Five thousand years of looking up

Newgrange waits for the midwinter sunrise

Farmers in Ireland’s Boyne Valley raised the great mound of Newgrange around 3200 BC, before the stones of Stonehenge and the pyramids of Giza. Above its entrance they left a narrow opening, the roof-box. For a few mornings around the winter solstice, light from the rising Sun slips through it, runs along the 19-metre passage and lights the floor of the inner chamber for about a quarter of an hour. Nobody wrote down why. But the builders clearly knew where the Sun rose on the shortest day, and built that knowledge into stone.

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Sky: Newgrange, Ireland, 12 January 3000 BC (Julian calendar): sunrise on the winter solstice (the sky map reaches back to 3000 BC).

The Great Pyramid points to true north

The sides of King Khufu’s pyramid at Giza, finished around 2560 BC, face the four cardinal directions to within a few arcminutes — far better than any magnetic compass could manage. Egyptian surveyors most likely took north from the sky, perhaps by watching stars circle the pole or by marking the shadow of an upright pole. No bright star sat exactly at the pole then. The nearest was Thuban in Draco, which the slow wobble of the Earth’s axis has since carried far away; today the pole belongs to Polaris.

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Sky: Giza, Egypt, 1 February 2560 BC (Julian calendar): the northern sky, with Thuban near the pole.

Stonehenge frames the solstice Sun

On Salisbury Plain the great sarsen stones of Stonehenge were raised around 2500 BC. The monument is laid out along a single axis: north-east towards the midsummer sunrise and, the other way, south-west towards the midwinter sunset. Many archaeologists now think the winter sunset mattered more, because the people who built it seem to have gathered for feasts nearby in midwinter. Either way, its builders followed the Sun’s turning points along the horizon, the backbone of any farming calendar. Crowds still meet the midsummer sunrise there every June.

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Sky: Stonehenge, England, 14 July 2500 BC (Julian calendar): sunrise on midsummer morning.

Babylon keeps a diary of Venus

Tablet 63 of the Babylonian omen series Enūma Anu Enlil lists the days on which Venus vanished into the Sun’s glare and reappeared, through the 21-year reign of King Ammisaduqa. It is one of the oldest records of planetary observations anywhere. The surviving copies were made about a thousand years later; the best known is in the British Museum. Because Venus repeats its cycle almost exactly every eight years, historians still use the tablet to argue about the dates of the reign — and of early Mesopotamian history with it.

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Sky: Babylon, Iraq, 8 March 1640 BC (Julian calendar): Venus as the morning star before sunrise.

The Nebra sky disc

In 1999 treasure hunters with a metal detector dug up a bronze disc on the Mittelberg hill near Nebra in Germany; police recovered it in 2002. About 32 centimetres across, it is inlaid with gold: a Sun or full Moon, a crescent, scattered stars and a tight group of seven often taken for the Pleiades. Arcs added later along the rim span about 82°, the same angle through which sunset swings along the horizon between the solstices at that latitude. Buried around 1600 BC, it is often called the oldest concrete picture of the sky. Some researchers read it as a rule for keeping a lunar calendar in step with the Sun.

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Sky: Mittelberg near Nebra, Germany, 23 March 1599 BC (Julian calendar): a thin crescent Moon beside the Pleiades in spring twilight.

An eclipse stops a battle

Herodotus tells of a war in Asia Minor between the Lydians and the Medes that ended when day suddenly turned to night in the middle of a battle. Both armies took it as a sign and made peace. He adds that Thales of Miletus had foretold the year of the eclipse. Modern calculations match it to the total solar eclipse of 28 May 585 BC, which crossed today’s Turkey late in the afternoon. Whether Thales really predicted it, and how — perhaps from Babylonian eclipse cycles — is still argued over. It is one of the earliest events in history that can be dated to the day.

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Sky: Kızılırmak (Halys) valley, Turkey, 28 May 585 BC (Julian calendar): the eclipsed Sun low in the west.

Eratosthenes measures the Earth

Eratosthenes, chief librarian at Alexandria, heard that at noon on midsummer day the Sun shone straight down a well at Syene, today’s Aswan. At Alexandria on the same day a vertical stick still cast a short shadow: the Sun stood about 7° from overhead, a fiftieth of a full circle. So the distance between the two cities had to be a fiftieth of the Earth’s circumference. With roughly 5,000 stadia between them, he got 250,000 stadia. Depending on which stadion he used, that is between 2% and 16% off the true 40,000 km.

