On April 8, North America will experience its second total solar eclipse in seven years. The moon will glide over the surface of our sun, casting a shadow over a swath of Earth below. Along this path, the world will turn dark as night.

Skywatchers in Mexico will be the first to see the eclipse on the mainland. From there, the show will slide north, entering the United States through Texas, then proceeding northeast before concluding for most people off the coast of Canada.

Why eclipses happen is simple: the moon comes between us and the sun. But they are also complicated. So if you’ve forgotten all of your eclipse facts, tips and how-to’s since 2017, we’re here to explain it for you.

But before we dive in, there is one thing to know that is more important than anything else: It is never safe to look directly at the sun during an eclipse (except for the few moments when the moon has fully obscured its surface). At all other times, watch the event through protective eye equipment. Read on to learn about how to watch an eclipse safely.

A solar eclipse occurs when the moon orients itself between Earth and the sun, shielding the solar surface from our view.

In cosmic terms, it is unusual that this happens: the moon is about 400 times smaller than the sun, but it is about 400 times closer to us. That means that when these two celestial bodies are aligned, they appear to be the same size in the sky.

Annular solar eclipses occur when the moon is farther from Earth and appears too small to completely shield the sun’s surface. Instead, the outer part of the solar disk remains uncovered — a “ring of fire” in the sky.

Partial solar eclipses happen when Earth, the moon and the sun are imperfectly aligned. The moon only obscures a chunk of the sun. There will be two in 2025.

Earth can also get between the moon and the sun, creating a lunar eclipse. This can be observed once or twice a year.

In any given place along the eclipse path, the event will last around two hours or more.

The event will commence with a partial solar eclipse, as the moon takes a small bite out of the sun’s edge, then consumes more and more of its surface. According to NASA, this can last anywhere from 70 to 80 minutes.

The phase of the eclipse where the moon has completely blocked the sun’s surface is called totality. This is the only time the event can be viewed with the naked eye.

The length of totality varies by location. In April, some places will experience this phase for more than four minutes; others, for only one to two minutes.

During totality, the sky will get dark as night and the temperature will drop. Wispy white strings of light from the sun’s outer atmosphere, or corona, will suddenly be visible. Lucky viewers may even spot a thin, reddish-pink circle around the edge of the moon. That’s the chromosphere, an atmospheric layer below the sun’s corona. Its color comes from the presence of hydrogen throughout the layer.

After totality, the sun will slowly peek out from behind the moon again — another partial eclipse that will last the same amount of time as the first one. The moon will recede until the sun is back to normal brightness in our sky.

In general, avoid looking directly at the sun without special equipment to protect your eyes. Inexpensive options for watching the eclipse include paper solar viewers and glasses. If you are using equipment purchased for a past solar eclipse, make sure to inspect it. Toss anything with scratches or other signs of damage.

According to NASA, it is not safe to look at the sun through any optical device while using paper glasses or viewers. To watch the eclipse through cameras, binoculars or telescopes, buy a special solar filter.

The only time you can view a solar eclipse with the naked eye is during the moments of totality. Once the moon begins to reveal the surface of the sun again, return to watching the event through protective equipment to avoid injury.

In general, staring directly at the sun, even for a few seconds, can cause permanent damage to your eyes. This can range from blurry or distorted vision to something even more serious, like blind spots. Because there are no pain receptors in the retina, you won’t feel it while it’s happening.

The same is true during an eclipse — except during the brief moments of totality, when the moon has hidden the face of the sun. At all other times, use protective eye equipment to view the event.

If it’s too late to get glasses or viewers, there’s always a do-it-yourself option: a pinhole camera to indirectly experience the eclipse. You can create one using cardstock, a cardboard box, a kitchen strainer or even your fingers. These designs project an image of the eclipse onto the ground or some other surface that is safe to look at.

The total eclipse will sweep across large portions of Mexico, the United States and eastern Canada. For the most dramatic show, it’s best to experience the eclipse along the path of totality, which is where the moon will completely blot out the sun.

Viewers near Mazatlán, a beach town on the Pacific shoreline of Mexico, will be the first place to experience totality on North America’s mainland. Various sites in Mexico along the eclipse’s path will experience the longest duration of totality — as long as four minutes and 29 seconds.

Cities across the United States, including Dallas, Indianapolis and Cleveland, will most likely be hot spots for the upcoming eclipse. Other notable locations include Carbondale, Ill., which also saw totality during the solar eclipse in 2017; small towns west of Austin, Texas, which are projected to have some of the best weather in the country along the eclipse path; and Niagara Falls, if the skies are clear. Six provinces of Canada are in the path of totality, but many of them have a very cloudy outlook.

