Galileo's Telescope: Four Moons That Changed Everything

In the fall of 1609 Galileo Galilei heard about a curious Dutch invention — a spyglass — and built a better one. In the next few months, he pointed it at the sky, and everything he saw broke the Aristotelian model of the cosmos in some specific way.

The instrument, in numbers

Galileo's best telescope had an aperture of about 37 mm, a focal length of ~980 mm, and yielded about 30× magnification. It was a simple two-lens design: a plano-convex objective and a plano-concave eyepiece — what we now call a 'Galilean' refractor. The field of view was tiny (about a quarter of a degree, less than half the moon), the image was upright, and chromatic aberration made every bright object bloom with color.

By any modern standard, this is a bad telescope. But it was 30× better than the human eye, and no one else had one yet. He wrote up his first winter of observations in Sidereus Nuncius (Starry Messenger), published in March 1610.

The Moon: not a perfect sphere

Aristotle's cosmology said the heavens were made of a fifth element, incorruptible and perfectly smooth. Galileo drew the Moon with mountains, craters, and shadows. He calculated the heights of some lunar mountains from the length of their shadows near the terminator — a technique any observer with a small telescope can still repeat tonight — and got answers within 20% of modern values.

A world with mountains and valleys was another world, not an ethereal glowing disk. The distinction between 'earthly' and 'heavenly' matter — foundational to Aristotelian physics — was already in trouble.

Jupiter's four moons

On the night of January 7, 1610, Galileo noticed three small 'stars' in a line with Jupiter. He watched them for the next several nights. They didn't stay put — they moved with respect to Jupiter, sometimes disappearing behind or in front of it. Within a week he understood: they were satellites orbiting Jupiter itself.

That was the killer discovery. If Jupiter had moons orbiting it, then not everything in the universe orbited Earth. Jupiter was a planet with its own miniature system — a working analog of the Copernican Sun-and-planets. Any argument that 'Earth cannot move because the Moon would be left behind' evaporated: Jupiter clearly does the same thing at speed, dragging four moons with it.

The phases of Venus

Venus was harder to see. Galileo pointed the telescope at it in the fall of 1610, and over several months watched it shift from a nearly full disk (small, on the far side of the Sun) to a crescent (larger, near Earth on the near side). This is the exact pattern Copernican theory predicts and Ptolemaic theory forbids. In the Ptolemaic system, Venus is always between us and the Sun, so it should only ever show crescent phases.

Venus's full range of phases is arguably the most decisive single observation in the geocentric-heliocentric argument. Galileo wrote the discovery to Kepler in an anagram (a common way to time-stamp a discovery without revealing it prematurely), and published in 1611.

Sunspots and the Milky Way

Two more observations rounded out the picture. Galileo (and, independently, Thomas Harriot in England) saw sunspots on the surface of the Sun — another blemish on a supposedly perfect celestial body — and tracked their motion to derive the Sun's rotation period at about 27 days.

And he turned the telescope on the Milky Way, which had always looked like a milky glow to the naked eye. What he saw resolved into countless individual stars — the first hint that the universe was much more populated than anyone had suspected.