Nowadays we take the existence of the solar system as a given. It wasn’t always like that. In fact we have only really known of its existence for about 400 years. Before then the general assumption was that the universe was centred on the Earth.
The question is: what changed?
First, we have to understand some facts about how the ancients viewed the universe.
The first known civilisations were not only surprisingly good at astronomy but they also passed their records to later civilisations.
Thus the ancient Greeks were aware of the earlier astronomy of Babylon and India, where there had been civilisations for thousands of years before ‘classical’ Greek civilisation.
In particular, they all know that while most of the thousands of stars they could see in those light-pollution-free days had fairly fixed positions, a handful moved around the sky. The Greeks worked out that there were five of these wandering stars, Mercury Venus, Mars Jupiter and Saturn. This was not a trivial achievement: the Babylonians had separate morning and evening planets. Legend has it that Pythagoras was the first to realise that they are one and the same planet, which we call Venus.
All five of these planets are bright objects, which are very easy to find in the night sky.
The two photos below, taken by the author, show Mars against the stars 25 nights apart.
These five planets (and those discovered much later) move in characteristic ways. Mercury & Venus are never visible at midnight. They are evening and morning stars. Mars, Jupiter and Saturn can in principle be visible at any time of night. All three of them travel along a characteristic looping path. They perform one loop a year. (Mars only performs one every two years. We now know that this is because in the ‘missing’ years it is on the other side of the Sun from us.)
This movie, due to Wikipedia contributor Eugene Alvin Villar, shows the path taken by Mars during 2003.
At least one Greek writer, Aristarchus of Samos, worked out that this behaviour could be explained by having the planets orbit the Sun. We only know about his work because Archimedes referred to it. His manuscript did not survive; and his ideas did not become mainstream.
The explanation of planetary motion that did become mainstream was that they all go around the Earth, orbiting in circles-within-circles, called epicycles. So there was no ’solar’ system at all. The Sun was merely one more object which was thought to orbit the Earth.
Source: http://upload.wikimedia.org/wikipedia/commons/2/29/Ptolemaic_elements.svg
It is often thought that these ideas were overthrown in a great burst of discovery around the year 1600. In fact this is an exaggeration, although somewhat true. Ptolemy’s data were gradually refined throughout the Middle Ages.
Copernicus had access to better data than Ptolemy. Contrary to popular belief, he did not have the Sun at the centre of the universe. He had everything orbit an empty point in space. And his model did not eliminate epicycles. Quite the opposite: he had more epicycles than Ptolemy. His motive was a belief that Ptolemy ‘fiddled’ his model by not insisting on uniform circular motion; he rectified this problem.
Johannes Kepler, the Imperial Mathematician at Prague, benefited from the immense amount of data collected by his predecessor Tycho Brahe, who had collected precise data over 10 Mars orbits, or about 20 years. ‘Precise’ meant that his positions were accurate to about three arcminutes or 1/20 of a degree.
Kepler did not at first know how Mars moved; and it took several years of trial and error for him to work out that it orbits the Sun along an ellipse with the Sun at one focus. People knew a lot less about ellipses then than now, and knew virtually nothing about gravity, so Kepler was a real pioneer trying to find his way through unknown intellectual territory. What is so remarkable about him is the sheer originality of what he did.
He had the wit to realise that if Mars and Earth orbit the Sun, he knew from Tycho’s data how long a Mars orbit takes (687 days), and knew that if Earth is at E1, then the next time Mars reaches the same place in its orbit, Earth will be at E2 because 687 days is a little under two years. By comparing the triangles S-E1-M and S-E2-M in the diagram, Kepler could work out the distance to Mars from geometry.
Once he had done this, he knew that the epicycles, the circles-within-circles, just weren’t there.
Kepler was aware of Copernicus’ work, and of that of his contemporary Galileo. Galileo was not good at appreciating the work and ability of other people, which could make him come across as arrogant; and which made him some powerful ecclesiastical enemies. He hadn’t time of day for Kepler. Despite this fault he was an extremely able man. Fortunately the one time when he did pick up and run with someone else’s idea was when he heard about the newly invented telescope. He wasn’t the only one to try this new invention on the heavens, but he was perhaps the boldest, certainly the most thorough, and was able to communicate what he discovered very effectively. In a very short space of time, he discovered that Jupiter has satellites, and that Venus shows phases. The former discovery showed conclusively that not all orbiting bodies orbit the Sun. The phases of Venus can only be understood if it orbits the Sun, not the Earth. If Venus and the Sun orbit the earth, the phases of Venus must appear to us to be either always crescent if Venus is the nearer; or always gibbous if the Sun is the nearer to us. In fact, as the picture below shows, the phase is gibbous when Venus is behind the Sun relative to us and crescent when it is nearer to us than the Sun.
The phase of Venus changes from gibbous to crescent. Photos: John Clark
Galileo also showed that the Earth’s gravity causes all objects to fall at the same speed, regardless of the mass of the object.
