On Aug. 21, millions of Americans will turn their eyes upward to witness a rare natural phenomenon: a complete solar eclipse.
The eclipse is the first in decades that will be completely visible in the United States, no matter where you live. No one outside of the US will be able to see it, which is why it’s been dubbed “The Great American Eclipse” and why eclipse excitement in America is reaching a fever pitch. The total solar eclipse will only be viewable for a few minutes—and only along the eclipse’s “path of totality“—but there will be much more time to catch a glimpse of the partial eclipse that day.
Here is what you need to know about the most anticipated astronomical event of the summer:
What is a solar eclipse?
A solar eclipse happens when the moon passes between the sun and the Earth, with the moon’s disk covering the face of the sun.
That Earth experiences eclipses at all is actually kind of a cosmic coincidence: Though the diameter of the sun is about 400 times greater than the moon’s, they appear as the same relative size in our sky because the sun is so far away. When the moon passes between Earth and the sun, it blocks its light while exposing the corona (the sun’s atmosphere). If the moon were smaller, or if the sun was closer to Earth, the moon’s disk wouldn’t cover the sun completely, and we would never experience solar eclipses.
Solar eclipses happen at the new moon phase because that is when the Earth, the moon, and the sun align. The elliptical path of the moon around the Earth intersects the Earth’s elliptical path around the sun twice each lunar month because their orbits are titled 5º from each other instead of being in the exact same plane. The intersecting points of the Earth and moon’s elliptical paths are called lunar nodes. A solar eclipse occurs when the moon passes between the Earth and sun at a lunar node, blocking the sun’s light from hitting Earth. During a total solar eclipse, the sky darkens and the air gets colder by about 10ºF (5.5ºC).
A solar eclipse is not the same as a lunar eclipse, which is more common to see and happens when the Earth passes between the sun and the moon, blocking sunlight from reaching the moon.
When the moon formed over 4 billion years ago, it was much closer to Earth, so it appeared much larger in the sky. Early solar eclipses were darker, with the moon not only blocking the sun, but also its corona. The moon is slowly receding from the Earth at a rate of about an inch per year. In the distant future, Earth won’t experience total solar eclipses because the moon will be so much smaller relative to the sun in the sky.
What are the kinds of solar eclipses?
There are a four kind of eclipses: total, annular, partial, and hybrid. The size of the moon and the sun in the sky do actually vary slightly because the moon orbits the earth in that elliptical path, and the earth orbits the sun in the same way. They can present as bigger or smaller depending on where they are in their respective orbital paths.
- An annular solar eclipse occurs when the moon passes between the earth and the sun, but the moon’s disk doesn’t completely cover the sun’s body. The last annular eclipse in the US occurred in May 2012 and the next will be in October 2023. Because the sun’s disk is still visible during an annular eclipse, viewers on earth can’t see its corona. It is never safe to look directly at an annular solar eclipse.
- In a partial solar eclipse, the umbra (the moon’s shadow) misses the Earth, so the sun appears only partially eclipsed to viewers, no matter where on Earth they are. Partial eclipses are also very unsafe to look at directly without proper viewing equipment.
- A total solar eclipse happens when the orbital paths of the sun and the moon care aligned, so the moon completely covers the sun’s disk. The umbra hits Earth directly, and sky-gazers can see the sun’s corona radiating from behind the moon. This is the type of eclipse happening Aug. 21.
- A ‘hybrid’ solar eclipse (also called an annular-total eclipse) occurs when an annular eclipses changes into a total eclipse, or vice versa.
How often do solar eclipses occur?
Solar eclipses are not as rare as you might think. There are, on average, 2 to 3 every year. About once every 18 months, a total solar eclipse is visible from some location on Earth.
The maximum number that can happen in one year is five, a very rare occurrence. The last time Earth experienced five solar eclipses in a year was 1935 and the next time will be in 2206.
When was the last total solar eclipse in the US?
The last complete solar eclipse in the US happened in July 1991 and was only visible in Hawaii (apparently the sky was cloudy that day—eclipse viewers had much better luck in Mexico). The last total eclipse in the continental US occurred in February 1979 and was visible in many parts of the northwestern US and Canada.
