Eclipses occur throughout the Solar System, and indeed wherever there are planets that have satellites like our Moon orbiting around other stars. But on Earth, the apparent size of the Moon is about the same as the apparent size of the Sun. The Sun is obviously much more mega huge than the Moon, but it is also much farther away. By some cosmic coincidence, both of these ratios are 400, resulting in a Sun and Moon that appear the same size, giving them roughly equal importance in our mythic imagination.
Does this coincidence require a scientific explanation? Is there some physical process which can explain why these two numbers are the same, or is it down to statistics?
Let me give another example. The Moon both orbits around the Earth and rotates about its own polar axis, just like the Earth both revolves around the Sun once per year and rotates once per day. But for the Moon, these periods are the same: it rotates at the same speed which it orbits the earth, with the result that one side of the Moon always faces Earth, and we can never see its far side from the Earth's surface. When I first learned about this "coincidence" I thought, what are the chances?
But there is a physical explanation for why the orbital speed and rotational speed of the Moon are the same, and it's called tidal locking. There are other tidally locked systems in our Solar System, and once we figured out the subtler implications of Newtonian gravity and celestial mechanics, science could provide an explanation for this apparent coincidence. (1 point for Science!)
Another example involves the Universe itself. As the Universe evolves and expands, the densities of matter and radiation in the Universe decrease, but the density of radiation decreases faster. In the early Universe, the density of radiation was higher than matter, but most of the time the Universe has been matter dominated -- that is, until very recently, when the Universe began accelerating. We don't know what the "dark energy" causing this acceleration is, but the simplest explanation gives it a density that is constant over time (unlike matter and radiation whose densities decrease). But why is this period of acceleration happening so close to now, when we are able to observe it? This is called the cosmic coincidence problem, and in general any successful theory of dark energy is expected to explain it, though many theories ignore it. (1 point for WTF Is Going On, Nature?)
So, what is it about these coincidences that they require physical explanation? If the phenomenon of tidal locking didn't result from our theories of gravity, would we still be searching for an explanation of why the orbital period and rotational periods of some satellites are the same? What if only our Earth-Moon system had this property? (I realize I am mostly asking questions here, not answering, but I am doing philosophy. Abandon Certainty with me for a bit.)
I have the feeling that if only the Moon had the same orbital and rotational period in our Solar System, and if gravity offered no scientific explanation, we would still be looking for some way to explain the coincidence, either scientifically or statistically. It would be too special, and too human-centric. "Because humans are special" lacks explanatory power, and since the Copernican revolution, any scientific explanation which puts humans in a special place is met with great skepticism. This is the heart of why the cosmic coincidence problem is troubling, and why some scientists would rather believe that multiple universes exist than that we occupy a special place in our own.
The eclipse coincidence -- that the Sun and Moon have the same angular size from Earth -- strikes me as similarly human-centric. But there is no physical explanation, and scientists aren't actively searching for one. I say scientists because people are certainly putting explanations out there on the internet (you see, it was necessary for the development of consciousness...), but as long as two unrelated things have happened at the same time, people have come up with explanations. Human brains are great at finding patterns, even when they don't exist.
That is not to say that scientists don't think about it. I mean, the Sun and the Moon have the same apparent size, isn't that cool?? But it's generally accepted that there is no underlying physical explanation, that the coincidence is purely statistical. Unlike with tidal locking, other celestial bodies in our Solar System don't have this apparent size coincidence, so any physical explanation would have to be unique to the Earth-Moon system. Further, the Moon is actually receding away from us (at the whopping speed of 4 centimeters per year), so in the past it appeared bigger than the Sun, and in the future it will appear smaller. Even today, the Moon actually changes its apparent size as it moves through its elliptical orbit. Given the billions of stars in our Galaxy, and the thousands of planets in other solar systems (just that we're aware of), it's likely that this apparent size coincidence is not unique.
Where does that leave our understanding of the explanation of coincidences? I have presented three examples:
- The Sun and Moon are observed to have the same apparent size viewed from the Earth. This can be explained statistically: given enough planetary systems, such a coincidence can be expected.
- The Moon has the same orbital and rotational period. There are many examples of this type of coincidence in our Solar System, and it can be shown to follow from laws of nature.
- The acceleration of the expansion of the Universe started relatively recently in its cosmic history, which seems to place humans in a special time. Both scientific and statistical explanations are being sought: either the coincidence will be explained by a theory of dark energy, or there are so many universes that such a coincidence can be expected in one of them (ours).
More importantly, what is the probability that I will wake up in time for the eclipse tomorrow AND that there will be clear skies, given that I live in England AND I hate mornings? Such a confluence of events would be rare indeed.
