Copernicus’ revolution and Galileo’s vision, in pictures | 2/19/2018 | Eleanor Imster
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Galileo’s sketches of the moon, showing its phases. Image via Wikimedia.

It’s not a stretch to say the Copernican revolution fundamentally changed the way we think about our place in the universe. In antiquity people believed the Earth was the center of the solar system and the universe, whereas now we know we are on just one of many planets orbiting the sun.

Shift - View - Century - Theory - Observations

But this shift in view didn’t happen overnight. Rather, it took almost a century of new theory and careful observations, often using simple mathematics and rudimentary instruments, to reveal our true position in the heavens.

We can gain insights into how this profound shift unfolded by looking at the actual notes left by the astronomers who contributed to it. These notes give us a clue to the labor, insights and genius that drove the Copernican revolution.

Imagine - Astronomer - Antiquity - Night - Sky

Imagine you’re an astronomer from antiquity, exploring the night sky without the aid of a telescope. At first the planets don’t really distinguish themselves from the stars. They’re a bit brighter than most stars and twinkle less, but otherwise look like stars.

In antiquity, what really distinguished planets from stars was their motion through the sky. From night to night, the planets gradually moved with respect to the stars. Indeed “planet” is derived from the Ancient Greek for “wandering star.”

Motion - Mars - Weeks

The motion of Mars over many weeks.

And planetary motion isn’t simple. Planets appear to speed up and slow down as they cross the sky. Planets even temporarily reverse direction, exhibiting “retrograde motion.” How can this be explained?

Page - Arabic - Copy - Ptolemy - Almagest

A page of an Arabic copy of Ptolemy’s Almagest, illustrating the Ptolemaic model for a planet moving around the Earth. Image via Qatar National Library.

Ancient Greek astronomers produced geocentric (Earth-centered) models of the solar system, which reached their pinnacle with the work of Ptolemy. This model, from an Arabic copy of...
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