The Planets

I am delighted to read The Planets by Andrew Cohen and Brian Cox. After the end of the Cassini, Juno and New Horizons missions is a perfect time for a popular survey of what we know of the solar system, and it is relaxing to know that if Professor Cox says Saturn’s rings are less than a hundred million years old he has good grounds for saying it, unlike many political statements.

I was thinking that it does not affect my life, but that is untrue. If Saturn did not exist, Jupiter would not be orbiting where it is, and Earth might not exist. If humans did not have the curiosity and the engineering and political skills to find these things out, the human race would be significantly different, and if I were not interested in such things I would not be me. Even a minor change such as one more or less 100km moon of Saturn would have a butterfly-effect, perhaps big enough to end life on Earth.

And our understanding is influenced by the culture. As a small child I could recite the names of the planets even as I could count to a hundred, but thought of them as more or less unchanged since the system formed. I remember calling Neptune a gas giant, then an ice giant. The thought of the system as changing, planets moving much closer to the Sun or further away is based on new data, including from exoplanets, but seeing wild change as possible rather than incremental improvement affects and is affected by how we see society, and what is politically possible.

None of it is certain. Theories explain data, but are subject to change, as new data are collected or new possibilities imagined.

I find the whole very beautiful, the images, the striving and achievement, and the sharing.

Cox is a romantic and inspiring writer. He can be sharp: I think one of the reasons why anthropogenic climate change is so difficult for a certain type of person to accept is that atmospheres seem ethereal and tenuous and incapable of trapping enough heat to modify the temperatures on a planet significantly. For such people I suggest a trip to Venus, where they will be squashed and boiled and dissolved on the surface of Earth’s twin.

The character of the scientist requires not only comfort with but attraction to the unknown; an acceptance and delight in the complexity of Nature… the search for certainty is a fool’s errand, and the lesson is to find delight in not knowing while simultaneously committing to extending the domain of the known. That’s the key to science, the key to happiness and the only reasonable response to the existential challenge of existence.

We live in a solar system of wonders, of planets of storms and moons of ice, of landscapes and vistas that stir the imagination and enrich the soul.

Unfortunately Andrew Cohen is not so inspiring, but with such material he rarely fails to fascinate. It could be better edited, though, I spotted some errors and I am no expert.

The idea that a system allowing complex life-forms to evolve might be rare, requiring precise events to happen in the changes of orbits of the planets and even a large moon to hold the angle of rotation fairly constant is hard. It is up to us to sustain life in our galaxy. The engineering triumphs and alien wonders give me hope.

The book takes me back to childhood, to simple wonder at the strangeness of space and the brilliance of the people finding its secrets. I devoured it.

The Equant point

Ptolemy was wrong. The Sun does not go round the Earth. Why did his view dominate our understanding for over 1300 years? Why was it so hard to change our minds?

Through the mediaeval period, Ptolemy superseded Aristotle’s understanding of celestial motion because he was more accurate. His theory could be used to predict where a planet would appear at future times. He was not only wrong about the Sun going round the Earth, but also about the celestial spheres: the planets were fixed to spheres, made of the etherial fifth element quintessence, and nesting within each other, else, how would they not fall to Earth? He was also wrong in holding to an Aristotelian idea, that because the Heavens are perfect, the planets must move in circles, because the circle is perfect.

The planets do not appear to move in perfect circles. They speed up and slow down, which is not perfect. Ptolemy’s answer was the Equant point. The Earth was not thought to be at the centre of the sphere on which the planet moved, but off-set. The equant point was also not at the centre: it was that place from which the planet would appear to move at a constant rate in its circle.

Ptolemy was right that Mars was closest to Earth, then Jupiter, then Saturn, because he decided that the slower moving planets were further away: but he had no reason for deciding that order.

He was a practical scientist, making new instruments for measuring the precise position of the planets more accurately than before. He was a mathematician, devising the mathematical models which enabled astronomers for centuries after to predict where the planets would appear.

Why would astronomers follow this false scheme? Because predicting the courses of the planets was a complex task. The student would learn the accumulated knowledge of humanity in making those predictions, the practical skills of observing and the mathematics, and so would have Ptolemy’s views inculcated. It was how it was. The observations slowly became more accurate with better instruments, diverging from Ptolemy’s mathematics; and it was important to measure the Heavens, because Easter fell on the first Sunday after the first full moon after the vernal Equinox: we must therefore know when the Equinox is.

Copernicus placed the Sun at the centre, but retained the Greek idea that orbits were circular. Tycho Brahe had the Sun moving round the Earth, but Mercury and Venus moving round the Sun: the spheres, then, could not be, as they would be moving through each other. Johannes Kepler theorised that the planets moved in ellipses rather than circles, and Newton calculated how gravity affected their movement. But Mercury does not fit Newton’s laws, and this could be observed by the 19th century: it was theorised that there was another planet, Vulcan, within the orbit of Mercury whose gravity influenced it. Einstein’s theory explained the orbit of Mercury without need for another planet.

Ptolemy and the muse Astronomia