c. 747 BCEBabylon's nightly diaries begin
Babylonian scribes began systematic astronomical diaries — nightly positions of the Moon and planets, alongside weather, river levels and market prices — that continued for roughly seven centuries, the longest unbroken run of scientific observation ever made. From such records they learned to predict eclipses with the 18-year Saros cycle, and Ptolemy anchored his astronomy to their eclipse observations from 747 BCE onward.
c. 129 BCEHipparchus catalogs the sky
On Rhodes, Hipparchus completed a catalog of about 850 stars with positions and brightnesses on the six-step magnitude scale still in use today. Comparing his measurements with Greek and Babylonian records over a century older, he discovered precession — Earth's axis slowly cones around the sky over roughly 26,000 years — an effect amounting to barely a degree per human lifetime.
964Al-Sufi's Book of the Fixed Stars
Working at the Buyid court in Isfahan and Shiraz, the Persian astronomer Abd al-Rahman al-Sufi published a critical revision of Ptolemy's star catalog with his own magnitude estimates and Arabic star lore. It contains the first surviving record of the Andromeda galaxy, drawn as a "little cloud" — an object whose true nature would take until the 1920s to settle.
1543Copernicus moves the Earth
De revolutionibus orbium coelestium, printed in Nuremberg, put the Sun at the center and made Earth a planet. By the account of his friends, the first printed copy reached Copernicus in Frombork on the day he died, 24 May 1543. The book's real revolution unfolded over the following seventy years in the hands of Tycho, Kepler and Galileo.
1610Galileo publishes the telescope's sky
In March 1610 Galileo's Sidereus Nuncius reported what his improved telescope had shown since January: mountains on the Moon, the Milky Way resolved into countless stars, and four moons circling Jupiter — bodies visibly orbiting something other than Earth. He named them the "Medicean stars" after Florence's ruling family and had secured a Medici court position within months.
1838Bessel measures a star's distance
At Königsberg, Friedrich Bessel measured the annual parallax of the star 61 Cygni: about a third of an arcsecond, placing it roughly ten light-years away. The measurement — which rivals Thomas Henderson and Wilhelm Struve nearly beat him to — ended a two-thousand-year argument over whether the distances to stars could ever be known.
1912Leavitt's law creates the cosmic ruler
From photographic plates of the Small Magellanic Cloud, Harvard computer Henrietta Swan Leavitt showed that a Cepheid variable star's pulsation period reveals its true brightness, turning Cepheids into distance markers. Every cosmic distance measured since, including Hubble's, stands on a relation published in a three-page circular issued under the observatory director's name.
1929The universe is expanding
Using Mount Wilson's 100-inch telescope, redshifts largely measured by Vesto Slipher and Milton Humason, and Leavitt's law for distances, Edwin Hubble showed that galaxies recede at speeds proportional to their distance. Georges Lemaître had derived the same law theoretically in 1927 in a French-language Belgian journal; in 2018 the IAU voted to rename it the Hubble–Lemaître law.
1967Bell Burnell finds the pulsars
On 28 November 1967, Cambridge PhD student Jocelyn Bell Burnell confirmed that a quarter-inch of recurring "scruff" on her radio survey's chart paper was a source pulsing every 1.337 seconds — the first pulsar, a spinning neutron star. The 1974 Nobel Prize for the discovery went to her supervisor Antony Hewish, an omission still argued about.
1995A planet around another sun
On 6 October 1995, at a conference in Florence, Michel Mayor and Didier Queloz announced 51 Pegasi b, a Jupiter-mass planet racing around its star every 4.2 days, detected from the star's tiny wobble at the Haute-Provence observatory in France. More than 5,000 exoplanets have followed, and the pair shared the 2019 Nobel Prize in Physics.
