106 values, each with its units, its uncertainty, and where it came from.
All 351 constants →
sTime since the Big Bang, 13.797 billion years or 4.35 × 10¹⁷ seconds, from fitting the ΛCDM model to the microwave background.
mThe Sun–Earth yardstick, fixed by the IAU in 2012 as exactly 149 597 870 700 m and no longer tied to Earth's actual orbit.
radObliquity of Mars, 25.19° — a near-twin of Earth's tilt today, but chaotic over millions of years for want of a stabilising moon.
radObliquity of Uranus, 97.77° — the planet rolls around its orbit on its side, poles sunward, unlike anything else in the solar system.
kgMaximum mass a white dwarf can support by electron degeneracy pressure, about 1.44 solar masses or 2.86 × 10³⁰ kg.
KTemperature of the relic radiation from the Big Bang, 2.725 48 K — the most perfect blackbody spectrum ever measured, anywhere.
kg/m³Density 3H₀²/8πG that makes the universe spatially flat, about 8.5 × 10⁻²⁷ kg/m³ — some five hydrogen atoms per cubic metre.
mDistance to Alpha Centauri A, 4.13 × 10¹⁶ m or 4.365 light-years — the nearest Sun-like star, and the nearest whole stellar system.
mDistance to Barnard's Star, 5.64 × 10¹⁶ m or 5.96 light-years — the fastest-moving star in the sky, crossing a Moon-width per lifetime.
mDistance to Betelgeuse, about 168 parsecs or 548 light-years — uncertain by ten per cent, because the star is bigger than its own parallax.
mDistance to the nearest star beyond the Sun, 4.017 × 10¹⁶ m — 4.246 light-years or 1.302 pc, from a Gaia parallax of 768.07 mas.
mDistance to Sirius, 8.14 × 10¹⁶ m or 8.60 light-years — the brightest star in the night sky owes half its rank to simple nearness.
radObliquity of the ecliptic at J2000, 23.4393° or 0.409 rad — the tilt of Earth's spin axis that causes the seasons.
mSemi-major axis of the WGS 84 reference ellipsoid, exactly 6 378 137 m — the equatorial radius every GPS receiver is built around.
m/sMean speed of the Earth along its orbit, about 29.78 km/s — roughly 107 000 km/h, and it varies with distance from the Sun.
mMean radius (2a + b)/3 of the WGS 84 ellipsoid, about 6371 km — the single figure used when a spherical Earth is good enough.
mEarth's actual mean orbital distance, 1.000 002 61 au — close to the astronomical unit but a measured quantity, not the definition.
—Eccentricity of Earth's orbit at J2000, 0.0167 — nearly circular, yet enough to vary sunlight at the top of the atmosphere by 6.8 per cent.
mSemi-minor axis of the WGS 84 ellipsoid, 6 356 752.3 m — 21.4 km shorter than the equatorial radius because the Earth is spinning.
mJupiter's equatorial radius at the 1-bar level, 71 492 km; the polar radius is 66 854 km, a 6.5 per cent flattening from fast rotation.
mNeptune's equatorial radius at the 1-bar level, 24 764 km — slightly smaller than Uranus while noticeably heavier, the denser twin.
mSaturn's equatorial radius at the 1-bar level, 60 268 km — nine and a half Earths across, not counting the quarter-million-kilometre rings.
mUranus's equatorial radius at the 1-bar level, 25 559 km — four Earths across, measured almost entirely from one 1986 flyby.
m/sSpeed needed to escape Jupiter from the 1-bar level, 59.5 km/s — a well so deep the planet has kept every gas since it formed.
m/sSpeed needed to escape Mars from the surface, about 5.03 km/s — less than half Earth's, which is why a return mission is even thinkable.
m/sSpeed needed to escape Mercury from its surface, about 4.25 km/s — too low to hold an atmosphere against 700 K daytime heat.
m/sSpeed needed to escape Neptune from the 1-bar level, 23.5 km/s — a well that even governs who stays put in the Kuiper belt beyond.
m/sSpeed needed to escape Saturn from the 1-bar level, 35.5 km/s — the well Cassini deliberately fell into at the end of its mission.
m/sSpeed needed to break free of Earth's gravity from the surface, about 11.19 km/s, ignoring atmospheric drag and the planet's rotation.
m/sSpeed needed to leave the Moon's gravity from its surface, about 2.38 km/s — roughly a fifth of Earth's escape velocity.
m/sSpeed needed to escape Uranus from the 1-bar level, 21.3 km/s — deep enough to keep hydrogen for the age of the solar system.
m/sSpeed needed to escape Venus from its surface, 10.36 km/s — nearly Earth's, which is why Venus kept a crushing atmosphere and its water did not survive anyway.
km/(s·Mpc)Present expansion rate of the universe, 67.4 km/(s·Mpc) from the cosmic microwave background — a galaxy 1 Mpc away recedes at 67 km/s.
sExactly 365.25 days of 86 400 SI seconds — the conventional astronomical year that defines the light-year and the Julian century.
mMean distance of Jupiter from the Sun, 778.57 million km or 5.204 au — the orbit that organises the architecture of the solar system.
mDistance light travels in one Julian year of 365.25 days — exactly 9 460 730 472 580 800 m, since both c and the year are defined.
