Our view of the Earth, our home in space, and the Universe itself, has changed radically in the last four hundred years.

For millennia, we used to think that the Earth was at the centre of creation, and everything that we see in the sky orbits around us. We used to think that Gods and Demons governed the way the planets move about the heavens and that supernatural forces controlled our fate. Some people still believe this to be the case although how, in light of so-much scientific evidence, remains puzzling.

Thank goodness for science and maths.

The Earth-centred Universe

The Greek astronomer, Ptolemy, initially postulated that the Earth was at the centre of everything and that the stars and planets were fixed to a series of concentric glass spheres arranged in some kind of special geometry that was nearly impossible for us mere humans to comprehend. That said, even Ptolemy himself thought that this idea was weak and later openly acknowledged that a sun-centred system - or Heliocentric system - was better at explaining the changing seasons and the motion of the planets.

However, a Universe without God at the centre of everything didn't suit some folk and the idea of a Earth-centred universe was more-or-less-universally adopted by various religious bodies around the world and, sadly, it stayed that way for centuries. It's the same with this daft idea of a flat earth. The problem with authoritarian bodies is that if you disagree with their established doctrine then you could end up either imprisoned or, like heretic astronomer Giordano Bruno, tied to a stake in the middle of the marketplace and burned. Hence, these silly ideas went unchallenged for hundreds of years.

The idea of an Earth-centred Universe is very obviously flawed and yet, strangely, the idea of a sun-centred or Heliocentric system, is a much better fit. It's also quite old.

Eratosthenes and his shadows

Eratosthenes (276 BCE - 194 BCE) was the head librarian in Alexandria, Egypt, then the centre of learning in the ancient world. From the records available to him, Eratosthenes noted that at midday in the city of Syrene, the sun's rays shone directly down a well, casting no shadow. However, on the same day in Alexandria, he had seen that the sun would cast the shadow of a stick on the ground. Eratosthenes measured the length of the shadow and calculated the angle of the sun's rays to be 7.2 degrees.

The distance between Syrene and Alexandria was well known. It was on a major trade route and checked annually to determine the extent of Egypt's territory for agricultural and taxation-related purposes. In Eratosthenes' time, the distance would have been measured in units known as a stadia, which is roughly 154 meters in today's money. Yes, this is where the term Stadium as in football stadium comes from. Today, we've measured the distance between Syrene and Alexandria as 787 kilometres.

Eratosthenes realised that if the Earth was spherical then 7.2 degrees represented one fiftieth of the circumference of a circle (7.2 / 360 = 0.02 = 1/50). Thus, by multiplying the distance between Syrene and Alexandria by 50, he could calculate the circumference of the Earth.

Eratosthenes came up with a result of 39,375 km, which is 1.4% less than the real number, 40,076 km. Not bad, eh?

Al-Buruni's Method

The Arabic astronomer's had a similar method for estimating the Earth's circumference although it was less accurate than that used by Eratosthenes. In the Arabic method, as pioneered by the mathematician Abu Rayhan al-Biruni, you simply climbed to the top of a mountain and measured the dip angle of the distant horizon. It's less accurate because atmospheric distortion means that you can't easily judge the actual angle of the horizon but it does serve as a useful backup of Eratosthenes' result.

You can even do this yourself. All you need to know is the height of your mountain above sea level, an accurate protractor and a clear horizon.

Anyway, the point is, the next time your mate down the pub tries to convince you that the Earth is flat and that everything NASA tells you is a lie, then just tell him about Eratosthenes and his sticks. And if that doesn't work and you can't convince him using Maths and reason and a knowledge of history clearly demonstrates that the Earth is round then you should just find another friend because nobody likes to hang around with an idiot.

The Heliocentric Universe

Science and a Polish astronomer, Nicolas Copernicus, changed our view of the Universe forever. Although some revisionists have tried to suggest that it was the Islamic astronomers who first proved that the Earth went around the Sun, it was actually Copernicus and his mathematics which proved the point emphatically (although it's known that Copernicus referenced a lot of Arabic texts in his final publication...).

