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A total solar eclipse will cross North America on Monday, offering millions a rare opportunity to see afternoon skies temporarily darken as the moon blocks the face of the sun.

The eclipse's path fortuitously cuts across Mexico, 15 U.S. states and a small part of eastern Canada. In all other states in the continental U.S., viewers will be treated to a partial solar eclipse, with the moon appearing to take a bite out of the sun and obscuring part of its light.

Here's everything you need to know about the rare celestial event.

What is a solar eclipse?

Solar eclipses occur when the sun, moon and Earth align. The moon passes between Earth and sun, temporarily blocking the sun's light and casting a shadow on Earth.

A total solar eclipse is when the moon fully obscures the sun, whereas a partial solar eclipse means it blocks just a portion of the sun's face.

Solar eclipses occur only with the new moon. Because the moon's orbit around Earth is tilted, the three bodies don't always line up in a way that creates an eclipse.

"Imagine if the moon's orbit were in the plane of Earth's orbit around the sun — if that were the case, then every new moon, you'd have a total solar eclipse and every full moon, you'd have a lunar eclipse," Neil DeGrasse Tyson, director of the Hayden Planetarium at the American Museum of Natural History, told NBC News. "So, because things don't always align, it lends to the rarity of the event and the specialness of the event."

Where and when will the eclipse be visible?

This year's eclipse will follow a slightly wider path over more populated areas of the continental U.S. than other total solar eclipses have in the recent past.

NASA estimates that 31.6 million people live within what's known as the path of totality, where the total solar eclipse will be visible. An additional 150 million people live within 200 miles of the path, according to the agency.

The path travels through Texas, Oklahoma, Arkansas, Missouri, Illinois, Kentucky, Indiana, Ohio, Pennsylvania, New York, Vermont, New Hampshire and Maine. Tiny parts of Michigan and Tennessee will also be able to witness totality if conditions are clear.

After the eclipse crosses into Canada, it will pass over southern Ontario, Quebec, New Brunswick, Prince Edward Island and Cape Breton, at the eastern end of Nova Scotia.

Those outside the path of totality can still take part in the astronomical event by viewing a partial solar eclipse — visible throughout all 48 states of the contiguous U.S. — or a NASA livestream.

The timing, including how long totality lasts, depends on the location, but some spots will see the moon fully cover the sun for up to 4 minutes and 28 seconds.

Below is a list of timings for some cities along the path of totality, as provided by NASA. A number of other resources, including NationalEclipse.com and TimeandDate.com, can also help people plan.

• Dallas: Partial eclipse begins at 12:23 p.m. CT and totality at 1:40 p.m.
• Little Rock, Arkansas: Partial eclipse begins at 12:33 p.m. CT and totality at 1:51 p.m.
• Cleveland: Partial eclipse begins at 1:59 p.m. ET and totality at 3:13 p.m.
• Buffalo, New York: Partial eclipse begins at 2:04 p.m. ET and totality at 3:18 p.m.
• Lancaster, New Hampshire: Partial eclipse begins at 2:16 p.m. ET and totality at 3:27 p.m.

This composite image of 13 photographs shows the progression of a total solar eclipse, from right to left, at Madras High School in Madras, Ore., on Aug. 21, 2017.Aubrey Gemignani / NASA

How to safely view a solar eclipse

It is never safe to gaze directly at the sun, even when it is partly or mostly covered by the moon. Special eclipse glasses or pinhole projectors are required to safely view solar eclipses and prevent eye damage. Failing to take the proper precautions can result in severe eye injury, according to NASA.

Eclipse glasses are thousands of times darker than normal sunglasses and specially made to enable wearers to look at the sun during these kinds of celestial events.

Sky-watchers should also never view any part of the sun through binoculars, telescopes or camera lenses unless they have specific solar filters attached. Eclipse glasses should not be used with these devices, as they will not provide adequate protection.

However, during the few minutes of totality, when the moon is fully blocking the sun, it is safe to look with the naked eye.

Tyler Hanson, of Fort Rucker, Alabama, watches the sun moments before the total eclipse on Aug. 21, 2017, in Nashville, Tenn.John Minchillo / AP file

Beware of fake eclipse glasses. On legitimate pairs, the lenses should have a silver appearance on the front and be black on the inside. The manufacturer's name and address should be clearly labeled, and they should not be torn or punctured. Check, as well, for the ISO logo and the code "IS 12312-2" printed on the inside.

If you don't have eclipse glasses, you can make a homemade pinhole projector, which lets sunlight in through a small hole, focuses it and projects it onto a piece of paper, wall or other surface to create an image of the sun that is safe to look at.

All you need is two pieces of white cardboard or plain white paper, aluminum foil and a pin or thumbtack. Cut a 1- to 2-inch square or rectangle out of the center of a piece of white paper or cardboard. Tape aluminum foil over that cut-out shape, then use a pin or thumbtack to poke a tiny hole in the foil.

During the eclipse, place a second piece of white paper or cardboard on the ground as a screen and hold the projector with the foil facing up and your back to the sun. Adjusting how far you hold the projector from the second piece of paper will alter the size of the image on the makeshift screen.

What to look for while viewing the total solar eclipse

For people along the path of totality, there are some fun milestones to keep track of as the total solar eclipse unfolds.

As the eclipse progresses and the sun gets thinner in the sky, it will start to get eerily dark, according to Tyson.

The "diamond ring effect" is shown following totality of the solar eclipse at Palm Cove in Australia's Tropical North Queensland in 2012.Greg Wood / AFP - Getty Images file

When the last beams of sunlight are about to become obscured, look out for the "diamond ring effect": The sun's atmosphere will appear as an illuminated halo, and the last light still visible will look like the diamond of a giant ring.

As the sunlight decreases even further, an effect known as Baily's beads will be created by the moon's rugged terrain. Tiny "beads" of light will be visible for only a few seconds around the dark moon, as the last bits of sunlight peer through the moon's mountains and valleys.

When the moon is fully blocking the sun, it is safe to remove eclipse glasses and look at the total solar eclipse with the naked eye.

The Baily's beads effect is seen as the moon makes its final move over the sun during the total solar eclipse on Aug. 21, 2017, above Madras, Ore.Aubrey Gemignani / NASA

Some lucky sky-watchers may even catch a glimpse of a comet.

Comet 12P/Pons-Brooks — nicknamed the "devil comet" because an eruption last year left it with two distinct trails of gas and ice in the shape of devil horns — is currently visible from the Northern Hemisphere as it swings through the inner solar system.

The comet can be seen in the early evenings by gazing toward the west-northwest horizon. During the eclipse, when skies darken during totality, it may be possible to see the comet near Jupiter, but its visibility will depend on whether it's in the middle of an outburst and thus brighter than normal.

Most likely, all eyes will be on the alignment of the moon and sun.

"Most people won't even notice," Tyson said. "But if you know to look, it's there."

When is the next solar eclipse?

The next total solar eclipse will be in 2026, but it will mostly pass over the Arctic Ocean, with some visibility in Greenland, Iceland, Portugal and northern Spain. In 2027, a total solar eclipse will be visible in Spain and a swath of northern Africa.

The next total solar eclipse visible from North America will be in 2033, but only over Alaska. Then in 2044, a total solar eclipse will cross Montana, North Dakota, South Dakota, parts of Canada and Greenland.

The next total solar eclipse to cross the continental U.S. coast-to-coast in will occur in 2045. The path of totality for that eclipse will cut through California, Nevada, Utah, Colorado, New Mexico, Oklahoma, Kansas, Texas, Arkansas, Missouri, Mississippi, Louisiana, Alabama, Georgia and Florida.