Metro

Here’s everything you need to know about the solar eclipse

Photo taken on September 1, 2016, in Saint-Louis, on the Indian Ocean island of La Reunion, shows the moon covering the sun, leaving a ring of fire effect around the moon, during an annular solar eclipse. Stargazers in south and central Africa were treated to a spectacular solar eclipse on September 1, 2016 when the Moon wanders into view to make the Sun appear as a "ring of fire", astronomers say. The phenomenon, known as an annular solar eclipse, happens when there is a near-perfect alignment of the Earth, Moon and Sun. But unlike a total eclipse, when the Sun is blacked out, sometimes the Moon is too far from Earth, and its apparent diameter too small, for complete coverage. / AFP PHOTO / Richard BOUHETRICHARD BOUHET/AFP/Getty Images

RICHARD BOUHET/AFP/Getty Images

With a total solar eclipse crossing through the United States on Aug. 21, here are a few reminders about what’s to come and how to get the most of your astronomical experience.

What’s happening?

On Monday, Aug. 21, a total solar eclipse is going to interrupt your afternoon. Starting around 1 p.m. on the East Coast, the moon will begin to obscure the sun and, depending on where you are, might completely cover it.

Advertisement

This is called totality.

The path of totality — where you’ll be able to witness a total eclipse of the sun — is 70 miles wide and stretches across the United States, starting in Oregon and cutting diagonally down to South Carolina.

Get Fast Forward in your inbox:
Forget yesterday's news. Get what you need today in this early-morning email.
Thank you for signing up! Sign up for more newsletters here

Though you’ll be able to be see the effects of the eclipse outside that path, the best views will be within it, said Glenn Schneider, an astronomer who has witnessed 33 total solar eclipses.

“There’s no such thing as an almost total eclipse,” Schneider said. If you’re witnessing the eclipse from outside the path, you won’t be able to see the phenomenon of a total solar eclipse — the moon completely covering the sun — but instead will be getting glimpses of a partial covering.

If you’re staying in the Boston area, Schneider said the eclipse will feel like going outside when the sun is covered by very thin clouds — a definite dimming, but not nearly as drastic as it will be within the path.

Advertisement

This is why New Englanders, and others living outside the path of totality, are making cross-country trips for the best view.

Though the moment of totality will last just two minutes and 40 seconds at most in its peak near Southern Illinois, some people have been planning for it for weeks, months, or even years.

Why is everyone so excited about it?

Mainly because the last time a total solar eclipse was visible in the continental United States was in 1979, when totality covered areas including Washington state, Montana, and North Dakota. Before that, the last time a total solar eclipse stretched from coast to coast was in 1918.

In addition to its rarity, its beauty and magnitude are unlike any other experience, Schneider said.

“It is going to be, without question, the most incredible and incomparable natural — or perhaps any — phenomenon you will see in your lifetime,” he said. “It’s impossible to describe.”

There are several distinct features that make a total solar eclipse a spectacular sight. First, viewers will see drops of light called “Baily’s beads,” which will appear at the edge of the moon when sunlight contrasts with the ridges from mountains and valleys on the moon’s surface.

After that, there’s the diamond ring effect, which happens seconds before totality, when the solar corona becomes visible and creates what appears to be a jewel of light around the moon.

If you’re watching from the path, totality will feel eerie. Day will start to seem like night, despite what the clock says. The temperature will drop, birds might stop chirping, and Schneider said people will be awestruck, entranced with what’s going on above them.

What do I need to see it?

This is important: To watch the solar eclipse safely, you need solar eclipse glasses, which are different from sunglasses, even if you’re viewing the eclipse outside the path of totality.

Solar eclipse glasses have especially dark lenses and block out more light than traditional sunglasses. A solar eclipse is a unique experience for our eyes — we aren’t used to seeing bright beams of sunlight contrasted with a night sky.

According to the National Aeronautics and Space Administration, the only time it’s safe to view a solar eclipse without these glasses is during the moment of totality.

An especially important time to wear the glasses is right after totality ends.

When the moon is covering the sun, your pupils will have dilated to adjust to the darkness and let more light in. However, as totality is ending, there’s a flash of sunlight when the moon moves off the sun. This flash is intense enough to damage your retinas.

Glasses are available online, along with a list of reputable brands by the American Astronomical Society to help you avoid buying the fake glasses now flooding the market.

How does a total solar eclipse happen?

For a solar eclipse to occur, the sun, moon, and Earth have to align, which happens around the new moon every lunar month.

We don’t see a total solar eclipse with every new moon because there are other stipulations, too.

Usually, the moon’s shadow misses the Earth because the moon’s orbit is on a 5-degree tilt compared with the earth’s orbit. So for an eclipse to occur, the moon must land on a specific point where the moon’s orbit intersects the earth’s orbit.

In addition, because the moon orbits in an ellipse and not a circle, the moon has to be its closest distance from Earth in order for a total solar eclipse to occur.

It might seem strange that the moon is able to cover the sun because it’s 400 times smaller. However, the sun happens to be 400 times farther away from Earth than the moon, making the sun and moon appear to be nearly the same size from our perspective. This is why, despite the size difference, the moon appears to completely cover the sun during a total solar eclipse.

For more information about the upcoming solar eclipse, and spots to view it along the path of totality, visit eclipse2017.nasa.gov.

Kiana Cole can be reached at kiana.cole@globe.com. Follow her on Twitter at @kianamcole
Loading comments...
Real journalists. Real journalism. Subscribe to The Boston Globe today.