Copernicus never lived to see his great work published. Like so many of his learned colleagues, Copernicus was a Clergyman who feared excommunication. Disseminating heretical publications, which disagreed with established doctrine, was a punishable offense and so Nicholas' works only came to light after his death.

A generation later, the German astronomer Johannes Kepler analysed precisely the orbits of the major planets and was able to derive three key equations which tell us exactly how a planet moves around a star or how a moon orbits a planet.

Our current view of the solar system

We can divide the solar system into five regions as defined by the bodies which reside therein. These are:

The Inner Region - populated by the rocky planets, Mercury, Venus, Earth and Mars
The Circumstellar Disc - the main asteroid belt
The Gas Giants - A region populated by planets made up mostly of cold, compressed gas, planets such Jupiter, Saturn, Uranus and Neptune
The Kuiper Belt - a region populated by dwarf planets, asteroids and comets. Pluto is a member of this family.
The Oort Cloud - this region lies beyond the Kuiper Belt and extends into interstellar space

We'll talk more about the planets and other bodies themselves in another module.

Galactocentrism

If we can accept that the Sun is at the centre of the solar system then we also have to ask another question:

Where is the sun in space?

Galileo Galilei, the famous Italian Astronomer, had noted that the Milky Way, a hazy band of light seen in the night sky formed from stars that cannot be individually distinguished by the naked eye, could be resolved into individual stars with the aid of a telescope. English astronomer Thomas Wright had hypothesised that the Milky Way was, in fact, a galaxy adrift in space, like so many others that astronomers had observed through telescopes. In an attempt to validate this theory, Thomas simply counted the number of stars as they are distributed across the sky, eventually noting that there were significantly more stars in one direction than another.

In 1755, philosopher Immanuel Kant used Wright's ideas to speculate that the Milky Way might be a rotating body of a huge number of stars, held together by gravity. The resulting disk of stars would be seen as a band on the sky from our perspective inside the disk. Wright and Kant also conjectured that some of the small fuzzy patches that were also visible in the night sky might be separate galaxies themselves, similar to our own.

The Shape of the Milky Way

The first astronomer to tackle the shape of the Milky Way galaxy head on was William Herschel. We talk about William extensively in another module. Herschel repeated Wright's earlier experiment although with greater precision, using a star's brightness as a measure of its proximity. However, Herschel erroneously concluded that Earth and the Sun were close to the centre of the Milky Way.

As telescope technology improved, astronomers were able to measure the speed of the closest stars in relation to the Earth's movement. Two opposing streams were soon discovered, corresponding to the arms of our spiral galaxy, which move in opposite directions. Astronomers also noted that many of these small unresolved fuzzy patches out in space actually had a form and structure of their own, and that many of these structures had a spiral form. It was soon theorised that the Milky Way might also have some form of spiral structure.

Astronomers noted that supernova in these distant spiral forms were considerably less bright than they should have been, implying that they were at far greater distances than we first imagined. In other words, these spiral structures are not part of the Milky Way but are, in fact, separate and independent entities in their own right.

The Shape of the Universe

Armed with a knowledge of stars and a rock-solid classification system - see the section of the Harvard Computers for more information on stellar classification - Edwin Hubble was able to photograph stars in these distant galaxies and, from those photographs, determine their distance.

Furthermore, by analysing the spectra of these stars, Hubble was able to measure the speed of those galaxies as they moved through space. He came up with an astonishing conclusion - all of the galaxies he observed were moving away from each other. The further away from us then the faster they appeared to be moving. This implied that, at some point in time, these galaxies (or whatever these galaxies had evolved from) was original much closer together and may, at some point in the past, have existed in the same place.

Independently of Edwin Hubble, a Dutch Priest and physicist, Georges Lemaitre, suggested that the Universe was expanding. In 1931, Lemaître went further and suggested that the evident expansion of the universe, if projected back in time, meant that the universe was smaller in the past, perhaps concentrated into a single point. Such a primeval atom, defining where and when the fabric of time and space came into existence, is the origin of the so-called Big Bang Theory.