Noon at AlexandriaNoon at Syene

Sky: Alexandria, Egypt, 26 June 240 BC (Julian calendar): noon on midsummer day.
Sky: Aswan (Syene), Egypt, 26 June 240 BC (Julian calendar): noon on midsummer day, the Sun almost overhead.

Hipparchus notices the sky creeping

Working on Rhodes, Hipparchus compiled a catalogue of several hundred stars and compared his positions with ones recorded about 150 years earlier. The bright star Spica had moved by about two degrees relative to the autumn equinox. He concluded that the whole framework of equinoxes creeps slowly against the stars. We call it precession and know its cause: the Earth’s axis wobbles like a top, once in about 26,000 years. Hipparchus also built theories of the Sun and Moon good enough to predict eclipses, and is often credited with ranking stars by brightness.

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Sky: Rhodes, Greece, 20 April 130 BC (Julian calendar): Spica due south on a spring evening.

Two planets become one

On the evening of 17 June 2 BC, Venus and Jupiter, the two brightest planets, drew so close in the western sky — a few hundredths of a degree — that to most eyes they would have merged into a single brilliant point, near the star Regulus in Leo. Some writers have proposed this as the Star of Bethlehem. It is only one candidate among several, and it depends on dates for King Herod and the birth of Jesus that historians still debate. As a sight, though, it was genuinely rare.

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Sky: Jerusalem, 17 June 2 BC (Julian calendar): Venus and Jupiter in the western twilight.

Ptolemy writes the Almagest

In Alexandria, Claudius Ptolemy gathered Greek astronomy into one great book, the Mathematical Syntaxis. Arabic translators later called it al-Majisti, “the greatest”, and in Latin it became the Almagest. It describes an Earth-centred universe in which each planet rides a small circle, an epicycle, carried around a larger one — a clever device that reproduces the backward loops planets make near opposition. Its catalogue lists 1,022 stars in 48 constellations, most of which we still use. For some 1,400 years it was the standard textbook of the sky, from Byzantium and Baghdad to medieval Europe.

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Sky: Alexandria, Egypt, 7 February AD 150 (Julian calendar): Mars at opposition, high in the south at midnight.

Al-Sufi describes a “little cloud”

The Persian astronomer ʿAbd al-Raḥmān al-Ṣūfī, working at the Buyid court in Isfahan and Shiraz, revised Ptolemy’s star catalogue with his own estimates of brightness and drew every constellation twice: as it looks in the sky and as it appears on a globe. His Book of Fixed Stars, completed around 964, contains the earliest known description of the Andromeda Galaxy, which he called a little cloud. Many star names we use today, such as Aldebaran and Altair, reached Europe through Arabic books like his. A crater on the Moon, Azophi, carries his Latinised name.

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Sky: Isfahan, Iran, 25 October AD 964 (Julian calendar): the Andromeda Galaxy high on an autumn evening.

A guest star in Taurus

On 4 July 1054, astronomers of the Song court at Kaifeng reported a “guest star” near the star we call Zeta Tauri. For 23 days it was bright enough to see in daylight, and it stayed visible at night for almost two years. It was a supernova: a massive star exploding about 6,500 light-years away. The wreckage is the Crab Nebula, M1, which a small telescope shows as a faint grey smudge; at its heart spins a neutron star that flashes 30 times a second. That July the guest star rose in the east shortly before dawn.

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Sky: Kaifeng, China, 4 July 1054 (Julian calendar): the guest star rising before dawn.

Ulugh Beg’s giant sextant in Samarkand

Ulugh Beg, grandson of the conqueror Timur and ruler of Samarkand, was a scholar first. In the 1420s he built a three-storey observatory around a huge meridian arc, set into a trench in the hill, with a radius of about 40 metres. With it his astronomers timed the Sun, Moon, planets and stars as they crossed the meridian. Their catalogue of 1437 gave fresh positions for 1,018 stars, the first major catalogue since Ptolemy built on new measurements, and their length of the year was off by less than a minute. Ulugh Beg was murdered in 1449; the ruins were found again in 1908.

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Sky: Samarkand, Uzbekistan, 15 January 1437 (Julian calendar): Sirius crossing the meridian on a winter night.

