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Small facts
132 things worth knowing, each with a link to where it comes from. Most were measured. Where a figure was worked out by arithmetic, or comes from a model that nobody can check directly, it says so.
The Sun
Sunlight takes 8 minutes 19 seconds to reach the Earth. If the Sun went out, nobody would know for that long.
Worked outNASA/JPL astrodynamic parameters
The Sun turns about 4 million tonnes of its mass into light every second, and has done for 4.6 billion years. It has barely made a dent.
Worked outNASA NSSDC Sun fact sheet
The visible surface of the Sun is at 5,772 K, about 5,500 °C.
The centre of the Sun is at about 15.7 million K. Nobody has measured it directly: it comes from models that are checked against the Sun's vibrations and the neutrinos it gives off.
From a modelNASA NSSDC Sun fact sheet
The Sun is 109 Earths wide, and about 1.3 million Earths would fit inside it.
Worked outNASA NSSDC Sun fact sheet
The Sun does not turn in one piece. Its equator goes round in about 25 days and the regions near its poles in about 36.
Gravity at the Sun's visible surface is 274 m/s², 28 times what you feel on Earth.
The Sun's outer atmosphere, the corona, reaches about 2 million °C, hundreds of times hotter than the surface underneath. Why is still not fully explained.
The Sun and everything round it circle the centre of the Milky Way at about 830,000 km/h. One lap takes about 230 million years.
Worked outReid and others 2019, Astrophysical Journal 885, 131
Voyager 1 left the bubble blown by the solar wind in August 2012, about 121 au from the Sun, after 35 years of travel.
The Earth and the Moon
The Moon averages 384,400 km away, but swings between about 363,300 and 405,500 km.
The Moon is moving away from the Earth by nearly 4 cm a year, about as fast as fingernails grow.
Gravity on the Moon is 1.62 m/s², one sixth of the Earth's.
The Moon is 27% as wide as the Earth. No other planet has a moon so large compared with itself.
Worked outNASA NSSDC Moon fact sheet
The far side of the Moon is not dark. It gets as much sunlight as the near side. It was first seen in 1959, in photographs from the Soviet probe Luna 3.
Twelve people have walked on the Moon, all between July 1969 and December 1972. They brought back 382 kg of rock and soil.
With no air to spread the heat, the Moon's surface reaches about 127 °C in full sunlight and falls to about -173 °C in the dark.
The Moon orbits the Earth in 27.3 days but takes 29.5 days to go from one full Moon to the next, because the Earth has moved on round the Sun in the meantime.
The Sun is about 400 times wider than the Moon and about 400 times further away, so they look the same size. That match is why total eclipses of the Sun are possible.
Worked outNASA, eclipses
The Earth goes round the Sun at 29.8 km/s, about 107,000 km/h. You have travelled about 2.5 million km since this time yesterday.
Worked outNASA NSSDC Earth fact sheet
The Earth turns once in 23 hours 56 minutes 4 seconds. The other four minutes of the day are the extra turn needed to face the Sun again after moving along its orbit.
The Earth is nearest the Sun in early January and furthest in early July. Seasons come from its tilt, not its distance.
The Earth is the densest planet in the Solar System, at 5,513 kg per cubic metre: five and a half times water.
The Earth's spin makes it bulge. It is 43 km wider across the equator than from pole to pole.
Worked outNASA NSSDC Earth fact sheet
The Earth and Moon both circle a shared balance point. It lies inside the Earth, about 4,670 km from the centre, so the Earth wobbles round it once a month.
Worked outNASA NSSDC Moon fact sheet
Mercury, Venus and Mars
Mercury spins three times for every two trips round the Sun. As a result, one day there, from sunrise to sunrise, lasts 176 Earth days: two of its own years.
Mercury's ground reaches about 430 °C by day and drops to about -180 °C at night, the widest swing of any planet.
Mercury, the planet nearest the Sun, very probably has water ice. Radar and the MESSENGER spacecraft found the signs of it in craters at the poles whose floors never see sunlight.
Mercury is mostly core. Its metal centre fills about 85% of the planet's radius.
Mercury is the fastest planet, averaging 47.4 km/s, and goes round the Sun in 88 days.
Venus is the hottest planet, at about 464 °C over its whole surface, day and night. It is hotter than Mercury despite being nearly twice as far from the Sun.
The air pressure on the surface of Venus is 92 times the Earth's, the same as being about 900 metres down in the sea.
Venus takes 243 Earth days to spin once and 225 to go round the Sun, so it turns more slowly than it orbits. It also spins backwards.
Venus is the brightest thing in the sky after the Sun and Moon. Its unbroken cloud reflects about three quarters of the sunlight that reaches it.
Olympus Mons on Mars is the largest volcano known on any planet. Its summit is about 21 km above the average level of Mars's surface, nearly two and a half times the height of Everest above the sea.
