A solar eclipse will be seen throughout the United States on August 21, and eclipse chasers around the world are preparing for the almost once-in-a-lifetime event. The last time a solar eclipse was seen from coast to coast was in 1918.

An eclipse is the alignment of the Earth, the Sun, and the Moon. It occurs around every 18 months or so when the Moon passes directly between the Earth and the Sun on its orbit around the world. When the three bodies line up perfectly, the Moon covers up the disc of the Sun, and people in the direct path of the Moon’s shadow will see the Sun go dark.

Image credit: National Geographic
Image credit: National Geographic

Even though solar eclipses occur every 18 months, this is the first time in 99 years that the event will be seen from coast to coast in the U.S.

The Moon will completely cover up the Sun this month

An eclipse is a special event because the Moon appears to cover the Sun. However, is a matter of perspective, as the Sun is roughly 400 times the size of the Moon.

“The Moon is small compared to the Sun, but it’s much closer, so we’re at a unique point where the Moon can perfectly cover the disc of the Sun,” Noah Petro, a planetary geologist at NASA’s Goddard Space Flight Center, told The Verge.

Not all eclipses are the same, as sometimes the Sun is completely covered (called a total eclipse), while sometimes the Moon only partially covers the Sun (a partial eclipse).

This total eclipse will be seen everywhere in the U.S., although it will look clearer depending on the place where you are. NASA published a map where the path of totality will be seen. The path of totality is about 70 miles wide –from Oregon to South Carolina– and it’s the area where the Moon’s shadow will track across Earth’s surface, which is the result of the Moon moving in its orbit and the planet rotating. Anyone in the path of totality will be able to see the Sun completely covered up for a short period.

Solar filter sunglasses are only ones that work to see the eclipse

The eclipse will begin in the middle of the Pacific Ocean at 11:46 A.M ET. In the continental U.S., it will be seen on the coast of Oregon around 12:05 P.M ET. The shadow will travel across the path of totality at 1,500 miles per hour, and it will finish its trip through the U.S. at 4:06 P.M ET, when it stops being seen at the coast of South Carolina.

It’s necessary to have a pair of solar filter sunglasses to look at the eclipse, as staring at the Sun is bad. Solar filter sunglasses block more than 99.99 percent of sunlight, ultraviolet, and infrared radiation. NASA and the American Astronomical Society have listed manufacturers who make safe glasses for viewing the Sun.

Image credit: RMSC
Image credit: RMSC

NASA says the only time the Sun can be watched with the naked eye is during a total eclipse when the Moon is completely covering it. However, it’s important not to see the Sun the moments before it is completely covered because sunlight can still cause severe damages to the eyes, such as retinal burn.

“Until that point, the Moon completely blocks out the Sun, just a little bit of sunlight is still dangerous to your eyes,” Petro told The Verge.

NASA also said it’s possible that smartphone cameras with small lenses could be damaged by the Sun. If you’re worried about your camera, experts recommend you put your solar filter glasses over the lens and take pictures of the solar eclipse like that.

Total solar eclipse will be seen on August 21

Scientists have perfected the science to predict solar eclipses. Better technologies and fancy equipment have aided astronomers to predict these events correctly. However, it was not always the case. Thousands of years ago, astronomers were beheaded if they failed to predict when one of these events would occur.

For instance, in 2134 BC, a total eclipse occurred over China, but Emperor Chung K’ang said that “the Sun and Moon did not meet harmoniously.” That mistake caused the beheading of two court astronomers, called Hsi and Ho, who failed to predict the eclipse.

Scientists and astronomers have far better resources now than they did thousands of years ago, and a NASA astronomer said it had been a gradual improvement.

“It has been centuries of gradual improvement that just piled up over time,” former NASA astronomer Fred Espenak told BuzzFeed News. “The floodgates really opened when digital computers came online.”

The total solar eclipse will be seen on August 21. If you’re not in the path of totality, you can see the total solar eclipse in a live stream from NASA, as it will be broadcasted on NASA TV for about 4 hours.

Source: The Verge