By now, you've heard about the total solar eclipse that will be crossing the country Aug. 21. Here's what you need to know to watch it, and understand it:
What happens during a solar eclipse?
Once a month, the moon's orbit takes it directly between Earth and the sun — we know it as the new moon, when the night sky is darkest because the moon is out at the same time as the sun. During a solar eclipse, the moon passes directly in front of the sun, blocking some or all of its light from reaching Earth.
This NASA video explains the moon's role in eclipses:
What will it look like?
Viewed from Central Maryland, about 80 percent of the sun will be obscured. It will appear as an upward-pointing arc, like the top of a fingernail.
In other areas of partial eclipse, different parts of the sun will be visible. This NASA video illustrates how the view varies across the country and will change as the eclipse advances:
In the eclipse's "path of totality," the sun will shrink to a sliver before going completely dark for about two and a half minutes. Then, a sliver will reappear and grow as the eclipse passes. The path is only about 70 miles wide, stretching from Oregon to South Carolina, and you will only experience a total eclipse if you're inside of it.
When should I look for it?
In Baltimore, a partial eclipse will begin at 1:18 p.m., reach its greatest extent at about 2:43 p.m. and end at 4:01 p.m. But the eclipse will begin well before it's visible here.
A partial eclipse will already be occurring when the sun rises in Hawaii, and will come into view just after 9 a.m. on the Oregon coast, so when it's about noon in Baltimore. The period of total eclipse, for locations along its path, is less than three minutes long and will stagger across the continent over the course of about 90 minutes. When total eclipse begins in western Oregon, the partial eclipse will just be starting here in Baltimore. When the period of total eclipse ends for viewers near Charleston, S.C., it will be 2:49 p.m. both there and here in Baltimore.
How can I look at it safely?
Even when the sun is mostly obscured, direct exposure to its rays can cause eye damage. Eclipse glasses, solar filters for telescopes or homemade eclipse viewers are a necessity. The American Astronomical Society recommends buying eclipse glasses from one of the reputed vendors it lists on its website. NASA shares several ways to make homemade eclipse viewers here, here, here and here.
How rare is a total solar eclipse?
There are usually six or seven total solar eclipses per decade, somewhere in the world. There are many more partial eclipses, when the moon never fully covers the sun's face, and annular solar eclipses, when the moon appears slightly smaller than the sun, creating an apparent "ring of fire" in the sky at the point of greatest eclipse.
Of course, most of those eclipses aren't visible from the United States. A total solar eclipse's path of totality has touched the United States eight times since 1900, perhaps most notably in 1918. That eclipse took a somewhat similar path as this month's will, from Oregon to Florida.
Total solar eclipses will cross the continental U.S. twice more in the next 30 years, on April 8, 2024, and Aug. 12, 2045. The path of totality for the 2024 eclipse will be only about 300 miles from Baltimore at its closest, visible from Texas to Maine. The 2045 eclipse will track from northern California to Florida.
Why don't they happen more often?
The moon passes between Earth and the sun once a month, but on a plane that is about 5 degrees askew from the plane on which the Earth orbits the sun. That means that most of the time, at the new moon, the moon's shadow is cast just above or below Earth. Twice a year, the moon's orbit crosses that sun-Earth plane, which astronomers call the ecliptic, but that doesn't usually coincide with the new moon. In addition, the moon's orbit is slightly elliptical, so sometimes when an eclipse occurs, the bodies aren't aligned just right for a total eclipse, so all we get is a partial one.
If you ask Brandon Lawton, an outreach astronomer at the Space Telescope Science Institute, we are lucky to live on a planet where such dramatic eclipses are possible at all, though. The sizes and distances of the sun and moon mean both take up about half a degree of the sky, about the width of a pinky finger at arm's length. Without that, eclipses wouldn't be nearly as dramatic, if possible at all.
"It doesn't happen anywhere else in the solar system," he said.