Copernicus puts the Sun in the middle

Nicolaus Copernicus, a church canon in Frombork on the Baltic coast, spent decades on a Sun-centred model of the planets. His book On the Revolutions of the Heavenly Spheres was printed in Nuremberg in 1543, the year he died; a story says the first copy reached him on his deathbed. Setting the Earth in motion explained at a stroke why planets seem to loop backwards and why Mercury and Venus never stray far from the Sun. Copernicus kept perfect circles, so his predictions were no better than Ptolemy’s. The gain was the idea, and it changed everything.

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Sky: Frombork, Poland, 24 May 1543 (Julian calendar): Jupiter and Saturn in the late-May twilight.

Tycho’s new star

On the evening of 11 November 1572, at Herrevad Abbey in Scania, then part of Denmark, the young nobleman Tycho Brahe looked up and saw a brilliant star in Cassiopeia where none had been before. It soon rivalled Venus. He measured it for months, and it never shifted against the background stars, so it had to lie far beyond the Moon — in the region that followers of Aristotle held to be perfect and unchanging. His little book On the New Star made him famous. It was a supernova, roughly 8,000 to 10,000 light-years away.

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Sky: Herrevad Abbey, Sweden, 11 November 1572 (Julian calendar): the new star high in the north-east.

Kepler’s star in Ophiuchus

In October 1604 another new star flared up, this time in Ophiuchus, a few degrees from Jupiter, Saturn and Mars, which had gathered low in the evening sky. Observers in Italy noticed it on 9 October. Johannes Kepler, imperial mathematician in Prague, followed it from 17 October for a year and wrote a book about it. Like Tycho’s star it showed no parallax. It is the last supernova in our own galaxy seen with the naked eye; the next one anybody saw, in 1987, was in the Large Magellanic Cloud, a neighbouring galaxy.

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Sky: Prague, Czechia, 17 October 1604: the new star beside Jupiter, Saturn and Mars at dusk.

Galileo finds moons around Jupiter

On 7 January 1610, in Padua, Galileo Galilei turned a home-made telescope that magnified about 20 times towards Jupiter and saw three small “stars” in a line beside it. Night after night they changed places, and by 13 January he had found a fourth. They were moons circling Jupiter: proof that not everything goes around the Earth. In March he published The Starry Messenger, with the mountains of the Moon and the countless stars of the Milky Way. A pair of binoculars held steady can show the same four moons today.

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Sky: Padua, Italy, 7 January 1610: Jupiter in the east in the first hour of the night.

Huygens finds Titan and solves Saturn

Saturn puzzled the first telescope users: Galileo saw it with “ears” that later vanished. In The Hague, Christiaan Huygens and his brother Constantijn ground better lenses, and on 25 March 1655 he noticed a faint star beside Saturn that kept pace with it — Titan, Saturn’s largest moon. In 1659 he explained the ears too: Saturn is circled by a thin, flat ring that touches the planet nowhere. As Saturn travels its 29-year orbit we see the ring from changing angles, and twice per orbit it turns edge-on and almost disappears, most recently in 2025.

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Sky: The Hague, Netherlands, 25 March 1655: Saturn in the south-east on the evening of the discovery.

Rømer times the speed of light

Jupiter’s moon Io slips into Jupiter’s shadow every 42½ hours, a clock in the sky that astronomers hoped would help sailors find their longitude. At the Paris Observatory, the Danish astronomer Ole Rømer noticed that these eclipses came early when the Earth was nearer Jupiter and late when it was farther away. In 1676 he predicted that an eclipse due on 9 November would be about ten minutes late, and it was. Light, he argued, takes time to cross the Earth’s orbit. It was the first good evidence that light travels at a finite speed.

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Sky: Paris Observatory, France, 9 November 1676: Jupiter low in the south on 9 November 1676.

Newton’s Principia

Isaac Newton’s Mathematical Principles of Natural Philosophy, published in London on 5 July 1687 with Edmond Halley paying the printer, showed that one force rules a falling apple and the orbiting Moon. Gravity weakens with the square of the distance, and from that rule and his three laws of motion Newton derived Kepler’s laws of planetary orbits, the tides, the paths of comets and the slow wobble of the Earth’s axis. For the first time the heavens and the Earth obeyed the same physics.

