How Do We Know the Earth Goes Round the Sun?

Question posed by Colin.

I feel like including some history in this response; if you want just the evidence that points to a Sun-centred solar system, look for the bullet points!

If you look into the night sky over a long enough period of time, you can see various objects move. Many of these objects appear to move around us- the Sun is an example, rising in the East every morning, and setting in the West every evening. The Moon is another. Other objects move around the sky, coming back to the same point every now and then. And standing here, on the Earth, it's easy to assume that it is static, unmoving, eternal.

For many hundreds of years, human beings assumed this to be the case- the Earth stood still and the rest of the Solar System, even the Universe, made its way around us.

Somewhere around 300 BCE, ancient Greek astronomer and mathematician Aristarchos made some very controversial claims. The prevailing view of the universe at the time was that it was a sphere centred around the Earth, the radius of which being the distance between the Sun and the Earth. Everything else that could be seen in the heavens existed within this sphere. Aristarchos suggested that the universe was much bigger than this model suggested, and that it was centred on the Sun with the stars all fixed on the inner surface of this great sphere.

  • Aristarchos discovered that the Sun is larger than the Earth, and used this to argue that the Earth would orbit the Sun, rather than the other way around.
Aristarchos received very little support for this new view of the universe, and it was even suggested by some that he be charged and sentenced accordingly for what amounted to blasphemy.

Now we jump to the 16th century ACE, and Polish astronomer Nicolaus Copernicus. Late in his life he published a book, De revolutionibus orbium coelestium, in which he describes his view of a Sun-centred ("heliocentric") universe: the Sun at the centre, with seven planets in circular orbits, and the far distant stars of Aristarchos's idea.
  • Observing the motion of the planets from Earth over a period of months or years, they follow a strange route across the sky: they travel in a particular direction for a while, then they stop, travel in the opposite direction for a while, then they stop again and resume travelling in their original direction. This observed motion is very difficult to explain in an Earth-centred ("geocentric") solar system, and Copernicus argued that a system in which the Sun was stationary with the Earth and the other planets moving around it in circular orbits more easily explained this apparent weird motion of the planets.
In the 17th century, German mathematician and astronomer (and astrologer) Johannes Kepler was a defender of heliocentrism and did much work on reconciling scientific progress in this area with passages from the Bible that appeared to disagree with it - the main source of opposition to these ideas at the time was the church.

Kepler adjusted and refined Copernicus's model of the solar system having developed three laws of planetary motion, which include the idea that the planets move in elliptical orbits rather than circular ones.

  • Kepler's laws work well describing the motions of the planets in a Sun-centred system, and he used these along with detailed observations to publish the Rudolphine Tables, predicting future positions of the planets with respect to the stars with greater accuracy than ever before.
Towards the end of the 17th century, English polymath Isaac Newton defined his law of gravitation and showed how this was consistent with Kepler's three laws of planetary motion.
  • Newton's law of gravitation also helped to bolster the case for heliocentrism on its own: the Sun is much more massive than the other planets (even combined), so they must orbit around it.
In 1729, James Bradley, English astronomer and third Astronomer Royal, discovered and explained a small but by now measurable (due to improvements in technology) apparent movement of the stars as observed from Earth.
  • The stars all appear to move in tiny ellipses over the course of a year. This is called 'stellar aberration', and is attributed to the Earth's motion around the Sun. This provided the first relatively straightforward supply of evidence for the motion of the Earth.
On to 1838, and German astronomer Friedrich Bessel. He was the first to measure a star's parallax. To demonstrate what parallax is, hold a finger up in front of you at arm's length, and close one eye. Line your finger up against something further away from you; an ornament on the mantelpiece, or a lamppost, or something. Now, without moving anything, close the eye you were looking through and open the other one. Your finger is no longer lined up with whatever it is you lined it up with- it 'appears' to have moved.
  • This effect is known as 'parallax' and can be observed with stars if we can measure their position in the sky accurately enough. Instead of opening and closing eyes, we just observe from different positions around the Sun. Parallax is useful for measuring the distance to stars, but the fact that we see this effect at all shows that we must be moving around the Sun.
In 1842, Austrian physicist Christian Doppler discovered that wavelengths change in frequency for an observer who is moving with respect to the source of the wave.
  • This effect, the Doppler effect, is what causes the sound of a fire engine's siren to change pitch as it races past you. The same effect works with light waves as well as it does with sound waves, and we see stars change colour slightly over the course of a year as, due to our motion around the Sun, we spend some time moving towards them, and some time moving away.
So, to summarise:
  • Stellar aberration;
  • Parallax;
  • and the Doppler effect
All provide evidence that the Earth orbits the Sun, and not the other way around!

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