When did we first get direct proof that Earth orbits the Sun?

As we’ve discussed, stellar parallax provides direct proof that Earth orbits the Sun, and it was first successfully measured in 1838 by German astronomer Friedrich Bessel. However, it turns out that there is another observation that was made even earlier and also provides direct proof that Earth orbits the Sun. This observation involves something called the “aberration of starlight,” which was discovered in the 1720s by English astronomer James Bradley (1693-1762).

You can understand this proof of Earth’s orbital motion by analogy to holding an umbrella to stay dry in the rain. If the rain is falling straight down from the sky, then you will hold the umbrella straight over your head when you are standing still. However, if you are walking through the rain, your forward motion means that the “straight down” rain will be hitting you at a slight angle. As a result, you will need to tilt your umbrella slightly forward to stay dry. Notice that the direction in which you must tilt your umbrella tells you the direction you are walking in.

Bradley’s discovery in the 1720s was that telescopes have to be tilted slightly in order to observe the stars, and that the direction of the required tilt changes over the course of each year. Just as the tilt of your umbrella tells you the direction you are walking through the rain, the tilt of telescopes tells us the direction that Earth is moving relative to incoming starlight, and the pattern of changes over the year therefore offers direct proof that Earth orbits the Sun. (In case you are wondering, this is called the “aberration” of starlight because the word “aberration” means “a departure or deviation from what is expected,” and prior to its discovery, no one had expected starlight to be coming into telescopes at a slight angle).