Valles Marineris on Mars is a canyon system more than 4,000 km long and up to 7 km deep. On Earth it would stretch across the United States.
A day on Mars, called a sol, lasts 24 hours 39 minutes 35 seconds.
Gravity on Mars is 3.73 m/s², 38% of the Earth's. Someone who weighs 70 kg on the bathroom scales at home would read 27 kg there.
Worked outNASA NSSDC Mars fact sheet
The air on Mars is 95% carbon dioxide and its pressure at the ground is less than 1% of the Earth's.
Mars is red because the iron in its dust has rusted.
The giant planets
Jupiter has the shortest day of any planet, turning once in 9 hours 56 minutes, even though it is eleven Earths wide.
Jupiter has 318 times the mass of the Earth, more than twice that of all the other planets put together.
Worked outNASA NSSDC planetary fact sheets
Jupiter's Great Red Spot is a storm wider than the Earth that has been watched for well over a century.
Jupiter's magnetic field traps a bubble so large that its tail stretches past the orbit of Saturn.
Jupiter is heavy enough that it and the Sun circle a point just outside the Sun's surface. Seen from another star, the Sun would appear to wobble once every 11.9 years.
Worked outNASA NSSDC Jupiter fact sheet
Saturn's average density is 687 kg per cubic metre, less than water. It is the only planet that light.
Saturn's main rings are about 280,000 km across but mostly only about 10 metres thick. A sheet of paper with the same proportions would be nearly 3 km wide.
A six-sided jet stream about 30,000 km across circles Saturn's north pole. It was first seen by Voyager in the early 1980s and was still there when Cassini left in 2017.
Saturn spins so fast, once in about 10.7 hours, that it is visibly squashed: about a tenth wider across the equator than from pole to pole.
Saturn has 292 known moons in NASA's list, more than all the other planets combined.
All four giant planets have rings. Saturn's are the only ones bright enough to see in a small telescope.
Uranus is tipped over by 97.8°, so it rolls round the Sun on its side. Each pole gets about 42 years of daylight and then 42 years of night.
Worked outNASA NSSDC Uranus fact sheet
Uranus has the coldest air measured on any planet, down to about -224 °C. It is colder than Neptune, which is much further from the Sun.
Uranus was the first planet found with a telescope. William Herschel spotted it from his garden in Bath in 1781.
Only one spacecraft has ever visited Uranus or Neptune: Voyager 2, which flew past Uranus in 1986 and Neptune in 1989.
Neptune has the fastest winds measured on any planet, more than 2,000 km/h.
Neptune was found by arithmetic. Its position was worked out from its pull on Uranus, and in 1846 it was seen within a degree of where the sums said to look.
Neptune takes 165 years to go round the Sun. It finished its first full orbit since being discovered in 2011.
Sunlight at Neptune is about 900 times weaker than on Earth. Noon there is like a dim twilight here.
Worked outNASA, Neptune facts
Moons
NASA's list holds 460 moons of the planets and Pluto. Saturn has 292, Jupiter 115, Uranus 29, Neptune 16, Pluto 5, Mars 2 and the Earth 1. Mercury and Venus have none.
Of the 460 known moons, 391 have been found since the year 2000. Nearly all of those are a few kilometres across and were picked up as faint dots by large telescopes.
About two thirds of the known moons of the four giant planets go round backwards, against their planet's spin. They are captured strays, not moons that formed in place.
The most distant moon known is a small body called S/2021 N 1. It circles Neptune about 50 million km out, a third of the distance from the Earth to the Sun, and takes 27 years to go round once.
Galileo found the four large moons of Jupiter in January 1610. They were the first things ever seen to orbit something other than the Earth.
Ganymede is the largest moon in the Solar System, 5,262 km across. It is wider than the planet Mercury, though less than half as heavy.
Ganymede is the only moon known to make its own magnetic field.
Io is the most volcanic world known, with hundreds of volcanoes. Some throw lava fountains dozens of kilometres high.
Io goes round Jupiter in 1.77 days, Europa in 3.55 and Ganymede in 7.16: each takes almost exactly twice as long as the one before. The regular tugs that result are what keep Io's inside molten.
Under its shell of ice Europa is thought to hold a salt-water ocean with about twice as much water as all the Earth's oceans together.
From a modelNASA, Europa
Callisto has the most heavily cratered surface known. Almost nothing has smoothed it over in four billion years.
Titan is the only moon with a thick atmosphere. The air is mostly nitrogen, like ours, and the pressure at the ground is about one and a half times the Earth's.
Titan is the only place besides the Earth known to have rivers, lakes and seas on its surface. They are liquid methane and ethane, at about -179 °C.
Enceladus is only 504 km across, yet it sprays jets of water into space from cracks near its south pole. The Cassini spacecraft flew through them and found salt and simple organic chemicals.
