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ATLANTA — When the total solar eclipse traces a path across Mexico, the United States and Canada on April 8, spectators can anticipate a multitude of awe-inspiring moments.
During a total solar eclipse, the moon completely blocks the face of the sun for a brief period known as totality — and 32 million people in the U.S. who are located along the 115-mile-wide path of totality for April's event will have a chance to enjoy this full expression of the celestial spectacle.
Another total solar eclipse won't be visible across the contiguous U.S. again until August 2044 and an annular eclipse, in which the moon can't completely block the sun, won't appear across this part of the world again until 2046.
"Until you've actually seen (a total eclipse), it's almost impossible to describe," said Dr. John Mulchaey, Carnegie Institution for Science's deputy for science and the director and Crawford H. Greenewalt Chair of the Carnegie Observatories. "When you see totality, you can see how it's had such a huge impact on humans through thousands of years. It's one of the most beautiful things most people will ever experience."
But the phases surrounding totality — including a couple phenomena stunning enough to have earned their own names — are pretty memorable, too, eclipse experts say. Here's what to look out for.
What to watch for during the eclipse
The moon doesn't suddenly appear between Earth and the sun — the event begins with a partial eclipse, in which the moon appears to take a "bite" out of the sun. Depending on your location, the partial eclipse can last between 70 and 80 minutes, according to NASA.
"About 15 to 20 minutes before totality, the sky starts getting this really weird gloomy color," Mulchaey said. "It's almost like a gray because the sun's way high in the sky, but it's almost entirely blocked out. It's not like twilight, sunset or sunrise when (the sun is) low in the sky. It's above you. And all of a sudden, you're losing most of the sun's light, and it feels very weird."
Two breathtaking phases occur within the final moments before totality, Mulchaey said.
When the moon begins to cross in front of the sun, the star's rays will shine around valleys on the moon's horizon, creating glowing drops of light around the moon called Baily's beads. The phenomenon was named for English astronomer Francis Baily, who noted them during an annular eclipse on May 15, 1836.
As totality nears, Baily's beads will quickly disappear and make way for the "diamond ring," a nickname for how it looks when a single point of light remains — like a glistening giant diamond ring.
Both of these phases last less than a minute, Mulchaey said.
Then, it's time for totality.
Prepare for totality
The totality phase of the April 8 eclipse is expected to last twice as long as it did in 2017 because the moon is currently closer to the sun. Those squarely along the center line of the path will see a total eclipse that lasts between 3½ and four minutes, according to NASA.
"All of a sudden, totality happens, and the corona shows up," Mulchaey said. "Even though it's dark out, it's somehow glorious."
The corona is the sun's ultra hot outer atmosphere, which emits a glow that can be seen around the moon during the eclipse. Typically, the corona is hard to see because the sun's surface is so much brighter. During the total eclipse, the corona will resemble white streams of light, according to NASA.
During the 2017 eclipse, the sun was approaching solar minimum, or the quiet phase of our star's 11-year activity cycle. Now, the sun is nearing solar maximum, when the sun is exceptionally active, Mulchaey said. The corona will likely appear brighter and fuller, and there may be a chance to spot flaring loops of solar activity resembling streamers within the corona during the eclipse.
Spectators may also be able to see a region of the solar atmosphere called the chromosphere, which will appear as a thin, pink circle around the moon.
Bright stars or planets like Venus may shine in the dark sky, and the air temperature will drop as the sun disappears. The sudden darkness also causes animals to behave in unusual ways.
"We may start to see nocturnal behavior, like crickets chirping or bats emerging, and animals stopping daytime behaviors, like birds going to roost or flying insects landing," said Dr. Andrew Farnsworth, visiting scientist at the Cornell Lab of Ornithology.
After totality ends, the diamond ring and Baily's beads will briefly reappear before the partial eclipse returns as the moon slowly moves across the sun.
Why eclipses occur
While eclipses occur throughout the solar system, none of them are exactly like the ones experienced on our world.
The moon is about 400 times smaller than the sun, but the moon is also about 400 times closer to Earth than the sun is, creating a "beautiful coincidence" that results in eclipses when the three celestial bodies align, Mulchaey said.
This alignment is called syzygy, or when three objects line up in space.
In the distant past, the moon was much closer to Earth, which means the totality likely didn't appear as it does now. And within another 60 million years or so, the moon will be so far away that it will never cover the sun, making this a rare moment in time, Mulchaey said.
