How Do We Know the Earth Orbits the Sun?
Several years ago Nobel Laureate Harold Kroto asked a group of scientists if any of them believed the Sun went around the Earth. No one did. He then asked who thought the Earth went around the Sun. Every hand went up, albeit some cautiously fearing a trick.
Kroto finally asked who could point to evidence supporting that belief. A few thought they could. To the others he said: Do you just take it on faith? How much else do you accept without evidence?1
So how do we know that the Earth orbits the Sun?
It’s Not a New Idea. The notion that the Earth is hurtling through space initially seems farfetched — we sense no such motion whatsoever. But that’s because as passengers on spaceship Earth we are traveling in the same direction and at the same speed as the planet. It’s like sitting in an airplane — we only know we are moving when we look out the window.
Theories that the Earth circles the Sun have been around for millennia. A Greek astronomer, Aristarchus of Samos, suggested it in the third century B.C.3 Its teaching was suppressed, however, by European political and religious authorities after the fall of Rome so many people today think it is a purely modern invention.
The Theorists. The officially sanctioned model of the Solar System said the Sun and planets all went around the Earth. But the complex paths of the Sun, Moon and planets against the backdrop of fixed stars (see illustration below) couldn’t be explained by simple elliptical orbits. So to make the theory fit the facts, astronomers devised complex systems of orbits within orbits. Unfortunately none of them really worked.
Many astronomers knew that planetary motion was better described by a Sun-centered or heliocentric model, but they kept their views private fearing persecution. One, Nicolaus Copernicus, began developing a heliocentric model in 1514, but he didn’t publish it until he was nearing death in 1543. Once released his treatise spread across Europe with a speed that demonstrates the power of the then new printing press and publishing industry.
Debate and inquiry ensued. In Germany, Johannes Kepler formulated three laws of planetary motion, which explain Copernicus’ system mathematically. In England, Sir Isaac Newton tied Kepler’s formulas to his own laws of gravity and motion to create a universal theory that explains how things move both on Earth and in space.
Copernicus, Kepler and Newton devised principles that explain what we see, but none of them presented actual observations of the Earth orbiting the Sun.
The Observer. The development of the telescope in the early 1600s enabled astronomers to see things up close, including some objects that do not orbit the Earth.
Galileo famously observed four moons orbiting Jupiter. He also saw that Venus exhibited phases like the Moon’s, proving that it orbited the Sun. His publication of those discoveries resulted in his being called before the Inquisition and ordered to refrain from thinking, teaching or defending such heretical ideas.
Galileo’s discoveries clearly showed that there were celestial bodies that do not orbit the Earth, but they didn’t prove that the Earth itself was moving or orbited the Sun.
The Proof. So how do we know when we have moved? The answer is by seeing that the relative positions of stationary objects have changed. For example, if we see that a tree is in front of a window when we are across the street from it, but not when we are next door, we know that we have moved because the tree and house have not. On a celestial scale the same is true if we see a similar change in the positions of stars relative to each other. This shift is called parallax.
The problem is that the stars are so far away that such changes in position are too small to detect even when viewed from opposite sides of the 8000 mile wide Earth. But the Earth’s orbit is 186 million miles across and that is large enough for a shift to be measured when stars are viewed from opposite sides of it.
In the 1830s two German astronomers, Wilhelm Struve and Friedrich Bessel, successfully measured this shift, thereby conclusively demonstrating that the Earth was not stationary. Interestingly, an eighteenth century British astronomer, James Bradley, discovered another proof of Earth’s motion known as aberration of light while unsuccessfully attempting to measure parallax.7
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Kroto challenged his colleagues to remind them that science is about skepticism and proof. Just because something like climate change can’t be irrefutably established today doesn't mean it isn't true and shouldn’t be studied.
The importance of these principles is illustrated by the award of the 2017 Nobel Prize in Physics to three scientists who detected the existence of gravitational waves. Albert Einstein theorized their existence in 1915, but the instruments of his day couldn’t measure them.8 A century later the quest for evidence paid off, both for science and three scientists.
- Account by Diane Wu on NPR Radio’s This American Life, Episode 630, Things I Mean to Know (Originally aired October 27, 2017). Transcript on This American Life web site.
- Plate from 1660 star atlas, Harmonia Macrocosmica, by Andreas Cellarius. Image from on-line collection of plates on the University of Utrecht’s web site.
- Article about Aristarchus of Samos in Wikipedia.
- Illustration from first edition of Encyclopaedia Britannica article on Astronomy based on diagrams by Cassini and Long. Image from Wikipedia.
- Photo of Galileo’s telescopes on display at the Museo Galileo in Florence, Italy. Image from Galileo Telescope web site.
- Image from on-line lab manual developed by the University of Iowa’s Department of Physics and Astronomy.
- See, e.g., A History of Astronomy — Part II on the European Space Agency’s web site.
- Dennis Overbye, 2017 Nobel Prize in Physics Awarded to LIGO Black Hole Researchers, N.Y. Times (October 3, 2017).
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