For much of human history, the only light known to humans came from the sky – the Sun, Moon and millions of stars. The ancient peoples regarded these as being ‘heavenly’ and out of this world. As the human race has developed increasingly clever and powerful instruments to observe the sky, many important discoveries have been made. Here is our list of what we think are the 10 greatest discoveries in astronomy.
1 – The Planets Move
The Middle East was effectively the cradle of astronomical observation with the ancient Mesopotamians building observation towers and recording what they saw on clay tablets over a period of at least a thousand years. Hundreds of these tablets have been recovered from archaeological sites and one of them, the Venus tablet of Amazonica, contains descriptions of the planet Venus moving against the starry background. It is the first ever record that a planet was moving.
The idea was taken on by the ancient Greeks, who created a mathematical model of the Solar System that stood for about 2000 years. This was the geocentric (Earth fixed and immovable at the centre of the Universe) model in which the stars were the most distant objects, fixed to a ‘Celestial Sphere’ and the planets all orbited the Earth. Their final version is known as the Ptolemaic system after the philosopher Ptolemy. It was also the Greeks who gave us the word Planet – ‘planetes‘ is ancient Greek for wanderer.
In the Western world, this model was not challenged until the 15th century, however, Islamic astronomers had built instruments and were making observations based on the idea that the Earth was not fixed at the heart of everything well before that.
2 – The Earth is not the Centre of the Universe
It is now taken for granted that we live on an insignificant lump of rock orbiting an unremarkable yellow dwarf star, however, it took humans a long time to realise this. Credit for this significant paradigm shift is accorded to Nicolaus Copernicus (1473 to 1543) whose work Dē revolutionibus orbium coelestium was published shortly before his death. He first seems to have developed the idea that the Sun was at the centre of the Solar System at the start of the 16th century. It is believed that he was worried about publishing the book since it went against the teachings of the Church. Doing that usually meant you were burned at the stake as a heretic.
Good scientific models should make good predictions; despite all of its ‘epicycles‘, the Greek geocentric model came up short. It was this realisation that drove Copernicus to re-think our solar system to come up with a better model.
The idea was not new, Greek Philosophers, the Indian Astronomer Aryabhata and several Islamic astronomers including Tusi and Qushji also had questioned the Ptolemaic idea of a fixed Earth and proposed the planets orbited the Sun.
3 – Planetary Orbits are not Circular
The orbits of the planets were always assumed to be perfect circles. This idea had constantly created issues with making predictions of planetary positions but it was completely unthinkable to believe anything else until Kepler came onto the scene.
This German mathematician was working for Danish observational astronomer Tycho Brahe at the time of his revelation; he was using Tycho’s accurate observations to compute the circular orbit of Mars. The whole process was driving him mad as nothing seemed to quite work. But in 1605, he finally hit on the idea that planets travel in ellipses with the Sun at one of the two foci. Thus Kepler’s First Law of Planetary Motion was born … giving astronomers the ability to make more accurate predictions of planetary positions. This was also the first really concrete evidence that Copernicus’ Sun-centred model was correct.
His model of the Solar System was far better than any previous and even predicted that mercury would pass across the face of the Sun on a periodic basis which was first observed by Pierre Gassendi in 1631. During the following centuries, there were aspects of planetary motion that even Kepler’s model could not predict. For solutions and a better model, the World had to wait for the genius of Albert Einstein.
4 – Jupiter has Moons!
This may not sound like it should be included in this list of the 10 greatest astronomical discoveries but at the time, it was Earth-shattering news for several reasons as you will see.
The date was 1609. The man was Galileo Galilei. The place was Padua. He had come across the telescope as a toy but developed it and refined it so much so that it became a scientific instrument. Early in 1610, he turned it towards the skies and was amazed. He could see mountains on the Moon, clusters of stars and intriguingly, four bright stars in a line alongside jupiter. Over the course of a week, he realised that these were travelling around the planet Jupiter itself.
This changed his life and the course of science forever.
He reasoned that since satellites could orbit a planet, why could the planets not orbit the Sun? Thus he became a solid supporter of the heliocentric theory. This was completely at variance with the teachings of the Catholic Church so he was in deep trouble. Luckily for him, he was far too important a person to burn to death so they placed him under house arrest.
In later times, he has been called the ‘Father of Modern Science‘ because his work also showed that to gain scientific knowledge, it was necessary to make observations and test them against relevant theories. If the theory didn’t match the observations, then a different theory needed to be developed. That of course is what is now called the ‘Scientific Method‘.
5 – Halley’s Comet has a Predictable Orbit
Fast-forward to the other end of the 17th century and meet Edmund Halley. At the time, the appearance of a comet (the name comes from the Greek ‘kometes‘ meaning ‘long haired star‘) was greeted with panic and despair as they were believed to be portents of evil and doom.
Halley chose twenty four comets from the past which had been recorded in sufficient detail to allow him to roughly work out their orbits. He noted that three of them (plus the comet that had been seen in his lifetime in 1682) seemed to follow the same orbit of about seventy six years. He figured that they were actually the same comet that had a predictable orbit and said that it would be back in 1758. Sure enough, the comet reappeared and now bears his name.
