by Chris Peterson » Sun Aug 21, 2016 2:56 pm
Jim Leff wrote:I was a kid during the total eclipse of March 7, 1970, and remember it lasting over three minutes. I was taught at the time that "total" meant total. So how can this total eclipse be a minute shorter?
The period of totality (along the center line) depends on many factors, such as where the Earth is in its orbit around the Sun (our orbit is not circular), where the Moon is in its around the Earth (also not circular), the angle the shadow line intersects the Earth's surface, and probably more. The geometry is complex.
At the simplest, just consider that that the distance from the Earth to the Moon changes, and therefore the apparent size of the Moon changes (there have been a number of APODs illustrating this). When the Moon appears larger, it will obscure the Sun for a longer time than when it appears smaller. When the Moon is at its farthest from us, it doesn't fully cover the Sun at all, and even a "total" eclipse won't be total, as in this annular "total" eclipse I shot through the clouds from San Diego in 1992.
[quote="Jim Leff"]I was a kid during the total eclipse of March 7, 1970, and remember it lasting over three minutes. I was taught at the time that "total" meant total. So how can this total eclipse be a minute shorter?[/quote]
The period of totality (along the center line) depends on many factors, such as where the Earth is in its orbit around the Sun (our orbit is not circular), where the Moon is in its around the Earth (also not circular), the angle the shadow line intersects the Earth's surface, and probably more. The geometry is complex.
At the simplest, just consider that that the distance from the Earth to the Moon changes, and therefore the apparent size of the Moon changes (there have been a number of APODs illustrating this). When the Moon appears larger, it will obscure the Sun for a longer time than when it appears smaller. When the Moon is at its farthest from us, it doesn't fully cover the Sun at all, and even a "total" eclipse won't be total, as in this annular "total" eclipse I shot through the clouds from San Diego in 1992.
[img]http://www.cloudbait.com/gallery/solar/annular.jpg[/img]