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Halley’s comet comes back on time

Edmond Halley noticed that the comets of 1531, 1607 and 1682 had followed nearly the same path, and suggested they were one object returning every 75 or 76 years. He predicted its return for 1758 but did not live to see it. In Paris, Alexis Clairaut, Joseph Lalande and Nicole-Reine Lepaute worked out how Jupiter and Saturn would delay it. On Christmas night 1758 Johann Georg Palitzsch, a farmer and amateur astronomer near Dresden, found it. It rounded the Sun on 13 March 1759, a month earlier than the calculation said, but a triumph all the same: comets obey gravity too.

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Herschel finds a new planet

William Herschel was a German-born musician in Bath who built his own telescopes. On the night of 13 March 1781, sweeping the sky from his garden with a 6.2-inch reflector, he noticed a “star” in Taurus that showed a tiny disc rather than a point. Days later it had moved. He took it for a comet, but its orbit turned out to be nearly circular and twice as far from the Sun as Saturn’s. Uranus was the first planet found with a telescope, and it doubled the size of the known solar system. It is just visible to the naked eye under a dark sky.

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Sky: Bath, England, 13 March 1781: Uranus in Taurus on the night of the discovery.

Ceres turns up between Mars and Jupiter

On the first night of the 19th century, 1 January 1801, Giuseppe Piazzi at Palermo Observatory was checking star positions in Taurus when he found one that moved. He followed it for six weeks until it was lost in the Sun’s glare. The young mathematician Carl Friedrich Gauss then devised new methods to compute an orbit from so few observations, and at the end of the year the object was found again almost exactly where he said. Ceres orbits between Mars and Jupiter. Within six years three more such bodies turned up; William Herschel called them asteroids. Since 2006 Ceres has been classed as a dwarf planet.

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Sky: Palermo, Italy, 1 January 1801: Taurus, where Ceres was moving among the stars.

Bessel measures the distance to a star

For centuries the lack of any yearly shift in the stars’ positions — parallax — was used as an argument against a moving Earth. The shift is real, just tiny. In 1838 Friedrich Wilhelm Bessel at Königsberg announced that 61 Cygni, a faint star racing unusually fast across the sky, sways back and forth by about a third of an arcsecond each year. That put it roughly ten light-years away; the modern figure is 11.4. At about the same time Thomas Henderson and Friedrich Struve reported parallaxes for Alpha Centauri and Vega. The distance to the stars had been measured at last.

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Sky: Königsberg (Kaliningrad), 1 October 1838: Cygnus overhead on an autumn evening.

Neptune is found where the mathematics said

Uranus kept drifting off its predicted path, as if something unseen were tugging at it. In Paris, Urbain Le Verrier calculated where that something must be and sent the position to Johann Galle at the Berlin Observatory. On the night of 23 September 1846, the day the letter arrived, Galle and the student Heinrich d’Arrest compared the sky with a new star chart and found an uncharted “star” within about a degree of the prediction. It was Neptune. In England John Couch Adams had made a similar calculation, and James Challis at Cambridge had even recorded the planet that summer without noticing.

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Sky: Berlin, Germany, 23 September 1846: Neptune in Aquarius near midnight.

The first photograph of a star

Photography was barely a decade old when, on the night of 16–17 July 1850, the photographer John Adams Whipple and William Cranch Bond, director of Harvard College Observatory, pointed the 15-inch Great Refractor at Vega and exposed a daguerreotype plate for about 100 seconds. The result was a small dot, but it was the first star other than the Sun ever caught in a photograph. Within a few decades photographic plates had replaced the eye for measuring the sky: they could soak up faint light for hours and kept a permanent record.

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Sky: Cambridge, Massachusetts, 16 July 1850: Vega almost overhead.

An eclipse puts Einstein to the test

Einstein’s general theory of relativity predicted that the Sun’s gravity bends starlight passing close to it — by twice as much as Newton’s physics allowed. The only chance to see stars right beside the Sun was a total eclipse. On 29 May 1919 the eclipsed Sun stood in front of the bright Hyades cluster in Taurus, and British teams photographed it from Sobral in Brazil and the island of Príncipe off West Africa. The stars appeared shifted by about the amount Einstein predicted. When the result was announced that November, he became world-famous almost overnight.