So why is this one of the 10 greatest discoveries in astronomy? It paved the way for a replacement of superstition and folklore with a rational explanation of the physical universe, a slow process which continues even today.
6 – the Milky Way is a Disc of Stars
The first realisation that the Milky Way was a disc of stars is thanks to musician-turned-astronomer William Herschel. Finding that telescopes were far too expensive to buy, he turned to DIY, he and his sister Caroline making their own mirrors from an alloy known as speculum metal (two-thirds copper with one third tin). The largest they produced was 1.25 metres diameter and had a tube that was about 12m long.
Herschel made several very significant contributions to astronomy but he was the first to determine the shape of our Galaxy, the Milky Way. Over a period of years, he made careful observations of the distribution of stars and came to the conclusion that the Milky Way was a disc of stars. This is quite a feat as his telescope, good as it was for the 18th century, allowed him to view only a tiny fraction of the billions of stars that make up the Galaxy in which we live.
Unfortunately, the ancient ideas still pervaded astronomy and he assumed that the Sun was at the centre of things despite being the first to detect that the Sun was actually moving through space.
7 -General Relativity
Isaac Newton’s law of universal gravitation worked perfectly adequately but failed to explain why the point of closest approach of Mercury to the Sun (perihelion) slowly advanced. Early in the 20th century Albert Einstein, then working as a clerk in the Swiss patent office, was as puzzled by this as the professional astronomers of the time.
Something else that niggled him was the fact that Newton believed that gravity was an instantaneous force. The size of the Universe was known by then and it made no sense that gravity could act immediately over such immense distances.
So he put the two ideas together, re-wrote Newton’s equation that had stood for over 300 years and came up with his ‘Theory of General Relativity’. Essentially, he said mass will curve space and time.
Think of the space-time continuum as being flexible. An astronomical object will bend it in the same way that a bowling ball will create a dent in a trampoline. Fire in a marble and it will appear to orbit the bowling ball. Newton would have said that gravity caused this but according to General Relativity, the effect is created by a combination of the forward motion of the marble combined with falling into the dent created by the bowling ball. The same idea works for stars, planets and moons.
General Relativity was given credence by observations of stars during a total eclipse. Einstein predicted the posutions of stars close to the Sun during totality would appear displaced sligtly from their normal positions. Observations backed up the prediction so this model of the universe was adopted.
8 – The Expanding Universe
During the 1920s, astronomer Edwin Hubble studied many of the ‘white nebulae’ found by William Herschel a century earlier. Using the 100 inch Mount Wilson telescope, Hubble found that they were made up from billions of stars like our own Galaxy and well outside of it too.
He measured the distances of as many of them as he could and found that their light appeared to be shifted to the red end of the spectrum. This redshift is caused by the stretching of light waves when an object is rapidly moving away from the observer. Not only were the redshifts huge, confirming they were very distant objects, the further away they were, the greater the redshift.
This has been dubbed Hubbles’s Law and was the first solid evidence to back up the Big Bang model of the formation of our universe.
He published his findings in 1929, two years after a Belgian priest and mathematician, Georges Lemaitre, had come to the same conclusion using Hubble’s own data together with earlier data prepared by another astronomer, Slipher. So how come Lemaitre didn’t get the credit? Firstly, his conclusion was published in French in an obscure Belgian Scientific publication. Secondly, it wasn’t until the 1980s that anyone realised this anyway.
9 – Radio Waves from the Centre of the Milky Way
Physicist Karl Jansky was working for Bell Laboratories, investigating sources of interference with the wavelengths used for ship-to-shore and transatlantic radio transmissions.
He discovered that one source of interference was from the layer of the atmosphere called the iononsphere, a second source was from thunderstorms but the third source appeared to originate from beyond the Earth. It rose with the Sun, crossed the sky with the Sun and disappeared at Sunset. Jansky continued his studies and found that gradually, the source went out of synchronisation with the Sun’s movements placing it beyond the Solar System.
He traced it to the constellation of Saggitarius and the source became known as Saggitarius A. He believed that he had discovered a new type of astronomical object at the centre of our Galaxy – he was right. We now believe the radio waves emanate from matter falling into a super-massive black hole that seems to exist at the centre of all spiral galaxies.
This discovery was more than just the new object, it was the dawn of a new and powerful way of looking at the universe – radio astronomy.
10 – Cosmic Microwave Background Radiation
Another discovery to come out of Bell Labs in the USA. Two physicists, Penzias and Wilson were allowed to use a redundant horn antenna to study the temperature of the gas surrounding the milky way. But they discovered that their readings were consistently about three degrees out and they could find no reason for this. At the time, there was a pair of pigeons nesting in the antenna which made them think that was perhaps the problem. A colleague shot the pigeons and they cleaned the pigeon droppings out of the antenna but there was no difference.
With the help of cosmologists working on the ‘Big Bang‘ theory of the origin of the universe, they realised that they had discovered the remnants of the bang itelf. The theory predicted that there would be faint thermal afterglow of about three degrees that could be observed throughout the sky. Although the rival ‘Steady State’ theory didn’t die there and then, it certainly suffered a mortal blow from which it would never recover.