Proving Copernicus Right
The name of 16th century Polish astronomer Nicolaus Copernicus became a household world because he proposed that the Earth revolves around the sun. But the man who finally gathered scientific proof of that theory was English astronomer James Bradley, born during this month in 1693.
Called the best astronomer in Europe by Isaac Newton, Bradley methodically observed the star Gamma Draconis and noticed slight seasonal shifts in its position, which he then observed in other stars as well. He called this effect "the aberration of light" and estimated its angle at 20 to 20.5 seconds; the modern value is 20.47 seconds. Eventually, Bradley realized that the displacement stemmed from viewing a stationary object from a moving one, the Earth--thus confirming Copernicus's concept.
Bradley later discovered a second process that causes stars to wobble in their places in the firmament. In an effect he called "nutation"--known to be true today--subtle changes in the angle of Earth's rotation, caused by the moon's pull on the equator, alter apparent stellar positions.