For a couple of hours on Monday, in case you have somehow missed the astronomical degree of excitement, the moon will get in the way of our view of the sun. Along a nearly 70-mile-wide path stretching from Oregon to South Carolina, for two to three minutes at the peak of the phenomenon, the sun will be covered entirely and the sky will go dark.
COMING MONDAY: Derrick Pitts, the Franklin Institute's chief astronomer, will take your solar eclipse questions live on Philly.com's Facebook page at 3 p.m., just after the eclipse's peak in Philadelphia.
Pennsylvania and New Jersey will experience a lesser spectacle, with the moon blocking three-quarters of the sun at its peak, in midafternoon. The sky will darken somewhat, to about the level of early evening. If the weather is clear, the sun will appear as though the moon were taking a giant bite out of it — but be sure to use certified solar-eclipse glasses for protection. Mere sunglasses are not enough. With the help of two astronomers, Villanova University's Ed Guinan and Rider University's John Bochanski, we provide a crash course in all things eclipse:
When is it happening here? In Philadelphia, the moon starts to edge in front of the sun at 1:21 p.m. Monday, obscuring more and more of it every minute until reaching a maximum of 75 percent coverage at 2:44 p.m. Then the moon continues steadily on its path, revealing the full sun once again at 4:01 p.m.
Why is this such a rare event? It might seem that our view of the sun would be blocked every time the moon travels around the Earth, once every 27 days. Not so. That is because the moon's path around the Earth is tilted by about 5 degrees with respect to the Earth's orbit around the sun. We perceive a solar eclipse only when the three celestial bodies are lined up just right.
What is a partial eclipse? Most solar eclipses are partial, with that bite-out-of-the-sun appearance no matter where they are viewed from the Earth's surface — again, because it is rare for the three bodies to line up perfectly. A handful of solar eclipses are "annular," meaning that the moon lies along the direct path between the earth and sun, but is too far away to block the sun completely — leaving an apparent "ring of fire" around the moon's outer edge. To get an idea why this happens, hold your thumb near your eye to eclipse, or block, your entire view of a skyscraper. Then extend the thumb to arm's length, and it will no longer cover the whole building.
How often can we see a total eclipse? Total solar eclipses, such as the one on Monday, occur somewhere on Earth every year or two, but most of them are visible only from the ocean. The last time a total eclipse could be seen in the United States was 1979, along a path from Oregon to North Dakota. The next one that will be visible from the U.S. occurs in 2024, along a path that includes Erie.
When does the total solar eclipse begin on Monday? NASA says the lunar shadow will start to creep across the Oregon coast at 9:05 a.m. Pacific time, but the show really begins at 10:15, when the view of the sun from that location will be 100 percent obscured for nearly two minutes. This period of "totality" lasts even longer elsewhere, with the point of greatest duration, 2 minutes 43 seconds, in Carbondale, Ill. The last U.S. location to see the total eclipse is Charleston, S.C., with the phenomenon ending at 2:48 p.m. Philadelphia time.
Why does the eclipse travel from west to east? The earth spins on its axis from west to east, so it might at first seem that the moon's shadow during an eclipse would slide backward in the opposite direction. But the moon is traveling from west to east as well, at a much higher speed than the Earth is spinning beneath it. The net effect: the lunar shadow moves in an easterly direction.
How can I see it safely? Amid high demand, some vendors of solar glasses are out of stock. Another option: NASA is streaming the event live at eclipse2017.nasa.gov, transmitting the signal from dozens of camera-equipped balloons that will be launched miles into the sky. That will include views of the sun as well as footage of the moon's shadow racing across the Earth's surface, at a top speed of more than 2,200 mph. Among the groups sending up a balloon is a team of engineering students from Temple University. The team plans to launch its balloon from a site west of Bowling Green, Ky., along the path of the total eclipse, said faculty adviser John Helferty, an associate professor of electrical and computer engineering. The students have devised a system to aim their airborne video camera from a laptop computer on the ground, he said. "We basically just use the left, right, up, and down keys to move the camera around," Helferty said.
Will the sun light up the "dark side" of the moon? With apologies to Pink Floyd, there is no dark side of the moon. There is a "back" side that we never see, but it is struck by the sun's rays just as often as the front, as it will be during the eclipse.
What's an umbraphile? Solar eclipse fanatics are known to travel the world in search of the best views. Villanova's Guinan, a professor of astrophysics and planetary science, confesses to being a moderate umbraphile, having seen eight total solar eclipses. (Umbra means "shadow" in Latin.) Sometimes plans go awry. In 1995, he and a group of astronomers traveled to Thailand to watch an eclipse from the grounds of an ancient temple, but the road was jammed with traffic. They ended up pulling off to witness the eclipse from an asphalt-making plant. "We wanted to see it in a beautiful setting, and here I saw it at an industrial site," he said.
Will eclipses keep happening forever? The moon is drifting 1.5 inches farther away from the Earth each year. In about 600 million years, it will no longer appear big enough to obscure the sun, should anyone still be around to view it. All solar eclipses from then on would be of the annular variety, with a ring of fire visible around the moon's dark silhouette