A Clockwork Sky

Tonight, Americans have a treat to look forward to: a total eclipse of the Moon. Sadly it won’t be visible from the UK (except perhaps very briefly near sunrise), otherwise I would certainly be venturing out to watch it.*

I have seen a total lunar eclipse before, back in the ’90s. That was remarkable, watching the shadow bite into the disc of the Moon. It started as a dark notch to one side, which then swelled. At first it looked black, due to the contrast with the brighter, still-sublit Moonscape. Then the shadow swept across the disk and the entirety of the satellite was submerged. At this point, the contrast was even and the colour changed dramatically: the Moon became a remarkable, coppery red.

Eclipses of the Moon were critical to early astronomy. There’s a surprising amount of basic science that you can do just by watching the darkening. First of all, they let us dispose of a myth: it’s called the Flat Earth. No educated person ever really believed this, except perhaps in the true distant recesses of prehistory. The Greek philosophers considered the sphericality of the Earth to be proved beyond reasonable doubt.

Lunar eclipses were one of the giveaways.

The reasoning is simple enough. During a lunar eclipse, whether annular or total, the Earth’s shadow is curved. If the Earth were a flat disc, then it would either cast a knife-sharp, linear shadow, or it would cast some sort of bent annulus. (In this geometry, projection effects – the angle the flat Earth makes with respect to the Sun and Moon – become critical to the outcome.) But we simply do not see this in practise. This is only at all sensible in a spherical-Earth context! No other arrangement can generate this outcome.

Also, comparing lunar and solar eclipses gave further clues to the relationship between Sun, Earth and Moon.

Solar eclipses are short. They’re impressive, but totality cannot last longer than a matter of minutes, and some of them last mere seconds. For lunar eclipses, however, totality can last into the hours. Also, we see the Moon eclipse the Sun, but never the Sun eclipse the Moon. From these facts, we can deduce three logical conclusions:

1) The Moon is closer than the Sun (or we’d see the Sun eclipse the Moon sometimes);
2) The Earth is larger than the Moon (because it takes longer for the Moon to track through the Earth’s shadow than the Moon can hide the Sun for).
3) To appear the same size at greater distance – i.e., be totally-eclipsable – then the Sun must be bigger than the Moon.

And the striking thing is, we can do these straight away, without any need for elaborate mathematics or expensive equipment. For the earlier history of astronomy – and indeed science itself – eclipses were of central importance.

But there’s more.

You see, we know some things in the modern age. One of them is that the Earth is a planet, orbiting with the Sun around their common centre of mass.** We also know that planets, the Earth included, move on elliptical orbits. That means their orbits are ovular rather than perfectly-circular. The degree of ‘ovular-ness’, technically called eccentricity, varies between planets. Luckily for us, our Earth has the third-lowest eccentricity in the Solar System. (If had a more eccentric orbit, we’d be subjected to substantial temperature swings between perihelion and aphelion.) Other planets have rather more. At aphelion, Mars is 249.2 million Km from the Sun; at perihelion this falls to 206.7 million Km, a rather staggering change of about 43 million kilometres.***

Lunar and solar eclipses provide some hint of eccentricity.

You see, not all eclipses are equal. On some occasions, the Moon does not wholly occlude the Sun. Instead, you have an annular eclipse – a donut-like ring of Sun glares from around the edges of the impertinent satellite, as if the Sun is peeved at the Moon for daring to interrupt its shining! These annular eclipses happen when either the Earth is closest to the Sun, and thus the Sun looks a bit bigger in our skies, or the Moon is toward the far end of its orbit and thus looks smaller. The annularity issue caused problems for the Ptolemaic model of the solar system. Ptolemy partly resolved the issue by introducing epicycles and deferents, but it was a kludgey and artificial solution. Even at the time, no-one really liked the fix. (Tycho Brahe’s observations of Mars in the 1500s finally exploded the Ptolemaic model once and for all.)

But the story of eclipses doesn’t end there.

After the advent of the telescope, the Galilean Moons were discovered in orbit around Jupiter. One clear piece of evidence for their association was the periodic eclipses they created on Jupiter. You can see these even through small telescopes. Also, given that Io, Europa, Callisto and Ganymede all have orbital periods on the order of a day to a few days, these eclipses could be seen regularly.

A final twist in the eclipse story comes with the Danish astronomer Ole Romer, who conducted observations of Jovian eclipse cycles in the late 17th Century. Romer noticed something odd. Sometimes the eclipses were early, and sometimes they were late. This was rather weird – the heavens were notoriously accurate in their timekeeping! Further investigation revealed an odd correlation with the orbital motion of the Earth: eclipses were early when the Earth was ‘closing’ on Jupiter and longer when it was moving away.

This led Romer to a revolutionary conclusion: light travels at a finite speed. The eclipses were late when the distance between planets was greater, and they were early when the distance was smaller.

This was incendiary stuff in its day. Up until that point, it had been firmly believed that light travelled at infinite speed. (This had been apparently supported by an experiment by Galileo Gallilei – actually, their result was due to a flaw in the experimental design, namely the fact that light is faster than the conduction speed of human nerves, and thus they couldn’t react to their changing lanterns fast enough. To be fair to the observers, they did what they could and they reported their results honestly, so they were merely wrong. And we don’t have a problem with wrong – it’s fraudulence that isn’t on!)

Christiaan Huygens took Romer’s observations and got a number out: 220,000 km/s. (The modern value is 299,792,458.0 km/s precisely.) Light was stupdendously fast, but not infinitely so. The Jovian eclipses had spoken, and there was no gainsaying their evidence! This speed caused some confusion – why should something be precisely that speed?

Today, we know the answer.

It’s because of what light is, physically-speaking. Light is electromagnetic radiation. As its name suggests, electromagnetic radiation consists of two things – a magnetic wave and an electric one. We know that electric fields can induce magnetic ones, and vice versa (one might call this the power station principle – without it, we wouldn’t have electricity). Anyway, if a very special set of conditions are met, it turns out that the change in a free-standing magnetic wave can induce and electric field around it, and the changes in the electric field induce a magnetic one, which in turn induces and electric wave … and the results can propagate indefinitely. This happens when the electromagnetic wave is moving at a speed of 299,972,458 km/s. This is why light travels at that speed! It has to, or else it couldn’t exist.

And the first hint of this key principle of electromagnetism was suggested to us by eclipses. Truly, they deserve their powerful reputation. If you have a chance to see one, I highly recommend that you take it! Just think of the vast legacy of knowledge that those blood-red moons have bequeathed to us. It is truly amazing, perhaps as amazing as the event itself.

*And I say that in full knowledge that I live not that far from from Amersham in Buckinghamshire, which last nitght shattered the Home Counties temperature record: a staggering -19.6 C. It wasn’t that much warmer here in Hertfordshire. I had condensation on the ceiling – the ceiling! – earlier.
**This is the rigorous version the lie-to-children that the Earth orbits the Sun – although this will do as a first-order approximation, as the Sun outweighs the Earth by a factor of around a million. Consequently, the centre of mass is around a million times closer to the Sun than to the Earth, i.e. it is under the Sun’s surface.
***Although eccentricity and accompanying changes in temperature are not a great factor in Earth’s annual weather, they are a substantial climatic effect on Mars.


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