In Part 1A of the eclipse drama, we talked about the Moon. Now for the Sun and Earth.
First The Sun.
The gigantic sphere of plasma* (2.7 million miles in circumference, weighing 330,000 Earth masses) that is our Sun is orbiting the center of the Milky Way Galaxy at about 514,200 miles per hour, a speed which takes it once around the galactic center every 230 million years. (Since the extinction of the dinosaurs, it’s made it a little more than a quarter of the way around.) It helpfully drags us around with it, enabling us to observe intergalactic space in many directions over time. Physically, it’s about as normal a star as you can find in the local neighborhood (less than 1,000 light years away). Its relative “normality” is nice for astrophysicists, who can learn a lot about other stars from observing this one close at hand. Despite being sort-of normal, the Sun has a lot of electromagnetic storms and bursts of plasma that can do monstrously scary stuff to us (discussed in another of my posts here). Yet, it is tame relative to many other stars, whose volatility may make the evolution of life around them a crapshoot.
We’ll talk about the speed of the galaxy itself relative to other galaxies in a later post (promise: before we’ve completely orbited the galactic center).
The Sun is so massive that the entire Solar System’s “center of gravity” is within the Sun or sometimes a bit outside, depending on the positions of the planets at any particular point in time. It’s technically called the “barycenter,” the center of mass of all bodies in the Solar System combined, and it moves around, mostly within the Sun, in concert with the movement of the planets, particularly the “gas giants” Jupiter and Saturn. When you think about the enormous distances between the Sun and the planets, this phenomenon staggers the imagination (mine at least). Jupiter is 483,000,000 miles from the Sun, Saturn almost twice as far. See this. (Scroll down half a page in this one.)
I gave the Sun second billing in our eclipsian drama, behind the more geometrically active Moon, but ahead of our local heroine, Earth—why so? Because the Sun is too huge to ignore, and (duh) because it is, equally with the Moon, the sine qua non of the eclipse. From the point of view of the Sun itself, all this fuss about the eclipse on the speck of rock we call Earth is making a mountain out of a molehill. (Sorry for the commonplace. If you have a better metaphor, let me know. There aren’t any mountains or molehills on the Sun. The Sun’s gravity would suck them down into the electromagnetic tempest on its surface in a microsecond.)
Final note on the role of the Sun: its enormous gravity could be responsible for inducing the alignments that produce total eclipses, despite the complications of orbits and inclinations discussed below. Even for the Sun, that’s no mean feat. And, the Sun had little to do with the magic 400 to 1 distance ratio
Finally, The Earth:
I call the Earth a player in the eclipse drama, rather than just seating for the audience, because it, too, strongly affects the geometry that makes total eclipses possible, and also makes them more rare than you would think at first blush.
The raw data for Earth: 24,900 miles in circumference, with a mass equal to about 25 thousand billion billion Dodge Ram pickups. (6000000000000000000000000 kilograms if you want to be technical). Since the mass of the Earth is 81 times that of the moon, we think of the moon orbiting the Earth rather than the other way around. Indeed, the barycenter of the Earth-Moon system is inside the Earth—similarly to the way the barycenter of our Solar System is (usually) inside the Sun.
From the point of view of a distant spectator, say the Sun, the two might just as well be orbiting each other. From the Sun you will see both bodies wobbling in their faraway waltz as they tug at each other unequally, with the Moon’s orbit around the Earth wobbling a teensy bit, and the Earth’s orbit “around” the Moon, wobbling an eentsy-teensy bit.
This wobbling, tiny though it is, is one of the factors that make total eclipses as rare as they are. Even if it is just a very minor factor.
The principal factor determining the frequency of solar eclipses is the shape of the orbits of the two bodies (Earth to Sun, Moon to Earth), and the inclination (tilt) of their axes of rotation to the planes of their orbits (Earth around Sun, Moon around Earth).
First, total eclipses are rarer than partial and annular eclipses (explained here. ) because the Moon traces an elliptical orbit around the Earth, so its distance from us varies from the magic ratio of (closely) 400 to 1 much of the time.
But there’s more. It’s not just the varying distance to the Moon, it’s the fact that the plane of the Moon’s orbit is tilted with respect to the rotation of the Earth (and also with respect to the plane of the Earth/Moon System’s with the Sun). Complicating things further is the tilt of the axis of Earth with respect to the plane of its orbit around the sun. You are probably familiar with the latter as an explanation for the seasons.
