Starry Night

Ptolemy writes the sky down

Most northern constellations were invented in Mesopotamia and later by the Greeks. The Babylonian text MUL.APIN, probably compiled around 1000 BC, already lists stars and constellations. Around AD 150 Claudius Ptolemy summed them up in the Almagest: 1,022 stars in 48 constellations. He put the Earth at the centre of the universe, and the world believed him for more than 1,200 years.

Leo is one of the oldest constellations of all. The Greeks saw in it the Nemean lion, killed by Heracles as the first of his twelve labours.

Copernicus moves the Sun to the centre

Nicolaus Copernicus’s book On the Revolutions of the Heavenly Spheres is printed in Nuremberg. In it the planets orbit the Sun and the Earth spins on its own axis. It appeared in the year of its author’s death. In 1616 the Church placed it on the Index of Forbidden Books.

Tycho and Kepler in Prague

The Danish astronomer Tycho Brahe measured the positions of stars and planets without a telescope, yet with unprecedented accuracy. In 1599 he came to Prague, to the court of Rudolf II. A year later Johannes Kepler joined him. Tycho died in Prague on 24 October 1601 and is buried in the Church of Our Lady before Týn. Kepler inherited his Mars measurements and, after ten years of calculation, published Astronomia nova in 1609: planets move in ellipses.

Tycho lost part of his nose in a duel in 1566 and wore a prosthetic one for the rest of his life.

Galileo points a telescope at the sky

On 13 March 1610 the Starry Messenger is published in Venice. In it Galileo Galilei describes what he saw through an instrument magnifying twenty times: mountains on the Moon, a host of stars invisible to the eye and four moons orbiting Jupiter. He first spotted them on 7 January 1610. So not everything revolves around the Earth.

Newton explains what holds it together

Isaac Newton publishes the Principia, with the laws of motion and the law of universal gravitation. Kepler’s ellipses fall out of them naturally. The same force that pulls an apple to the ground keeps the Moon in orbit.

Edmond Halley paid for the printing out of his own pocket. The Royal Society could no longer afford his salary, so it paid him in unsold copies of a book on the history of fishes.

A musician discovers a planet

On 13 March 1781 William Herschel, a musician and composer in Bath, England, noticed an object he first took for a comet. It was Uranus, the first planet discovered since antiquity. His observations were written down by his sister Caroline, who discovered several comets herself. Four years later Herschel was the first to propose a shape for the Milky Way based on measurement: a disc.

The first measured distance to a star

Friedrich Bessel measured the parallax of the star 61 Cygni in Cygnus, its tiny apparent shift caused by the Earth’s journey around the Sun. He got 10.3 light-years. Today’s value is 11.4. For the first time, humanity knew how far away the stars really are.

Henrietta Leavitt finds a yardstick for the universe

At Harvard College Observatory she measured the brightness of stars on photographic plates. In the variable stars known as Cepheids she noticed that the brighter they are, the more slowly they pulse. The rhythm therefore reveals true luminosity, and from that distance, far beyond the reach of parallax. She died in 1921. Three years later a Swedish mathematician tried to nominate her for the Nobel Prize and only then learned she was no longer alive.

Hubble makes the universe bigger. Twice

In 1923 Edwin Hubble found a Cepheid in the Andromeda nebula and wrote “VAR!” beside the mark on his photographic plate. Using Leavitt’s rule he worked out that the star lay far beyond the edge of the Milky Way. The nebula was a galaxy in its own right. In 1929 he published the relation between the distance of galaxies and the speed at which they recede: the universe is expanding. The Belgian Georges Lemaître had got there two years earlier.

The hiss left over from the Big Bang

In 1964 Arno Penzias and Robert Wilson picked up a faint hiss on an antenna at Holmdel in the United States. It came from every direction and would not go away. In July 1965 their paper appeared alongside an explanation by colleagues: it is radiation left over from the hot beginning of the universe. In 1978 they received the Nobel Prize.

At first they suspected the pigeons nesting in the antenna. They evicted them, cleaned out the droppings, and the hiss stayed.

The first planet around another sun

In October 1995 Michel Mayor and Didier Queloz announced the planet 51 Pegasi b, a gas giant comparable to Jupiter. They found it at the Haute-Provence Observatory in southern France, in the region where van Gogh painted his stars. Its parent star lies in Pegasus, just over 50 light-years from us. In 2019 they received the Nobel Prize.

A telescope beyond the Moon

The James Webb Space Telescope launched on 25 December 2021 and works 1.5 million km from Earth. Its first full-colour images were released on 12 July 2022. In them, a patch of sky the size of a grain of sand held at arm’s length contains thousands of galaxies. The old question remains: what else are we not seeing?