The crater Herschel on Mimas is 130 km wide, a third of the width of the moon itself. A much bigger impact would probably have broken Mimas apart.
Iapetus has one face as dark as coal and the other as bright as snow. The leading side sweeps up dark dust as the moon moves along its orbit.
Hyperion is about half empty space inside: its density is only 0.54 times that of water. It also tumbles as it orbits, with no fixed day.
Miranda, a moon of Uranus only 470 km across, has fault canyons as much as twelve times as deep as the Grand Canyon.
Triton is the only large moon that orbits backwards. It was almost certainly a world like Pluto that Neptune captured.
Triton's surface is at about -235 °C, among the coldest measured anywhere in the Solar System. Even so, Voyager 2 saw geysers there throwing dark plumes 8 km up.
Phobos goes round Mars in 7 hours 39 minutes, faster than Mars turns, so it rises in the west. It is creeping inward by about 1.8 metres a century and will be torn apart or hit the ground in about 50 million years.
Charon is half as wide as Pluto. The two keep the same faces towards each other all the time, and the point they both circle is out in the space between them.
Dwarf planets, asteroids and comets
Ceres is the largest thing in the asteroid belt, 939 km across, and the only dwarf planet in the inner Solar System. It was the first asteroid found, on the first night of 1801.
The asteroid belt is nothing like the crowded field of rocks in films. All its asteroids together have less mass than the Moon, and spacecraft cross it without any need to steer.
Pluto takes 248 years to go round the Sun on a tilted, stretched orbit that carries it from 30 to 49 au. For twenty years of each orbit, most recently 1979 to 1999, it is nearer the Sun than Neptune.
The only close pictures of Pluto come from a single day. New Horizons flew past on 14 July 2015 after a journey of nine and a half years.
Pluto crosses inside Neptune's orbit but the two can never collide. Pluto goes round the Sun exactly twice for every three orbits of Neptune, which keeps them far apart.
Eris is a shade smaller than Pluto, 2,326 km across against 2,377, but about a quarter heavier. Finding it in 2005 is what forced astronomers to decide what counts as a planet.
Haumea spins once every four hours, faster than anything else its size, and the spin has pulled it into the shape of a rugby ball.
Sedna never comes nearer the Sun than 76 au, well over twice Neptune's distance, and swings out to about 1,000 au. One orbit takes more than 11,000 years.
Halley's Comet comes back about every 76 years. It was last near the Sun in 1986 and is due again in 2061.
A comet's tail does not trail behind it. It always points away from the Sun, so a comet heading outward travels tail first.
The first landing on a comet was in November 2014, when the European probe Philae came down on comet 67P/Churyumov-Gerasimenko. It bounced twice before it settled.
In September 2023 the OSIRIS-REx spacecraft brought 121.6 grams of the asteroid Bennu back to Earth, the largest asteroid sample collected so far.
The most distant world ever seen close up is Arrokoth, a 36 km body shaped like a flattened snowman. New Horizons passed it on 1 January 2019, about 6.5 billion km from the Sun.
Three objects have been caught passing through the Solar System from other stars: 'Oumuamua in 2017, comet Borisov in 2019 and comet 3I/ATLAS in 2025. All were moving too fast for the Sun to hold.
Most shooting stars are made by pieces of comet dust no bigger than a grain of sand, hitting the air at tens of kilometres per second.
Nobody has ever seen an object in the Oort cloud. Its existence is worked out from comets that arrive from every direction on orbits thousands of au long.
From a modelNASA, the Oort cloud
Beyond the Solar System
The nearest star to the Sun is Proxima Centauri, 4.25 light years away. It is too faint to see without a telescope.
Voyager 1, launched in 1977, is the most distant thing people have made. It is about one light day away, after nearly fifty years of flight. The nearest star is 1,550 times further still.
Worked outNASA, where the Voyagers are now
From the whole Earth, about 9,000 stars are bright enough to see without a telescope. From one dark place at one moment you can see perhaps 2,500 of them.
Every single star you can pick out by eye belongs to our own galaxy, and nearly all are within a few thousand light years: our own small corner of it.
At the centre of the Milky Way is a black hole with 4.3 million times the mass of the Sun. It was weighed by watching stars orbit it, one of them in only 16 years.
The Andromeda Galaxy, about 2.5 million light years away, is the most distant thing most people can see without a telescope.
The universe is 13.8 billion years old. The figure comes from the faint glow left over from its first hot stage, mapped across the whole sky by the Planck spacecraft.
Everything made of atoms, every star, planet and person, comes to about 5% of what the universe contains. About 27% is dark matter and 68% is dark energy, and nobody yet knows what either is.
More than 6,000 planets have been confirmed round other stars. The first round a star like the Sun was found in 1995.