Isaac Newton, born the year Galileo died, picked this idea up and incorporated it into his theory of gravity. In modern language, Newton’s law of gravity states that all bodies, whether on Earth or not, attract one another with a force
where G is a constant, M is the mass of one body, m is the mass of the other, and r is the distance between them.
There is an important twiddly bit to this law. Newton was not the only person to work out the law, but he was the only one to prove the important corollary that spherical bodies attract as if they were point masses at their centres. Since the Sun and planets are roughly spherical, this means that the law can be applied to them. Proving this was a very great achievement. Newton could only do it because he had invented a whole new branch of mathematics: calculus.
Once this was established, it was a rather shorter step for Newton to show that his law implies that orbiting planets travel in ellipses with the Sun at one focus, as Kepler had discovered. Actually, he showed that they can also travel in parabolas or hyperbolas, in which case they never return: they only ever visit the Sun once.
Much if not most of Newton’s book on the Solar System is concerned with the orbits of comets. He was obviously very keen to show that they too orbit the Sun.
By the time Newton died in 1727, Earth, Jupiter and Saturn were known to have satellites; and planets and comets were known to orbit the Sun.
As telescopes improved, another planet was discovered. Its discoverer, William Herschel, at first thought it was a comet. No-one suspected that there were more than the five known planets. This is so strongly true that Galileo had noticed this planet, Uranus, as a moving object, but did not recognise it for what it was.
Another planet was discovered in 1801: Ceres. We now know it as the largest asteroid, but the astronomers of the time had a more pressing problem than finding out what Ceres is: it was about to disappear behind the Sun for Earthbound observers. The discoverer, Giuseppe Piazzi, had exactly 24 observations of its position. People were very keen to find it again once it had passed behind the Sun. Unlike the main planets, it is a very faint object that is not at all easy to find. Remember that Kepler needed data going back 20 years to work out the orbit of Mars. Enter another mathematician as great as Newton: Karl Friedrich Gauss. He worked out a way to estimate the orbit of Ceres from just three observations. This method revolutionised the science of orbits. Working out an orbit was now a ‘quick hit’, rather than a lifetime’s work as for Kepler and Newton.
A handful of asteroids were discovered over the next few years. Work stopped when the Napoleonic Wars disrupted life in Europe. They were all too small to resolve as anything more than points of light. This is still nearly true: the Hubble Space Telescope can just about resolve Ceres as a disc. Hence people began to call them asteroids, not planets. Discovery of asteroids resumed in the mid 19th Century and has been going on ever since. Nowadays asteroid discovery is mostly the preserve of dedicated amateurs. Most but not all of them lie between Mars and Jupiter.
Meanwhile the planet Neptune had been found in 1846 by analysing the orbit of Uranus. Its orbit was being perturbed by a very massive, as yet unseen, body. This fact together with a belief that the Sun-to-planet distances fitted a simple pattern, told people where to look. The distances were thought to obey the Titus-Bode law. Neptune’s distance from the Sun is a poor fit to the Titus-Bode law, which is not nowadays taken seriously except by a few diehards.
People tried the same trick to locate the next planet out from Neptune. Its position was guessed by working out perturbations in Neptune’s orbit. With great fanfare, in 1930, the planet was announced and named Pluto. Unfortunately, the perturbations in Neptune’s orbit were not real, but the result of a mathematical error. Even more unfortunately for Pluto, when a satellite was discovered in 1978, Newton’s law of gravity could be used to work out its mass. This mass was found to be tiny compared to the other planets.
What we know of the planets themselves has mostly come from spacecraft sent to observe them. I think it is fair to say that we now live in a golden age of discovery about planets unmatched since the days of Kepler, Galileo and Newton.
Since the year 2000, the next phase of Solar System exploration has been driven by the advent of high quality digital photography and computer image analysis. The sky can now be systematically and automatically searched for solar-orbiting objects beyond Neptune. There are lots of them, mostly tiny, but a few are comparable in size to Pluto.
Since Pluto is really too small to be a major planet, a new category of object called a dwarf planet was devised. The dust has not yet settled on this definition: it upset a lot of people, and the voting mechanism used by the International Astronomical Union to re-categorise Pluto has attracted fierce criticism.
Most of the trans-Neptunian objects have orbital radii (strictly speaking orbital semi-major axes) in the range 40-80 AU, where one AU is the Earth-Sun distance. Neptune is about 30 AU from the Sun. These objects are known as the Kuiper belt. They appear to be lumps of impure ice.
One of the dwarf planets, Sedna, is ten times further from the Sun than the others, at nearly 500 AU. It may not be a Kuiper belt object. The existence of a cloud, the Oort cloud, between stars has been hypotheisied, by Jan H. Oort and by Ernst Opik, the grandfather of British politician Lembit Opik. It has yet to be observed. Perhaps Sedna is the first Oort cloud object to be discovered. Stay tuned.
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Mars was about 109 million miles away when this picture was taken. Seeing conditions were poor: the air was very turbulent and the wind was blowing my telescope about. Nevertheless, you can just make out the North Polar ice cap.