Who would have thought that the Earth-Moon system could be so complicated? It’s almost as bad as the U.S. health care system. 🙂
Visualizing the eclipse thingy as having the Earth, and Moon always revolving in the same plane, with the moon coming between the two every 28 days, poses a puzzle as to why there aren’t more solar eclipses. This visualization is natural but almost completely wrong. If you have a six-year-old child or grandchild who saw the demonstration of the eclipse in school with the three bodies in question lying on a flat table top, ask them how they picture the scene when there is NOT an eclipse. My hunch is they picture the Earth and Moon rolling around in different positions on the tabletop, with the Moon square in front of the Sun every 28 days. That’s sure how I pictured it when I was six or seven, when they had a notable partial eclipse in my neck of the woods. In fact, almost never are the three bodies on the same plane, much less the same plane with the Moon directly between the other two.
In a nutshell: the inclination of the Moon’s orbital plane to the plane of Earth’s rotation, and the inclination of the Earth’s axis in respect to its orbit around the Sun, need to be closely enough matched for the moon to line up with the other two bodies, such that its night-dark shadow sweeps across the Earth’s surface at all. Change the tilt of the Moon’s orbit, or the tilt of Earth’s axis in respect to its orbit around the Sun, and you might have a total eclipse every thousand years rather than the 18 months or so that we see. (OK, with just the right changes you would have a total or annular eclipse every 28 days, but imagine the odds against that (they are, not to be cute, astronomical)).
As I mentioned above, the Sun’s enormous gravitational field might have induced the relatively frequent perfect alignments of the three bodies. Still . . . It’s rather spooky, almost enough to make one believe in Intelligent Design.** Despite all the zillions of stars and planets in the Universe, this kind of coincidence—a total eclipse—must be rare, and still rarer for it to occur where there are intelligent observers to witness it.
In the absence of total solar eclipses, those hypothetical intelligent observers will be deprived of a lot of knowledge we’ve acquired about our Sun and other distant stars thanks to total eclipses, which would slow their understanding of astrophysics, and star and galaxy formation.
That could explain why they aren’t already zooming around overhead in comical-looking vehicles, occasionally snatching up humans—alleged by some but believed by few–for gruesome experiments.
Cosmic connection
As the eclipse approached last summer, astronomers and eclipse fans were queried on their thoughts about the significance of eclipses as well as on the mechanics of the event as discussed above. One response, from an astronomer who had witnessed nearly thirty total eclipses over years of global travel, struck a deep chord. He remarked that reactions of eclipse observers vary wildly, and said, “I have even seen people fall to their knees and weep.”
I don’t know what I’d do if I ever witnessed totality—get as emotional as I am now, picturing the person he described on their knees, weeping, in a strange darkness, under stars shining in the middle of the day? I doubt it. I wouldn’t be brave enough. Think of how you’d look! After all, the eclipses are merely a matter of physics and geometry. Very, very interesting. . . but with emotional power that goes soul deep?
I think I get it. What happens—what happened for millions of people on August 21st—is a sense of cosmic connection. The brief arrangement, of three celestial bodies in a nearly impossible coincidence, with stars visible in a sky turned suddenly to blackness, opens a window into the mystery of all that is and our place in it—the existence of unthinkably huge objects in unthinkably vast space, some unthinkably hot and others unthinkably cold, moving at unthinkably enormous speeds, the unsolvable riddle of why there is something rather than nothing. . . .
Scale. Think about it. More on the Most Amazing Year in Space to come.
==================== footnotes follow ===================
* Plasma (the material of suns, not your blood plasma) is a fourth state of matter, similar to a gas, but with atoms and molecules so highly ionized by temperature that they create, and are shaped by, supremely powerful electromagnetic fields. Since a star like our sun consists mainly of hydrogen—the simplest element made of up one proton and one electron–its plasma is a soup of protons and electrons independently bouncing around willy-nilly, a tempest of unbound ions. Normal hydrogen gas comes in molecules of two hydrogen atoms bound together, but in a star’s plasma these bonds are so ephemeral they have no effect on the plasma’s behavior. Plasmas are a volatile, violent bunch, with fields that twist and stretch and curl back on each other.
Eruptions of plasma can result in scary Coronal Mass Ejections, the dangers of which I have discussed in another post to be found at uncertainty and CMEs
The unruliness of plasmas explains the difficulties we’re having managing fusion as an energy source here on Earth. We have produced electricity through (hot) fusion, but at a net loss. So far we can control it only by using giant magnets that soak up more energy than the fusion generates. To learn more about attempts with fusion to generate electricity, search on tokamak.
** Don’t.