It’s now less than a month before the August 21 total solar eclipse. Many of us have booked rooms or campsites and will drive hundreds of miles to reach the path of totality, where the moon will cover 100% of the sun. On a map, it looks like a black ribbon just 73 miles wide but stretching almost 2,500 miles from Oregon to South Carolina. For a few minutes at midday, daylight will fade and be replaced with a hybrid twilight that resembles a bright, full-moon night but with a deep blue sky and brighter pastel pinks in the distance above the horizon. I bet you can hardly wait to get under the moon’s shadow.
If you’ve picked a site out in the wide-open spaces with a 360° view, you’ll not only sense the eclipse shadow’s enormity but also its boundaries. If there are landmarks beyond the path of totality, a mountain for instance, some sun still shines there, and you’ll easily see this from where you’re standing.
The moon’s shadow is shaped like a long, tapering cone that extends some 235,000 miles to the ground during a total eclipse. That’s a few thousand miles less than the average distance of the moon to the Earth, but it makes sense when you remember that the moon has to be closer to the Earth than average for its shadow to touch.
This time-lapse was compiled from true color photos taken by the Japanese weather satellite, Himawari-8, during the March 9, 2016 total solar eclipse. It shows the moon’s shadow sweeping over Indonesia.
Throughout the eclipse, the moon is moving along its orbit at some 2,280 mph (1.02 km/sec), so the shadow it casts moves rapidly along the ground though more slowly than its orbital speed because the Earth’s rotation keeps trying to play catch-up. Our planet rotates in the same easterly direction as the moon orbits at a rate that varies by latitude, fastest at the equator and slowest at the poles. At 40° north latitude (central U.S.), Earth spins at 795 mph (1,279 km/hr), so if we subtract 795 from 2,280, we get 1,485 mph (2,390 km/hr). That’s how fast the shadow will be traveling at that latitude.
We all know that Earth’s a sphere. Not a perfect one but close enough. When the moon’s shadow first encounters the planet, it does so at such an oblique angle, that it moves much faster across the ground. The shadow gradually slows as it “climbs uphill” across the Earth’s curvature and then hurries away on the other side of the “hill” when it departs the Earth for outer space.
When the shadow first touches Oregon, it will be traveling at 2,240 mph (3,600 kph), then will gradually slow down as it climbs over the Earth’s curve, reaching its slowest speed in Tennessee at 1,323 mph before accelerating to 1,354 mph as it departs South Carolina.
The moon’s umbral shadow crosses 2,496 miles (4,017 km) of the U.S. in just 90.7 minutes at an average speed of 1,651 mph (2,657 km/hr). Combine this with the fact that the moon only barely covers the sun during totality, and you can understand why the total phase of an eclipse doesn’t last long. That’s why some people book special flights in fast-flying, eastbound planes to extend totality from just a few minutes to dozens!
Photos taken from spacecraft not only show the dark, umbral core of the moon’s shadow but also part of the penumbra, where the shadow gradually becomes less opaque until it blends fuzzily into the sunlit Earth-scape. The penumbral shadow isn’t as dark as the umbral because sunlight spills into it, the more sunlight the further you get from the path of totality. That’s why it gradually gets lighter as you move from the center to the edge. In other words, the penumbra defines where the eclipse is partial.
And why the fuzzy edge? For same reason a tree casts a fuzzy shadow. The sun is a disk, not a point of light. Light from one side of the sun can reach areas that are shadowed from the light of the other side of the sun and vice versa. A little sunlight spills into one side of the shadow and the other, lightening and softening its border. Only a point source like Venus can create a sharp shadow.
Since the August eclipse has everything to do with shadows it seems fitting to spend a little time exploring them. Seeing the shadow play from space will help us better understand what to expect on the ground.