August 21 is a big day in the American celestial calendar: the 2017 solar eclipse. Referred to by some as the “Great American Eclipse,” its track across the entire continental United States gives millions the opportunity to see the sun disappear for a few minutes — in some places, completely — behind the moon.
What makes the 2017 solar eclipse all the more special is it’s a total eclipse, where the sun is blocked out more completely than during a more common annular eclipse. Such a thing hasn’t been viewable from American soil in 38 years, so be sure to take a picture! It will be one of the most impressive celestial events of the year. So why should you be excited about this event? Read on and we’ll explain.
Why do solar eclipses happen?
Solar eclipses occur when the moon passes in front of the Sun, blocking out its light. While the moon is only a minuscule fraction of the size of the Sun, the Sun is 94 million miles away from us, and this allows the moon to block out the Sun’s rays across a small fraction of the Earth’s surface.
You might be wondering why solar eclipses are so rare. This is due to two factors. First, the moon must be in the “new moon” phase, which means it is between the Earth and the Sun. Because of this, we see the “dark side of the moon,” and the moon appears invisible. But with new moon phases happening every 29.5 days or so, why aren’t eclipses happening just as frequently? You can thank the tilt of the moon’s orbit with respect to Earth for that.
There’s about a five-degree difference at play, which explains why the moon’s position in the sky seems to change daily (this also has to do with Earth’s seasonal orbital wobbles, which explains why we have seasons). This means that during most new moons, the Moon’s shadow either falls above or below the Earth’s surface.
But generally, two times a year these two orbits line up just right, and an eclipse occurs. If you’re lucky enough to see it, it’s a beautiful sight. And if you’re an astronaut, you’re in for a treat; watching the eclipse from the moon is completely different.
What’s the difference between an “annular” and “total” solar eclipse?
Solar eclipses come in two forms, annular and total. The difference depends on the orbit of the moon. In space, orbits are not a perfect circle: instead they have some degree of eccentricity, which is a measure of how much the orbit deviates from that perfect circle. This eccentricity also governs the magnitude of the eclipse itself. You can also see this concept at work throughout the year as the moon appears to grow and shrink in size in the night sky.
Most of the time the moon is far enough away that it doesn’t cover the sun completely, allowing for a ring of sunlight to come through. While light is greatly reduced, there still is enough that areas in the path of totality are basked in eerie twilight. With total eclipses, it is different.
In a total eclipse, the moon blocks out the solar disk completely, leaving a fibrous halo. What you’re seeing here is the sun’s corona — one of the few times it is visible on the Earth’s surface to the naked eye. Bright stars may become visible, crickets may start chirping, and you might even experience a sudden (but small) drop in temperature. For a total eclipse, the moon must be close enough to block out the entire solar disk, which makes them less common than annular ones.
In either case, totality lasts only for a few minutes, although the process to and from totality takes several hours.
What should I expect from the 2017 solar eclipse?
This depends greatly on where you are. Every U.S. state will see some percentage of the Sun disappear, with the Lower 48 seeing the Sun at least 55 percent obscured. The first spot to see totality in the U.S. from the 2017 solar eclipse will be just north of Newport, Oregon at about 10:15 a.m. PT. (Pack a bag of eclipse gear and get going!) From there, it will race east and southeastward, exiting the U.S. coast at 2:49 p.m. local time at Cape Romain, South Carolina.
(Editors note: while it seems like a long time, we’re using local times. The ‘path of totality’ will actually make its way across the entire Lower 48 in just 94 minutes!)
Altogether, 14 states will be in the path of totality, with those passing closest to the center of that path experiencing generally around two and a half minutes of darkness. Carbondale, Illinois will take the cake though: the city is closest to the point of greatest totality, and will see the sun eclipsed for 2 minutes, 42 seconds!