A total solar eclipse occurs when the moon passes directly between Earth and the sun, casting a shadow on Earth’s surface that instantly turns day into surreal night. In this Expedition, we’ll look at dramatic images from the 2017 eclipse and explore the science behind it.
Dubois, Wyoming: Path of Totality
During a solar eclipse, the moon’s shadow falls across just a tiny spot on earth. As the moon moves in its orbit, that spot of shadow flies across the landscape, forming a narrow band called the “path of totality.”
Anyone inside the path of totality will experience the sun being completely blotted out by the moon. Since the moon’s shadow is round, totality lasts longest for places that lie under the widest part, called “greatest totality.”
The little town of Dubois, Wyoming, population 985, sits directly on the path of greatest totality for the 2017 eclipse.
Though the eclipse passes from coast to coast, and will last slightly longer in the eastern U.S., the arid West gives viewers the best chance for a clear, blue sky with no clouds to block the view.
Though the eclipse can be seen from almost anywhere, a mountain view gives observers a chance to see its effects on the broader landscape, such as streetlights going on in town and “dusk” light on the horizon.
All eclipses begin when the moon first begins to block the sun, a moment known as “first contact.” Only observers looking directly at the sun will be able to spot first contact. Because of the sun’s intense, dangerous brightness, you need special lenses to view this part of the eclipse.
A new moon
Solar eclipses only take place during a new moon. Before first contact, the dark side of the moon is invisible against the daytime sky.
Not much difference
First contact is only visible when looking at the sun using special lenses or viewers. At this point, so much of the sun is still shining that the surroundings do not appear darker.
The Crescent Sun
As the moon orbits Earth, it begins to cover more and more of the sun. The sun appears to go into a crescent “phase.” Because all parts of the sun’s surface are so overwhelmingly bright, this “crescent” is only visible by looking at the sun using special viewers or creating a pinhole image.
2017 Total Solar Eclipse (2017-08-21) by NASA/Bill IngallsNASA
Partial eclipses, and total eclipses seen from outside the “zone of totality,” look like this crescent. You need viewing lenses or a pinhole viewer to see the shadow on the sun.
Even with more than half the sun covered, the remaining sunlight is extremely bright. A sunny day appears hazy or overcast, but it is not yet dark.
Looking at the Sun without special lenses, even during a near total eclipse, is very dangerous. The partial dimming during the eclipse might let your pupils open wider, letting in even more damaging rays.
A pinhole viewer, or camera obscura, works like the pupil of your eye. Light passes through a tiny hole and projects an inverted image, in this case of the sun, onto a surface. This is a safe way to view the sun. This viewer also contains a lens, to magnify the image.
For those in the direct path of a total eclipse, the disk of the moon slips fully in front of the disk of the sun. At the very last moments, as the sky becomes dramatically dark, observers may see some beautiful effects.
Eclipse - Apollo 12 (1980-08-05)NASA
Just before totality, the very last limb, or edge, of the sun sparkles like a diamond. The sun’s bright atmosphere and the tiny edges still showing around the moon create a round “diamond ring” effect.
Just as the moon blocks the full disk of the sun, rays of light pass through valleys, craters, and other depressions on the moon’s surface. These form “Baily’s beads,” like gems on a round bracelet.
In just the right place at just the right moment, the moon blocks the sun entirely. The world changes dramatically—from day to night in an instant. Animals may become confused, or even try to bed down for the night. Stars come out.
Observers sometimes report an overwhelming sense of wonder, or even fear. This is what it is like to experience a total eclipse.
A Round Shadow
The moon’s shadow turns the daytime sky dark, but observers on hills or open plains may be able to see the still-lit areas around the edge appearing like a twilight sky.
2017 Total Solar Eclipse (2017-08-21) by NASA/Carla ThomasNASA
Under the moon’s shadow, the blue sky goes dark. Stars may appear. The constellations visible during an eclipse are those of the opposite season. (For example, during the August eclipse, observers will see constellations normally visible in late winter.)
During totality, air temperature drops dramatically. Animals may seem confused. Street lights and other automatic lights come on.
The Sun in Totality
Astronomers highly value total solar eclipses. It gives them—and us—a rare view of the corona, or sun’s atmosphere. The corona is normally not visible due to the overwhelming brightness of the sun itself.
ESA's PROBA-2 View of Europe's Solar Eclipse (2017-12-08)NASA
The corona is made up of jets of superheated plasma the blast off the surface of the sun. They are hotter than the surface—millions of degrees—and scientists still aren’t sure why. The eclipse may help them learn.
The corona is not an even ring, but has streaks of different densities. This is because the sun’s magnetic field affects the corona’s charged particles in an uneven way.