proper filters designed for direct viewing of the Sun.

One can also view the partial eclipse by projecting images of the Sun using simple pinhole or equivalent devices. You can put a small hole in a piece of cardboard and project an image of the Sun onto a sidewalk or a piece paper. Or try the fun example of putting one hand with fingers partially spread over the other hand with fingers partially spread forming a waffle pattern. Now put your back to the Sun and look at the shadow of your hands on the ground. The numerous holes between your fingers will form a series of images of the partial eclipse showing bright circles with dark “bites” taken out of them.

What Will You See

Once you are safely observing, you will gradually see the Moon cross in front of the Sun during the partial eclipse phase. It will look like larger and larger bites being taken out of the Sun. Then the Moon will begin to move off of the disk of the Sun. The farther you are from the path of totality, the smaller the amount of the Sun that will be blocked out at the maximum point. As the Moon slowly moves across the Sun over 2 to 3 hours, don’t forget to take a little time to ponder the celestial event: you’re seeing the effect of the Moon 400,000 km away passing in front of the gigantic Sun 150 million km away.

If you are along the path of totality, the show will be even more spectacular for the 2 minutes or so that the Moon completely covers the main disk of the Sun. Just before the Moon completely covers the Sun there is a moment referred to as the diamond ring when the last rays of the Sun illuminate just one part of the circle. And at the beginning and end of the total eclipse one also may see a sight known as Baily’s Beads, where Sunlight streams through topographic gaps in the edge of the Moon such as canyons.

But what about totality itself? Sounding like the trailer to a horror movie, but much more impressive and considerably less dangerous: day will turn into night, birds may stop singing, sunset will appear all around you, stars and planets will become visible in the darkened sky, and the otherwise hidden solar corona will become visible as white rays streaming out from around the dark lunar disk. A total solar eclipse is truly one of nature’s most amazing sights. As you get swept away, don’t forget that once the sunlight starts appearing from around the Moon, you need to put solar glasses back on. 

Let me emphasize that in terms of viewing there is a huge difference between being where the Moon blocks 100% of the Sun, the path of totality, or being where the Moon blocks 99% of the Sun. That little bit of sunlight will wipe out the more profound effects that would be seen on the totality.

Other Opportunities

Eclipses can be predicted very far in advance very accurately, but weather cannot. The specter of disappointment wields the weapon of clouds. Just a forewarning that if you do everything else right, weather can still block out an eclipse. So what are your opportunities if weather defeats you this time? Or if you just love it so much that you want to watch as many eclipses as you can? For every total solar eclipse, there are tours that will take you to the locations you need to go even if those are viewing from cruise ships in the middle of an ocean. There are people that love the experience so much that they will hunt down every possible solar eclipse.

The next total solar eclipse is July 2, 2019 and it will be visible from portions of Argentina and Chile. Those of us in America also don’t need to wait too long for another American total solar eclipse, at least as these things go. April 8, 2024 a total solar eclipse will cross the central United States as well as portions Mexico and Canada. Also, keep in mind that there will also be numerous online streaming events that you can follow live during the solar eclipse. 


Solar eclipses, particularly total solar eclipses, are spectacular displays put on by the cosmos. It may be challenging to get reservations along the path of totality at this point, but well worth the effort. Whether you are able to watch the partial eclipse, the total eclipse, or have to watch online, I hope you enjoy this cosmic spectacle.

For more information and resources about the upcoming eclipse, see The Planetary Society's North American Eclipse web page.

Portions of this article were originally published in Professional Pilot magazine.

Read more: solar eclipse, Earth, the Moon

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Bruce Betts

Chief Scientist / LightSail Program Manager for The Planetary Society
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