The total eclipse on Aug. 21 is the first to cross the entire US continent since 1918 and the first with a totality exclusive to the United States since 1776.
Check out how many solar eclipses have occurred in your lifetime with this neat tool from the Washington Post.
What are the best places to see the 2017 solar eclipse?
No matter where you live in the US, you’ll be able to see at least a partial solar eclipse on Aug. 21. The lucky Americans who live within the “path of totality,” the narrow area spanning the continent where the moon’s shadow will sweep across the Earth, will experience a total solar eclipse. Twelve US states fall within the path of totality: Oregon, Idaho, Wyoming, Nebraska, Kansas, Missouri, Illinois, Kentucky, Tennessee, Georgia, North Carolina, and South Carolina.
Some large US cities like Nashville, Tennessee and Columbia, South Carolina are directly in the eclipse’s path, but smaller towns are going to have the best views and are preparing for an influx of eclipse-related tourism. In fact, interest in the eclipse for people who fall within the totality path is so fervent that a frequency map of recent Google searches almost exactly mirrors the arc of the eclipse.
Check out NASA’S full interactive map of the solar eclipse’s path that shows the times for the partial and total eclipse to see if you’re one of the lucky few that live within the totality.
What time will the solar eclipse be on Aug. 21?
The eclipse will begin in the western US and move east. The partial eclipse will kick off at 9:05 am PDT in Oregon, which will experience the total eclipse at 10:16 am PDT. South Carolina will be last, with the viewing window for the total eclipse ending at 2:44 pm EDT. The eclipse will be viewable in any given location for about 3 minutes, and it will cross the country in about an hour and a half.
How do I safely watch the solar eclipse?
Sky-gazers should be very cautious—looking directly at the sun can permanently damage your eyes. NASA has an entire portion of its website dedicated to tips for watching the eclipse. According to NASA, looking directly at the sun during an eclipse is never safe unless during the brief total eclipse, when the moon’s disk blocks the sun’s entirely (this only occurs within the path of totality, and only for a few minutes). Otherwise, you should be using proper safety viewers for any kind of eclipse viewing.
A safe way to watch the partial solar eclipse is with special “eclipse glasses” or hand-held solar filters (you can also make a homemade pinhole projector as an alternative method). Regular sunglasses, unfiltered cameras, or homemade filters are not safe and shouldn’t be used. Eclipse glasses must meet some basic criteria to be considered safe:
- Glasses should have ISO 12312-2 certification (that is, passing a particular set of tests set forth by the International Organization of Standardization)
- They should have the manufacturer’s name and address printed somewhere on the product,
- They should not be older than three years, or have scratched or wrinkled lenses.
You should put your glasses on before looking at the eclipse and remove them after you’ve completely turned away.
NASA has a list of approved brands that manufacture glasses for safe eclipse viewing. Use caution: The huge interest in the solar eclipse has led to a cottage industry of fly-by-night manufacturers looking to turn a quick profit. There are hundreds of listings on Amazon from vendors trying to sell subpar and potentially dangerous glasses to eclipse enthusiasts, so be sure to do your due diligence.
What happens if you look at a solar eclipse?
If you look directly at solar eclipse without proper safety glasses, your eyes are exposed to huge amounts of ultraviolet radiation and you could experience photokeratitis, which is like sunburn in the eyes. It is incredibly uncomfortable and makes sufferers extremely sensitive to light. Staring directly at an eclipse can also cause solar retinopathy, in which excessive light hits your retina and causes permanent damage to rod and cone cells. Looking directly at the sun, even for a few seconds, can burn your retina and lead to partial or total blindness.
If you think you might have damaged your eyes by looking at the sun during an eclipse, you should visit an optometrist immediately.
While there isn’t data on eclipse-related eye injuries (doctors report cases anecdotally, but no one is specifically collecting the information), the risk is high enough that NASA created a PDF on eclipse safety you can print from its website.
When is the next total solar eclipse after 2017?
If you miss this summer’s total solar eclipse, you aren’t completely out of luck: The next total solar eclipse with a path of totality that falls within the continental US will happen on April 8, 2024 and be viewable from Texas to Maine.