In a week, one of the summer’s most hotly anticipated events will arrive, as Americans everywhere finalize plans to travel to a 70 mile wide strip of land stretching from Oregon to South Carolina.
“It’s the return of the corona after one hundred years,” said Jim Lattis, director and co-founder of the UW Space Place. The UW Space Place is the public outreach center for the Astronomy Department at UW-Madison.
Next Monday on August 21st, a total solar eclipse will stretch across the entire continental US. While the last total solar eclipse to be seen from the US was in 1979, a total solar eclipse stretching from coast to coast like the upcoming one in August hasn’t been seen in the US since 1918.
Last Tuesday, Jim Lattis gave a talk to community members and eclipse enthusiasts at the UW Space Place about the upcoming solar eclipse. The room was packed to the brim with people of all ages, and the solar glasses specially made for observing the eclipse sold out quickly, a testament to how the eclipse has captivated the public.
A total solar eclipse, Lattis explained, is when the moon completely blocks the sun. When the moon blocks the sun completely, it casts a relatively small shadow on the earth called the umbra. As the earth and moon orbit, the umbra will travel along a path called the path of totality, which is the only area where the total solar eclipse will be visible. Total solar eclipses can last as little as a few seconds to more than seven minutes. This eclipse will last a maximum of two minutes and forty seconds.
During the total eclipse, the sun’s corona will burst out from behind the moon, and the sky will darken enough that even stars may be seen. The sun’s corona is an area of hot research for astronomers. The sun’s corona is made up of streams of plasma stretching millions of miles into space, pervading throughout the solar system. In fact, the corona is what causes the dazzling displays of northern lights on Earth. The corona is several million degrees hot – far hotter than the surface of the sun, which is only 6000 degrees hot. This unusual phenomenon has scientists still debating how the corona is heated to such high temperatures while the sun’s surface remains so comparatively cool.
As one moves out of the umbra’s path and into the penumbra, which is a larger shadow caused by partial obscuring of light, the moon and sun no longer appear to be perfectly aligned. This is what we call a partial solar eclipse, where the moon only partially covers the sun. While the umbra will not cross through Wisconsin, the penumbra will cover the entirety of Wisconsin, allowing us to observe a partial eclipse.
In Madison, the moon will obscure about eighty five percent of the sun’s disk. The sun will appear as a narrow crescent at peak obscurity, although it must be observed through solar glasses unlike a total eclipse, as the sun will still be extremely harmful to the eyes. While the changes in the sky might not be as dramatic as a total solar eclipse, there is still a host of unique phenomena that can be easily seen
One such phenomenon that Lattis shared was the dappling of light. Normally, when the sun shines through the leaves of a tree, it will leave a shadow dappled with light. The dappled light is always circular because the sun’s disk is also circular. However, during the partial eclipse, dappled light will look like mini crescent moons dotting the ground, because the partial eclipse will have turned the sun’s disk into a crescent shape.
Another unique phenomenon is “shadow bands.” If a large white sheet is placed on the ground during the partial eclipse, bands of alternating light and shadow will be projected onto it. This variation between light and dark is caused by our atmosphere interfering with and breaking up the sun’s light. It’s essentially the same reason why stars twinkle, with the exception that the light from the partial eclipse is bright enough for us to noticeably observe the atmospheric variations.
The eclipse will begin at 11:50 a.m. in Madison and end at 2:38 p.m., with peak obscurity occurring at 1:15 p.m.
