In the Dark

I’m told that there was a partial eclipse of the Sun visible from the UK this morning, although it was so cloudy here in Cardiff that I wouldn’t have seen anything even if I had bothered to get up in time to observe it. For more details of the event and pictures from people who managed to see it, see here. There’s also a nice article on the BBC website. The BBC are coordinating three days of programmes alongside a host of other events called Stargazing Live presumably timed to coincide with this morning’s eclipse. It’s taking a chance to do live broadcasts about astronomy given the British weather, but I hope they are successful in generating interest especially among the young.

As a spectacle a partial solar eclipse is pretty exciting – as long as it’s not cloudy – but even a full view of one can’t really be compared with the awesome event that is a total eclipse. I’m lucky enough to have observed one and I can tell you it was truly awe-inspiring.

If you think about it, though, it’s a very strange thing that such a thing is possible at all. In a total eclipse, the Moon passes between the Earth and the Sun in such a way that it exactly covers the Solar disk. In order for this to happen the apparent angular size of the Moon (as seen from Earth) has to be almost exactly the same as that of the Sun (as seen from Earth). This involves a strange coincidence: the Moon is small (about 1740 km in radius) but very close to the Earth in astronomical terms (about 400,000 km away). The Sun, on the other hand, is both enormously large (radius 700,000 km) and enormously distant (approx. 150,000,000 km).  The ratio of radius to distance from Earth of these objects is almost identical at the point of a a total eclipse, so the apparent disk of the Moon almost exactly fits over that of the Sun. Why is this so?

The simple answer is that it is just a coincidence. There seems no particular physical reason why the geometry of the Earth-Moon-Sun system should have turned out this way. Moreover, the system is not static. The tides raised by the Moon on the Earth lead to frictional heating and a loss of orbital energy. The Moon’s orbit  is therefore moving slowly outwards from the Earth. I’m not going to tell you exactly how quickly this happens, as it is one of the questions I set my students in the module Astrophysical Concepts I’ll be starting in a few weeks, but eventually the Earth-Moon distance will be too large for total eclipses of the Sun by the Moon to be possible on Earth, although partial and annular eclipses may still be possible.

It seems therefore that we just happen to be living at the right place at the right time to see total eclipses. Perhaps there are other inhabited moonless planets whose inhabitants will never see one. Future inhabitants of Earth will have to content themselves with watching eclipse clips on Youtube.

Things may be more complicated than this though. I’ve heard it argued that the existence of a moon reasonably close to the Earth may have helped the evolution of terrestrial life. The argument – as far as I understand it – is that life presumably began in the oceans, then amphibious forms evolved in tidal margins of some sort wherein conditions favoured both aquatic and land-dwelling creatures. Only then did life fully emerge from the seas and begin to live on land. If it is the case that the existence of significant tides is necessary for life to complete the transition from oceans to solid ground, then maybe the Moon played a key role in the evolution of dinosaurs, mammals, and even ourselves.

I’m not sure I’m convinced of this argument because, although the Moon is the dominant source of the Earth’s tides, it is not overwhelmingly so. The effect of the Sun is also considerable, only a factor of three smaller than the Moon. So maybe the Sun could have done the job on its own. I don’t know.

That’s not really the point of this post, however. What I wanted to comment on is that astronomers basically don’t question the interpretation of the occurence of total eclipses as simply a coincidence. Eclipses just are. There are no doubt many other planets where they aren’t. We’re special in that we live somewhere where something apparently unlikely happens. But this isn’t important because eclipses aren’t really all that significant in cosmic terms, other than that the law of physics allow them.

On the other hand astronomers (and many other people) do make a big deal of the fact that life exists in the Universe. Given what  we know about fundamental physics and biology – which admittedly isn’t very much – this also seems unlikely. Perhaps there are many other worlds without life, so the Earth is special once again. Others argue that the existence of life is so unlikely that special provision must have been made to make it possible.

Before I find myself falling into the black hole marked “Anthropic Principle” let me just say that I don’t see the existence of life (including human life) as being of any greater significance than that of a total eclipse. Both phenomena are (subjectively) interesting to humans, both are contingent on particular circumstances, and both will no doubt cease to occur at some point in perhaps not-too-distant the future. Neither tells us much about the true nature of the Universe.

Let’s face it. We’re just not significant.