Sooner or later every child does it. On a clear night, away from the street lights, you tip your head back and the sky looks back: thousands of stars, a pale band of milky light, and now and then a streak of fire that is gone before you can point at it.
People have stood exactly like this for tens of thousands of years. The sky has barely changed in that time. What changed are the questions we learned to ask about it, and the tools we built to answer them. This page is the short version of that story. The other astronomy pages on Gears & Stars each take one idea from it and let you play with it.
Before writing · c. 40,000–1000 BC
The sky as a calendar
The first astronomers were hunters, herders and farmers who needed to know when: when the herds would move, when the river would flood, when to plant. The sky was the only clock that never broke. Some notched bones from the Ice Age may be tallies of the Moon's phases.
Later, people built the calendar into the landscape. Stonehenge, in England, lines up with the sunrise at midsummer and the sunset at midwinter. The Nebra sky disc, buried in Germany around 1600 BC, shows the Sun, a crescent Moon and the Pleiades star cluster in bronze and gold. It is one of the oldest known pictures of the sky.
Mesopotamia · c. 1700–300 BC
Writing it down
In Babylon, scribes watched the sky every night for centuries and pressed what they saw into clay tablets. One tablet lists the dates when Venus appeared and disappeared over 21 years. With records that long, patterns start to show.
The Babylonians found that eclipses repeat in a cycle of about 18 years, now called the Saros, and they could predict them without knowing what the Sun and Moon actually were. Their counting in sixties is still with us: it is why a circle has 360 degrees and an hour has 60 minutes.
Ancient Greece · c. 600 BC – AD 150
Geometry meets the sky
The Greeks asked new questions: not only when, but why, and how big. Around 240 BC, Eratosthenes heard that at noon on midsummer day the Sun shone straight down a well in Syene, in southern Egypt. In Alexandria, further north, a vertical stick cast a short shadow at the same moment, at an angle of about 7 degrees.
Seven degrees is about one fiftieth of a full circle, so the Earth's circumference had to be about fifty times the distance between the two cities. His answer was close to the real value of 40,000 km. All it took was a stick, a shadow and some geometry.
Aristarchus suggested that the Earth goes around the Sun, but few were convinced. Hipparchus catalogued around 850 stars and discovered that the whole sky slowly wobbles, a motion that takes about 26,000 years. Ptolemy's Almagest, written around AD 150, put the Earth at the centre and moved the planets on circles riding on circles. It was wrong, but it predicted the planets well enough to be used for 1,400 years.
Gears and stars
In 1901, sponge divers found a lump of corroded bronze in a Roman-era shipwreck off the Greek island of Antikythera. X-rays later revealed at least 30 gear wheels inside. The Antikythera mechanism, built in the 2nd or 1st century BC, is a hand-cranked computer of the sky: it showed the positions of the Sun and Moon, the phase of the Moon, and when eclipses would happen. Nothing as complex survives from the next thousand years.
From Baghdad to Beijing · c. 800–1500
Keeping the lamp lit
While much of this knowledge was lost in Europe, astronomers in the Islamic world translated the Greek books, checked them against the sky and corrected them. Many star names we still use, like Aldebaran, Betelgeuse and Vega, come from Arabic. In 964, the Persian astronomer al-Sufi described a "little cloud" near the stars of Andromeda. It is the first written record of another galaxy.
In Samarkand, the ruler and astronomer Ulugh Beg built an observatory around 1420 with an instrument cut into the hillside, about 40 metres in radius, to measure the stars more precisely than ever before. Far to the east, Chinese astronomers kept careful records too. In 1054 they noted a "guest star" bright enough to see in daylight for more than three weeks. Today we point telescopes at the same spot and see the Crab Nebula, the wreck of that exploded star.
Europe · 1543–1687
The Earth starts to move
In 1543, Nicolaus Copernicus published a book that put the Sun at the centre. Tycho Brahe, without a telescope, then measured the planets more precisely than anyone before him. His assistant, Johannes Kepler, fought for years with Tycho's numbers for Mars until he gave up on perfect circles: planets move on ellipses, faster when they are close to the Sun and slower when they are far.
In 1609, Galileo Galilei pointed a telescope at the sky. He saw mountains on the Moon, four moons circling Jupiter, and Venus showing phases like the Moon, which is only possible if Venus goes around the Sun. Then in 1687 Isaac Newton showed that one force, gravity, explains both a falling apple and the orbit of the Moon. For the first time, the heavens followed the same rules as the Earth.
1700–1900
Measuring the distances
Newton's laws gave the shape of the solar system, but not its size. Edmond Halley found a way: watch Venus cross the face of the Sun from places far apart on Earth, time it, and the difference reveals the distance to the Sun. Expeditions sailed around the world for the transits of 1761 and 1769. James Cook's first voyage went to Tahiti for exactly this reason.
Try it yourself
Pick two observing stations, time the transit and work out the distance to the Sun the way the 18th-century astronomers did. The Transit of Venus
Halley also predicted that a comet would return in 1758. It did, after his death, and now carries his name. William Herschel found Uranus in 1781, the first planet discovered since antiquity. In 1846, Neptune was found almost exactly where mathematics said it had to be, worked out from its pull on Uranus.
In 1838, Friedrich Bessel finally measured the distance to a star: 61 Cygni is about 11 light years away, more than 700,000 times further than the Sun. At the same time, spectroscopes split starlight into rainbows and showed dark lines that revealed what stars are made of: the same elements as the Earth.
1900–1970
The universe gets bigger
In 1912, Henrietta Swan Leavitt found that some pulsing stars, the Cepheids, reveal their true brightness by how fast they pulse. That turned them into measuring sticks. In 1923, Edwin Hubble found one in the Andromeda "nebula" and showed that it lies far outside the Milky Way. Andromeda was a galaxy of its own, and there were countless more.
By 1929, Hubble and others showed that galaxies are moving away from us, and the further away they are, the faster they go: the universe is expanding. Run the film backwards and everything starts in a hot, dense beginning, the Big Bang. In 1965, Arno Penzias and Robert Wilson found its afterglow by accident, as a faint hiss in their radio antenna that came from every direction.
From weapon to observatory
After the war, captured V-2 rockets were fired straight up from White Sands, New Mexico, carrying instruments instead of explosives. In 1946 they recorded the Sun's ultraviolet light, which never reaches the ground, and took the first photographs of the Earth from space, from about 105 km up. The same technology led to the rockets that put Sputnik in orbit in 1957 and people on the Moon in 1969. How the V-2 worked
Today
What we know now
Astronomers now work with telescopes in orbit, on mountain tops and even buried in Antarctic ice. We have close-up pictures of every planet, and Voyager 1, launched in 1977, has left the Sun's bubble of solar wind altogether. Since 1995 we have found more than 6,000 planets around other stars. In 2015 we detected, for the first time, ripples in space itself from two black holes colliding, and in 2019 we saw the shadow of one. The James Webb Space Telescope now sees galaxies whose light left them more than 13 billion years ago.
That last number is the humbling one. The rest is dark matter and dark energy, and nobody knows what they are. Some of the big open questions:
- What are dark matter and dark energy?
- Is there life anywhere else, even a microbe?
- What happened in the very first instant, and was there a "before"?
- How did the first stars and galaxies form so quickly after the Big Bang?
Every generation in this story thought it was close to the full picture. The sky keeps proving otherwise, which is the best reason to keep looking up.
Start with one of the great expeditions of the 18th century: measuring the solar system with a black dot on the Sun.
The Transit of Venus