When the Moon and Sun Align: The Great American Eclipse, Part I

Summer may be winding down for those readers in the United States, but don’t despair—there is at least one fantastic reason to be excited about August. THE SOLAR ECLIPSE IS COMING!

[We know many Physics Buzz readers don’t live near the viewing area of this eclipse. If you are one of them, check out Time and Date’s list of upcoming eclipses to see if there is one coming soon to your area. We are especially excited about the August 21 eclipse since the path of totality is within driving distance of the PhysicsCentral office!]

On Monday, August 21, a 70-mile wide strip of the country ranging from Oregon to South Carolina will go dark. Not all at the same time, though; the darkness will travel from west to east over the course of about 90 minutes.

Around 9am local time in Lincoln Beach, Oregon, viewers will see the moon start inching in front of the sun. At 10:16am, the moon will completely block out the sun and the area will be fully covered by the moon’s shadow for about two minutes. Look up during that time and you’ll see the wispy tendrils of the sun’s outermost atmosphere reaching out from behind the dark circle of the moon. After totality, the moon will begin creeping out of the frame until full sunlight returns.

The total solar eclipse of March 9, 2016, as seen from aboard the cruise ship Le Soleál in the Molucca Sea off the coast of Indonesia. This sequence runs from upper right to lower left. During the partial phases before and after totality, the camera lens was covered by a safe solar filter. No filter was used during totality, which is about as bright as the full Moon and just as safe to look at.
Image Credit: Rick Fienberg / TravelQuest International / Wilderness Travel (CC BY-NC-ND 4.0).

This incredible scene will play out across the country through parts of Idaho, Wyoming, Montana, Nebraska, Iowa, Kansas, Missouri, Illinois, Kentucky, Tennessee, Georgia, North Carolina, and South Carolina. Not only will the areas grow dark, the sounds will change—nocturnal animals may become more lively, while songbirds and grasshoppers go quiet. Roosters may start to crow as daylight returns and pets may be anxious about the sudden change in lighting.

People in the United States not on this path, called the path of totality, won’t completely miss out: along with people in parts of South America, Africa, and Europe, they will be treated to a partial solar eclipse. Depending on the location, the moon will block out 50% or more of the sun at the peak of the eclipse for those off path within the United States. To find out what you can expect at your current location and for details on where totality will occur, visit Solar Eclipse Across America.

Solar eclipses aren’t rare, but being within the path of totality is. Any given place on Earth is within the path of totality only about once every 400 years. Exactly when and where an eclipse happens depends on a complicated mix of factors—the orbits of the moon, Earth, and sun; the changing size that the sun and moon appear to be due to elliptical orbits; and the tilt of the moon’s orbit relative to orbits of the Earth and Sun (the American Astronomical Society has a nice explanation here).

In fact, we’re lucky that total solar eclipses happen at all. Consider that a total solar eclipse is only possible when the moon appears to be at least as big as the sun, otherwise it wouldn’t be able to block out all of the light. In actual size, the sun is much bigger than the moon. The diameter of the sun is about 864,576 miles, while the diameter of the moon is 2,159 miles. Do the math and diameter of the sun is 400 times as big as the diameter of the moon.

However, the sun is also much farther away from us—about 92,960,000 miles compared to the moon’s 238,900 miles. Put another way, the sun is, on average, nearly 400 times farther away from us than the moon. Together, the fact that the diameter of the moon is 400 times smaller than the sun but it’s 400 times closer means that the sun and moon have about the same angular size—the two discs appear to be about the same size to a viewer on Earth.

But let’s say that instead of in Oregon, you are in southern Texas during the eclipse. Why isn’t it as dark? The path of totality is the area within the innermost shadow of the moon, called the umbra. Looking at a bit of the geometry helps illustrate why the path of totality is so narrow—the size of the moon and its distance from the Earth mean that during an eclipse, the Earth passes through the very narrow end of the moon’s shadow. If you were to freeze the moon and sun during and eclipse and slide the Earth closer to the moon, the path of totality would grow larger.

People within the moon’s umbra (the darkest part of the shadow) will experience totality. People within the blue region of the penumbra will see a partial solar eclipse.
Image Credit: NASA.

If we moved the Earth right up against the moon so that as much of the Earth as possible is within the umbra, that would give us a circular shadow with roughly the same diameter as the moon. In other words, the path of totality would be about 2,159 miles wide instead of just 70 miles. (Note that this is a simplified view. Since the moon and Earth are both spheres, the moon’s shadow would already have narrowed some before reaching the surface of the Earth.)

To a person outside of the path of totality but within a region covered by the penumbra (see image), the eclipse will be visible but it will never reach totality. In other words, the entire moon will not appear directly in front of the sun. The farther you are away from the path of totality in either direction, the smaller the eclipsed part of the sun will be. If at all possible, say those who have witnessed a total eclipse, be in the path of totality. It’s supposed to be incredible! At least as long as the weather cooperates…

For more on eclipse science and safety, watch for upcoming Physics Buzz posts and check out these comprehensive websites dedicated to the August eclipse:

NASA Eclipse Across America
AAS Solar Eclipse Across America
Exploratorium Total Solar Eclipse
Great American Eclipse

Kendra Redmond

Read more about the Great American Eclipse

Part II: Dark Days Ahead

Part III: Get Your Science On

Part IV: Tales of Totality

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