On August 21, we Americans get to see a complete photo voltaic eclipse. As I discussed in a previous piece, we will see photo voltaic eclipses solely as a result of our planet, our Moon, and our Sun generally come collectively in a straight line in area. When the Moon passes between the Sun and Earth, these within the Moon’s shadow see an eclipse.
But the story doesn’t finish there. A uncommon alignment of occasions permits Earthlings to witness not simply photo voltaic eclipses, however what we would name excellent photo voltaic eclipses. Our Moon simply barely covers the Sun’s vivid photosphere. Such an eclipse depends upon simply the best sizes, shapes, and relative distances of the Sun, Moon, and Earth.
There’s no legislation of physics that dictates this structure. There are 65 main moons in our Solar System, and plenty of smaller ones. But only we enjoy excellent photo voltaic eclipses. If there have been Martians or Uranians, they wouldn’t see such eclipses.
The Moon is about 400 instances smaller than the Sun. But the Moon is additionally about 400 instances nearer to the Earth than is the Sun. As a consequence, the dimensions of the Moon on our sky matches the dimensions of the Sun. And since they seem as spherical disks, they match in each measurement and form.
What’s extra, the tight match between Moon and Sun solely occurs throughout a slim window of Earth’s historical past. The Moon is slowly shifting away from Earth — at roughly the same speed that fingernails grow.
In transient, the most effective time and place to view whole photo voltaic eclipses in our Solar System is simply when and the place there are observers to see them.
Let that sink in.
What Life Needs
Most astronomers chalk this as much as coincidence. But there’s extra to the story.
As it occurs, this exact association of Earth, Moon, and Sun helps maintain life on Earth. Let me clarify.
For lots of reasons, a planet virtually absolutely needs liquid water on its floor to host complicated life. Most locations within the Solar System, and within the universe, are both method too scorching or method too chilly. To be “habitable,” a planet must be within the “Goldilocks Zone” round its star: the place it’s not too scorching and never too chilly. Think of this zone as a slim, almost round ring of area round a star.
The Earth is, after all, safely contained in the zone. But how slim is it? To determine that out, we want solely take a look at our closest planetary neighbors. Mars is past the outer fringe of this zone. It’s chilly and lifeless. Venus is exterior the internal fringe of the zone. It’s over 850 levels F on its floor. Apparently, there’s not plenty of wiggle room. If Goldilocks have been a planet, she can be a fussy little princess.
The Moon additionally performs a job in preserving Earth liveable. A big, well-placed moon stabilizes the lean of the axis of its host planet. That offers the planet a extra secure local weather. The Moon additionally contributes to Earth’s ocean tides, which helps combine vitamins from the land to the oceans. The two moons round Mars — in all probability captured asteroids — are a lot too small to stabilize its rotation axis. As a consequence, Mars wobbles on its axis excess of the Earth does. That’s unhealthy information for Martians.