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Sky: Sobral, Brazil, 29 May 1919: totality, with the Hyades around the Sun.

Hubble: other galaxies, and an expanding universe

In October 1923, on a plate of the Andromeda “nebula” taken with the 100-inch Hooker telescope on Mount Wilson, Edwin Hubble found a Cepheid — a star whose rhythm of brightening reveals its true brightness, as Henrietta Leavitt had discovered. It proved that Andromeda lies far outside the Milky Way: it is a galaxy in its own right. In 1929, combining distances like this with speeds measured by Vesto Slipher, Hubble showed that the farther away a galaxy is, the faster it recedes. Georges Lemaître had reached the same result in 1927: the universe is expanding.

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Sky: Mount Wilson, California, 6 October 1923: the Andromeda Galaxy near the zenith.

Clyde Tombaugh finds Pluto

At Lowell Observatory in Flagstaff, Arizona, 24-year-old Clyde Tombaugh spent months comparing pairs of photographic plates taken days apart, flicking between them in a blink comparator to catch anything that moved. On 18 February 1930, on plates from 23 and 29 January, a faint dot in Gemini jumped. The discovery was announced on 13 March, and an 11-year-old in Oxford, Venetia Burney, suggested the name Pluto. In 2006 the International Astronomical Union reclassified it as a dwarf planet, one of many icy worlds in the Kuiper belt.

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Sky: Flagstaff, Arizona, 23 January 1930: Gemini, where Pluto was hiding (it is far too faint to show).

Sputnik opens the space age

On 4 October 1957 the Soviet Union launched Sputnik 1 from the steppe of Kazakhstan, at the site later known as Baikonur. It was a polished metal ball 58 centimetres across with four long antennas, and it circled the Earth every 96 minutes, beeping on frequencies any radio amateur could pick up. People around the world went outside at dusk and dawn to watch for the rocket’s core stage, which also reached orbit and shone far more brightly than the little satellite. Three and a half years later Yuri Gagarin became the first person in space.

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Sky: Baikonur, Kazakhstan, 4 October 1957: the night sky at the moment of launch.

Jocelyn Bell finds the pulsars

Jocelyn Bell, a 24-year-old research student at Cambridge, helped string 2,000 radio aerials across a field at Lord’s Bridge, then read their pen charts, some 30 metres of paper a day. In August 1967 she noticed a bit of “scruff” that kept coming back, and on 28 November a fast recording showed sharp pulses 1.337 seconds apart. They came from a neutron star — a collapsed stellar core as heavy as the Sun but only about 20 km across — spinning and sweeping a radio beam past the Earth like a lighthouse. Her supervisor shared the 1974 Nobel Prize; she was left out.

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Sky: Lord’s Bridge, Cambridge, England, 28 November 1967: Vulpecula, home of the first pulsar.

Apollo 11 lands on the Moon

On 20 July 1969 at 20:17 UTC the lunar module Eagle touched down in the Sea of Tranquility with Neil Armstrong and Buzz Aldrin aboard, while Michael Collins circled overhead in Columbia. About six and a half hours later Armstrong stepped onto the surface. The two men spent some two and a half hours outside, set up experiments — including a laser reflector that observatories still bounce light off to measure the Moon’s distance to within centimetres — and brought back 21.5 kg of rock and dust. In Houston it was mid-afternoon, and the Moon was up in the daytime sky.

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Sky: Houston, Texas, 20 July 1969: the daytime Moon at the moment of landing.

Voyager 2 reaches Neptune

Launched in 1977, Voyager 2 used a rare line-up of the outer planets to swing from Jupiter to Saturn, Uranus and finally Neptune. On 25 August 1989, twelve years after leaving Earth, it skimmed about 4,950 km above Neptune’s cloud tops. It found a huge storm, the Great Dark Spot, winds of more than 2,000 km/h, six new moons and dark plumes rising from the frozen surface of Triton. No spacecraft has been back to Uranus or Neptune since, so almost everything we have seen of them close up comes from this one visit.

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Sky: Pasadena, California, 24 August 1989: Neptune in Sagittarius, seen from Earth that night.

