The telescope revolution

A new eye on the sky

Many children today can tell you that the universe began with a big bang 13.8 billion years ago, and that stars, planets, and galaxies formed afterward. That picture is so familiar that it is hard to imagine what the sky meant four hundred years ago. Until the early 1600s astronomers had only their eyes, so their best tools were built big, for precision. At Dengfeng (登封) in China, the tower that the astronomer Guo Shoujing (郭守敬) built around 1276 is a giant sundial pin, casting the sun’s shadow onto a stone scale more than thirty meters long. In Europe, Tycho Brahe measured the planets with huge brass arcs, but still with the naked eye.

The first revolution in astronomy came from a spectacle-maker’s shop. In 1608 Hans Lipperhey, in the Netherlands, asked for a patent on a tube for seeing far, and within months spyglasses were on sale in Paris. In 1609 Galileo Galilei, a mathematics professor at Padua, heard of it, built one of his own, and pushed it to about twenty times. He was not the inventor, nor the first to turn a telescope on the sky: that summer the Englishman Thomas Harriot drew the moon through one, but never published the drawing. Galileo was the first to publish what the telescope showed. His little book Sidereus Nuncius, “The Starry Messenger,” appeared in March 1610 and changed what people thought the universe was.

Four things Galileo saw

The moon is rough. For almost two thousand years the heavens had been thought perfect, and the moon a flawless polished sphere. Through the telescope Galileo saw a surface “rough and uneven,” with mountains and round hollows, and bright specks in the dark part: peaks catching the sunrise. The moon goddess, as one joke puts it, turned out to have a bad case of acne. The moon looked like a place, not a lamp.

Jupiter has moons. On the night of January 7, 1610, in Padua, Galileo noticed three small “stars” in a strangely straight line beside Jupiter. Over the next nights they moved, but always stayed close to the planet, and by January 13 there were four. He had found bodies circling something other than the earth. Not everything in the sky turns around us.

Venus has phases. From the autumn of 1610, now in Florence, Galileo watched Venus change from a small, nearly full disk to a large crescent, much as the moon does. In Ptolemy’s earth-centered system, Venus always stays between the earth and the sun, so it could never look more than a crescent. A nearly full Venus is possible only if Venus sometimes goes behind the sun, so it orbits the sun. This was the first telescopic proof that a planet circles the sun, and strong support for the sun-centered theory (日心说 rìxīnshuō), though not yet proof that the earth moves.

The Milky Way is stars. The pale band the Chinese call the Silver River (银河 yínhé) broke up under the telescope into countless separate stars, too faint and too many to see by eye. In one small corner of Orion he found more than five hundred new stars. The sky held far more than anyone had counted.

Counting the stars

If the Milky Way is a swarm of stars, what shape is the swarm, and where are we in it? The first answer came in 1785 from William Herschel, a German-born musician living in England who built the best telescopes of his day. His idea was simple: count the stars. With his sister Caroline writing down the numbers, he counted the stars in 683 small patches in a ring around the sky. If the stars are scattered evenly and the telescope reaches the edge of the swarm, then the more stars you see in a direction, the farther the swarm goes that way. The counts were far higher along the Milky Way, so the swarm had to be flat. His map, the first drawn from observation, was a flattened slab with ragged edges: right to be flat, wrong in size and outline, and with the sun near the middle. If we imagine the Milky Way as Beijing, then in Herschel’s map the sun lives in the Forbidden City.

The sun is not in the middle, and the reason is instructive. Herschel’s telescope did not reach the edge. Clouds of interstellar dust dim distant stars so strongly that, in the plane of the disk, we can see only a small part of the way across it. Within that neighborhood the Milky Way looks about equally deep in every direction along its band. (In 1817 he admitted that his telescope “could not fathom” it.) The bias was in the sky itself. Try it below.

The man who moved the sun out of the center was the American astronomer Harlow Shapley. In 1918, using the 60-inch telescope on Mount Wilson, he mapped 69 globular clusters, tight round balls of hundreds of thousands of stars that hang around the Milky Way (about 150 are known today). They are bright enough to see across the whole galaxy. Their distances came from pulsing stars, with a new ruler from a woman at the Harvard Observatory.

Shapley found that the clusters gathered around a point toward the constellation Sagittarius, about 50,000 light-years away by his estimate. That point, he argued, was the center of the galaxy, and the sun was nowhere near it. His distance was about twice too large, partly because he too had not allowed for dust; a 2021 measurement puts the center about 27,000 light-years away, and the sun in a small side arm, among a hundred billion stars or more. But the main point stood. Copernicus had moved the center from the earth to the sun; now the sun was not even the center of its own galaxy.

Island universes

Then, in this course’s telling, Shapley made Herschel’s kind of mistake, one size up. Telescopes showed fuzzy patches of light called nebulae, “clouds.” Some were gas clouds or star clusters inside the Milky Way. Others were spirals, with no single stars visible. In 1755 the philosopher Immanuel Kant had guessed that such patches might be other Milky Ways; a century later the idea got its English name, “island universes,” in a Cincinnati astronomy magazine. Shapley thought not: for him our galaxy was essentially the whole universe, and the spirals were not galaxies like ours. Heber Curtis took the island-universe side, and on April 26, 1920, at the Smithsonian in Washington, the two presented opposing cases in a famous “Great Debate.” Astronomers were divided, and bad data propped up both sides: Adriaan van Maanen reported, wrongly, that the spirals visibly turned from year to year, which would make them small and near. Neither man convinced the other. Arguing, as this course puts it, cannot settle a scientific question; observation does.

The observation came on Mount Wilson, above Pasadena, California. On the night of October 5–6, 1923, Edwin Hubble photographed the Andromeda nebula with the 100-inch Hooker telescope, then the largest in the world, and marked a new speck “N,” for nova. Checking older plates, he found that it brightened and faded on a regular beat: a Cepheid, one of Leavitt’s pulsing stars. He crossed out the N and wrote “VAR!” Its period gave its true brightness, and its faintness then gave its distance: nearly a million light-years by his estimate, far beyond even Shapley’s giant Milky Way. Andromeda was a galaxy of its own. Later corrections to the Cepheid ruler, starting with Walter Baade’s in 1952, put it about 2.5 million light-years away, and recent estimates give the observable universe hundreds of billions of galaxies.

Look at the pattern, as this course reads it. First the earth lost the center, then the sun, and then the galaxy. Each time, better observation did more than reveal new objects: it removed a flattering assumption about our own place. Astronomers now call this the Copernican principle: do not assume that our place in the universe is special.