The Oort cloud is thought to begin between 2,000 and 5,000 times the Earth's distance from the Sun and to end somewhere between 10,000 and 100,000 times. Its far edge may be more than a light year and a half away.
From a modelNASA, Oort cloud facts
Voyager 1 covers about a million miles a day. At that speed it will not reach the inner edge of the Oort cloud for about 300 years, and may take 30,000 years to come out of the far side.
Worked outNASA, Oort cloud facts
The cloud is named after the Dutch astronomer Jan Oort. He proposed it to explain why comets on very long orbits come in from every direction, not along the flat plane the planets keep to.
The Kuiper belt is a flat ring, but the Oort cloud is thought to be a round shell, wrapped right round the Sun in every direction. That is why it is called a cloud and not a belt.
From a modelNASA, Oort cloud facts
The nearest star system is three stars. Alpha Centauri A and B are a close pair, and little Proxima goes round the two of them about once in 550,000 years.
Proxima Centauri has a planet of about the mass of the Earth, going round it every 11.2 days at a distance where water could be liquid. It is the nearest planet known outside the Solar System.
Of the 108 stars this site's catalogue holds within 20 light years, 85 give out less than a thirtieth of the Sun's visible light. Only six outshine the Sun. The typical star is small, red and far too faint to see by eye.
Worked outHYG star database v4.4, David Nash
Sirius, the brightest star in the night sky, is also one of the nearest, 8.6 light years away. It looks bright mostly because it is close. Rigel, in Orion, gives out about 2,000 times more light from a hundred times further off.
Worked outHYG star database v4.4, David Nash
The three stars of Orion's Belt look like neighbours, but they are not. Mintaka and Alnitak are about 700 light years away, and Alnilam, the middle one, is given as nearly 2,000. A constellation is a pattern seen from here, not a group in space.
Worked outHYG star database v4.4, David Nash
Deneb is one of the most distant stars that is easy to see, about 1,400 light years away. The light reaching your eye tonight left it around the year 600.
Worked outHYG star database v4.4, David Nash
Fewer than one in ten of the stars you can see by eye are within 100 light years. Even so, the whole cloud of them is a speck beside the galaxy: nine in ten lie within 1,000 light years, and the Milky Way's disc is 100,000 across.
Worked outHYG star database v4.4, David Nash
The Sun is about 26,700 light years from the centre of the Milky Way. The figure comes from following one star for 27 years as it swung round the black hole there.
GRAVITY Collaboration 2019, Astronomy & Astrophysics 625, L10
The Gaia spacecraft has measured the brightness of about 1.8 billion stars, and the distance and motion of about 1.5 billion of them. The distances come from parallax: the tiny shift in a star's place in the sky as the Earth goes round the Sun.
The Milky Way is a spiral galaxy with a disc of stars more than 100,000 light years across. We live inside it, about half way out from the centre, so every picture of the whole galaxy, including the one here, is a model.
For years the Milky Way and Andromeda were expected to collide in about five billion years. A 2025 study using the latest measurements found the chance of that within ten billion years is only about even.
From a modelSawala et al. 2025, Nature Astronomy 9, 1206
The Local Group has two big galaxies, the Milky Way and Andromeda, and one middling one, Triangulum. Nearly all the rest are dwarfs, and the faintest give out only a few hundred times the light of the Sun, less than many single stars. About a hundred galaxies are known in and around it.
The Large and Small Magellanic Clouds are two small galaxies about 170,000 and 210,000 light years away. Both can be seen by eye from the southern half of the Earth.
Our galaxy belongs to a gathering of about 100,000 galaxies called Laniakea, some 520 million light years across. It was mapped in 2014 by working out which way galaxies are being pulled.
The nearest big cluster of galaxies is the Virgo Cluster, 16.5 megaparsecs away, which is about 54 million light years.
How fast is the universe expanding? Measuring the distances of galaxies gives 73 km a second for every megaparsec. The oldest light gives 67. Each is too precise for the gap to be chance, and nobody yet knows which assumption is wrong.
Over four months in 2003 and 2004 the Hubble telescope collected light from one small patch of sky for a total of 11.3 days. The picture holds about 10,000 galaxies.
For its first 380,000 years the universe was a glowing fog that light could not cross. When it cooled to about 3,000 degrees, atoms formed, the fog cleared, and that light has been travelling ever since.
The oldest light set out as the orange glow of gas at 3,000 degrees. Space has stretched so much since that it now arrives as microwaves, at 2.7 degrees above absolute zero: a stretch of about 1,100 times.
The oldest light is almost perfectly even. The warm and cool patches on the map differ by about one part in 100,000. Those faint patches are the seeds that grew into every galaxy.
The oldest light was found by accident in 1964, as a microwave signal stronger than anyone expected. Arno Penzias and Robert Wilson shared a Nobel Prize for it in 1978.