The show begins at dawn, thousands of miles southwest of the Pacific shore of Mexico. The moon starts to conceal the sun near Mazatlán at 9:51 a.m. local time. Viewers near Mazatlán will experience totality at 11:07 a.m. for four minutes and 20 seconds.

Then the moon’s shadow will swoop through Mexico, crossing over the Texas border at 1:10 p.m. Eastern time. Totality in the United States will start at 2:27 p.m. and end at 3:33 p.m. Eastern time.

Canadians will experience the solar eclipse in the afternoon for nearly three hours. The eclipse concludes beyond Canada’s boundaries when the sun sets over the Atlantic Ocean.

The duration of totality depends on how far a given location on Earth is from the moon. Places with the longest totality are closest to the moon and farther from the sun. The speed of the lunar shadow is slowest over spots with the longest totality.

In April, the longest period of totality will occur over Durango, a state in Mexico, for a total of four minutes and 29 seconds. Along the centerline, the location of shortest totality on land is on the eastern coast of Newfoundland and Labrador in Canada, for about two minutes and 54 seconds. But totality is even shorter along the edges of the total eclipse path; in some places, it lasts less than a minute.

Solar eclipses may seem to happen slowly, but the moon’s shadow is racing across the surface of Earth. Exact speeds vary by location. Eclipse calculators estimate the shadow will move between about 1,560 and 1,600 m.p.h. through Mexico, and more than 3,000 m.p.h. by the time it exits the United States. The eclipse will reach speeds exceeding 6,000 m.p.h. over the Atlantic Ocean.

According to the American Astronomical Society, total solar eclipses happen once every year or so, but they can only be viewed along a narrow path on Earth’s surface. Many occur over water or other places that can be difficult to reach. A given location will experience totality once in about 400 years.

But some places get lucky: Carbondale, a college town in southern Illinois, saw the total solar eclipse in the United States on Aug. 21, 2017, and will experience another one this April. San Antonio experienced an annular eclipse last October, and is also in the path of totality for this year’s eclipse.

Yes, any planet in our solar system with a moon can experience a solar eclipse. In February, a Martian rover captured Phobos, one of the red planet’s moons, transiting the sun.

The moons on other planets, though, appear either smaller or larger than the sun in the sky. Only Earth has a moon just the right size and at just the right distance to produce the unique effects of totality.

As the eclipse approaches its maximum phase, the air will get cooler, the sky will grow dimmer, shadows will sharpen and you might notice images of crescents — tiny projections of the eclipse — within them. Along the path of totality, the world will go dark while the moon inches toward perfect alignment with Earth and the sun.

Animals will also react to the solar eclipse. Bees stop buzzing, birds stop whistling and crickets begin chirping. Some pets may express confusion. Even plants are affected, scientists found after the solar eclipse in 2017. They have diminished rates of photosynthesis and water loss similar to, though not as extreme as, what happens at night.

Viewers in locations away from the eclipse path will see the moon partially blot out the sun, though how perceptible the effects are depends on the site’s distance from the centerline. (The closer you are, the more remarkable it will be.) Still, it won’t be quite like experiencing the eclipse during totality.

Remember that you should always wear protective eye equipment while watching a partial eclipse.

If you can’t make it to the path of totality but still want to experience it, many organizations are providing live video streams of the eclipse, including NASA and Time and Date. The Exploratorium, a museum in San Francisco, will also offer a sonification of the eclipse and a broadcast in Spanish.

In the 1800s, a French astronomer discovered the element helium by studying the spectrum of sunlight emitted during an eclipse. These events also allowed the first scientific observations of coronal mass ejections — violent expulsions of plasma from the sun’s corona — which can cause power outages and communication disruptions on Earth. Scientists also confirmed Einstein’s theory of general relativity, which says that massive objects bend the fabric of space-time, during a solar eclipse in 1919.

And there is more to discover. This April, NASA plans to fly instruments on planes to capture images of the solar corona, and launch rockets to study how the drop in sunlight during an eclipse affects Earth’s atmosphere. A radio telescope in California will try to use the moon as a shield to measure emissions from individual sunspots.

The public is joining the fun, too. During the eclipse, a team of ham radio operators will beam signals across the country to study how solar disturbances can affect communications. Some people along the path of totality will record sounds from wildlife. Others will use their phones to snap pictures of the eclipse to help sketch out the shape of the solar disk.

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