What is a planet? (The Renaissance)

The Renaissance: 10 Planets, 1 Moon
There matters stood for more than fifteen hundred years. Planets were those strange wandering lights in the night sky, and everything else was a star, clouds, or birds. And then an upstart Italian named Galileo Galilei had the bright idea to turn a spyglass onto the stars and so changed everything. The spyglass had been developed just a few years before in the Netherlands, where it was primarily used as a way to spy on the neighbors. Galileo made some changes in the Dutch design so that it was easier to make and to use, and sold copies of his design to various nobles in order to pay for his late nights carousing with the stars.

A schematic of Galileo's telescope. Today, you can buy binoculars that have better magnification and resolution than Galileo's best telescope did, and use them to see the same things that he did all those years ago.

Galileo improved on the design in several ways. His sliding case made it easier to focus the spyglass, and made it something that could be collapsed and stored. The twin lenses at the front of the spyglass kept the light from smearing out into a rainbow  [2] and helped make the image sharper. In honor of his new design, the instrument was given a new name: the “far looker” or telescope. And Galileo”s revolutionary new design would soon change the very way that we say the universe.

Like everyone else, Galileo used the telescope to look at warships and to spy on his neighbors. But, unlike everyone else, Galileo also looked at those five moving specks of light in the night sky. And when he did, Galileo discovered something that set the planets apart from the other stars. When you look at a star through a telescope, no matter how powerful, the star just looks like a point of light. This happens because the width of even the largest star is much, much, much smaller than the distance to that star. But planets are close enough to us that they make a visible disc when seen through a telescope.

When seen through Galileo”s telescope, Mercury was a tiny disc, about half the size of a peppercorn. Venus was a little larger and went through phases, just like the Moon. The Moon was fascinating, with many pock marks that Galileo called star wounds or astroblemes. And Mars had a white tinge that moved from one pole to the other over the years.  But the greatest surprises were Jupiter and Saturn. Saturn was “jug eared” with what he thought were two companion planets that came and went over the years. And Jupiter had four companions that circled around him. Galileo named these new planets the Medicean stars in honor of his patron Cosmo de Medici; today they are known as the Galilean satellites Io, Europa, Callisto, and Ganymede. Suddenly, there were ten planets in the Solar System.

The Galilean view of the Universe. The Earth was demoted from the center, and Jupiter had four other planets orbiting it.

More importantly, these ten planets completely demolished the old Aristotelian and Ptolemaic geocentric universes. According to Aristotle, the planets were perfect and without blemish but anyone using a telescope could see that the Moon and Sun were covered with imperfections. And the phases of Venus could only be explained if it orbited the Sun and not the Earth. As Copernicus had shown [3], if any planet orbited the Sun while the Sun orbited the Earth then that planet should appear to move backwards in its orbit at some point. As Venus never did so, both Venus and the Earth had to orbit the Sun. Though there were many who protested and some who punished Galileo for his heretical views [4], the evidence was clear. The Earth was no longer the center of the universe; it was just another planet, orbiting around the Sun.

[2] Astronomers call this “chromatic aberration”. It happens because different colors of light are bent by different amounts when they pass through a prism. Sixty years later, Isaac Newton would use this fact to demonstrate that white light is actually made up of a rainbow of colors.

[3] Strictly speaking, Copernicus wasn’t the first person to propose a sun-centered universe. Aristarchus had proposed one in 200 BC, and had promptly been threatened with death for impiety. His ideas were taken up by others, including Giordano Bruno who was burned at the stake just sixteen years before Galileo made his discoveries. Copernicus avoided these problems through the clever expedient of waiting to publish his more radical ideas until just before he died. Copernicus is generally credited with being the father of modern astronomy because he not only championed the heliocentric system, he also detailed exactly what the differences in the two models would look like. This effectively gave Galileo a checklist of what to look for when he turned his telescope to the skies.

[4] What really got Galileo in trouble wasn’t his views so much as the way he had of rubbing everyone else’s nose in the fact that he was right and they were wrong. This worked fine just as long as he was creating wonderful new things and remembered not to make the boss look like an idiot; when he forgot those two cardinal rules, the cardinals reminded him that they ruled. The first time he published his support of Copernicus’s system (in 1615), he was investigated by the Inquisition and let off with a warning to present the idea as a possibility and not a certainty. Unfortunately, by 1632 he had forgotten the Inquisition and their warning; he published “A Dialogue Concerning the Two Chief World Systems” asserting that the heliocentric system was a fact. Worse still, he put all of the arguments made against it, including those made by Pope Urban VII, in the voice of a character named “Simplicio” (“Stoopid Idjit” when translated from the Italian). Needless to say, this did not go over well with the authorities, who decided to “put him to the question” (i.e., threaten him with torture and worse if he didn’t recant). Galileo took one look at the torture instruments, said “My bad!” and was given the lesser punishment of house arrest for the rest of his life.