The Hubble Space Telescope

The Space Shuttle Discovery carried the Hubble Space Telescope into orbit on 24 April 1990. Above the blurring air its 2.4-metre mirror should have given razor-sharp images, but the first pictures were fuzzy: the mirror had been polished very precisely to the wrong shape, off by about two micrometres at the edge. Astronauts fitted corrective optics in December 1993. Hubble went on to photograph the Pillars of Creation, help pin down the age of the universe and, in the Hubble Deep Field of 1995, show thousands of galaxies in a speck of sky near the handle of the Big Dipper.

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Sky: Baltimore, Maryland, 18 December 1995: the Big Dipper, near the Hubble Deep Field, before dawn in December 1995.

A planet around another Sun-like star

On 6 October 1995 Michel Mayor and Didier Queloz of the University of Geneva announced a planet circling 51 Pegasi, a Sun-like star about 50 light-years away. They found it at the Haute-Provence Observatory in France from the star’s tiny wobble, a back-and-forth of about 50 metres per second. The planet was a shock: about half as massive as Jupiter, yet whipping round its star every 4.2 days, far closer than Mercury is to the Sun. Planets had been found around a pulsar in 1992, but never around a star like ours. Thousands are known now. Mayor and Queloz shared the 2019 Nobel Prize in Physics.

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Sky: Haute-Provence Observatory, France, 6 October 1995: Pegasus on the evening of the announcement.

New Horizons flies past Pluto

After nine and a half years in space, NASA’s New Horizons shot past Pluto on 14 July 2015 at about 14 km per second, some 12,500 km above the surface. A world that had been a fuzzy dot became a place: a heart-shaped plain of nitrogen ice called Sputnik Planitia, mountains of water ice several kilometres high, a blue haze, and a large moon, Charon, with a dark red pole. The data took more than a year to trickle home. On New Year’s Day 2019 the probe flew past Arrokoth, a small double-lobed body even farther out in the Kuiper belt.

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Sky: Laurel, Maryland, 13 July 2015: Sagittarius, where Pluto was, on the night before the flyby.

The first picture of a black hole

On 10 April 2019 the Event Horizon Telescope team showed a blurry orange ring around a dark centre: the shadow of the black hole in the galaxy M87, 55 million light-years away and about 6.5 billion times the mass of the Sun. To see something so small from so far, radio dishes from Hawaii and Arizona to Chile, Spain and the South Pole had observed together in April 2017, synchronised by atomic clocks into a virtual telescope as wide as the Earth. In 2022 the team released a picture of Sagittarius A*, the black hole at the centre of our own galaxy.

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Sky: Maunakea, Hawaii, 10 April 2017: M87 high over Maunakea during the April 2017 observations.

Webb’s first images

The James Webb Space Telescope, launched on Christmas Day 2021, unfolded its 6.5-metre gold-coated mirror and a sunshield the size of a tennis court on the way to its post 1.5 million km from Earth. On 12 July 2022 the first full-colour images came out: the “Cosmic Cliffs” of the Carina Nebula, the Southern Ring Nebula, the galaxy group Stephan’s Quintet and, shown the evening before, a deep field crowded with distant galaxies. Webb sees mainly in infrared, so its colours are translated from light our eyes cannot detect.

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Sky: Santiago, Chile, 12 July 2022: the Carina Nebula on the evening of the release.

Totality across North America

On 8 April 2024 the Moon’s shadow raced from Mexico’s Pacific coast across Texas and the eastern United States into Canada. Along a path up to about 200 km wide, day turned to deep twilight for as long as four and a half minutes: the Sun’s corona appeared, pink prominences flared at the Moon’s edge and planets came out at midday. Tens of millions of people lived inside the path, and many more travelled to it. The next total eclipse seen from the contiguous United States comes on 23 August 2044.

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Sky: Dallas, Texas, 8 April 2024: totality over Dallas.

Rubin Observatory’s first look

On 23 June 2025 the Vera C. Rubin Observatory on Cerro Pachón in Chile released its first images: the Trifid and Lagoon nebulae glowing with gas and dust, and a vast field of galaxies in the Virgo Cluster. Its first test observations had also turned up more than 2,000 previously unknown asteroids. Its 8.4-metre telescope and 3,200-megapixel camera, the largest digital camera ever built, are designed to photograph the whole southern sky every few nights for ten years: a time-lapse film of the universe that catches anything that moves, flickers or explodes.

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Sky: Cerro Pachón, Chile, 23 June 2025: the Lagoon and Trifid nebulae overhead that night.