WPower output of Alpha Centauri A, 1.52 times the Sun's — a slightly older, slightly brighter solar twin, our best preview of the Sun's future.
WPower output of Sirius A, 25.4 times the Sun's — the true wattage behind the night sky's brightest star, mostly poured out in the ultraviolet.
WApparent luminosity of Vega, about 40 Suns — the star that defined magnitude zero, later caught cheating by spinning nearly apart.
mMean distance of Mars from the Sun, 227.92 million km or 1.524 au — the orbit whose stubborn eccentricity taught Kepler the ellipse.
kgMass of Jupiter, 1.898 × 10²⁷ kg — 318 Earths, and more than twice all the other planets combined; the unit for weighing exoplanets.
kgMass of Mars, 6.417 × 10²³ kg — about 10.7 per cent of Earth's, small enough that the planet lost most of its atmosphere.
kgMass of Mercury, 3.301 × 10²³ kg — the smallest planet, yet the second densest, with an iron core filling most of its volume.
kgMass of Neptune, 1.024 × 10²⁶ kg — 17.1 Earths, the densest of the giant planets and the one found with mathematics before a telescope.
kgMass of Proxima Centauri, 0.1221 solar masses — an eighth of a Sun, just clear of the hydrogen-burning limit, with a trillion-year future.
kgMass of Saturn, 5.683 × 10²⁶ kg — 95 Earths spread so thinly that its mean density, 687 kg/m³, is less than that of water.
kgMass of Sirius A, 2.063 solar masses — weighed by watching its 50-year waltz with the white dwarf companion Bessel predicted unseen.
kgMass of the white dwarf Sirius B — 1.018 Suns packed into an Earth-sized sphere, the heaviest white dwarf with a precision mass.
kgMass of the Earth, 5.9722 × 10²⁴ kg — the unit in which rocky exoplanets are weighed, and limited in precision only by G.
kgMass of the Moon, 7.346 × 10²² kg — 1.23 per cent of Earth's, the largest satellite-to-planet mass ratio in the solar system.
kgMass of Uranus, 8.681 × 10²⁵ kg — 14.5 Earths of hydrogen, helium and icy volatiles, tipped on its side at 98 degrees.
kgMass of Venus, 4.8675 × 10²⁴ kg — 81.5 per cent of Earth's, making it our closest twin in bulk and nothing like it in climate.
mSemi-major axis of the lunar orbit, 384 400 km centre to centre; the actual distance ranges from 356 500 to 406 700 km.
mVolumetric mean radius of Mars, 3389.5 km; the equatorial radius is 3396.2 km and the polar 3376.2 km, a 20 km flattening.
mVolumetric mean radius of Mercury, 2439.7 km — the smallest planet, barely a third of Earth's radius and still shrinking as its core cools.
mVolumetric mean radius of the Moon, 1737.4 km — just over a quarter of Earth's radius, and barely 0.3 per cent from a perfect sphere.
mVolumetric mean radius of Venus, 6051.8 km — 95 per cent of Earth's, measured by radar through clouds no telescope can pierce.
sThe civil day of exactly 86 400 SI seconds — a defined unit that Earth's actual rotation now overruns by a millisecond or two.
mMean distance of Mercury from the Sun, 57.91 million km or 0.387 au — though its eccentric orbit swings 24 million km either side of it.
mMean distance of Neptune from the Sun, 4495.1 million km or 30.05 au — the outer edge of the planetary system, four light-hours out.
WIAU nominal solar luminosity, exactly 3.828 × 10²⁶ W — the conventional unit in which every other star's power output is quoted.
mIAU nominal solar radius, exactly 6.957 × 10⁸ m — a fixed convention, since a gaseous Sun has no true surface to measure.
mDistance at which one au subtends one arcsecond — 648000/π au exactly, about 3.26 light-years, the working unit of stellar astronomy.
mJupiter's polar radius at the 1-bar level, 66 854 km — 4638 km less than the equatorial, squashed by a ten-hour rotation.
mNeptune's polar radius at the 1-bar level, 24 341 km — a 1.7 per cent flattening, the roundest figure among the four giant planets.
mSaturn's polar radius at the 1-bar level, 54 364 km — nearly 6000 km short of the equator, the most flattened planet there is.
mUranus's polar radius at the 1-bar level, 24 973 km — a 2.3 per cent flattening, with the odd twist that a pole often faces the Sun.
mRadius of Betelgeuse, about 764 solar radii or 3.6 au — placed at the Sun it would swallow Mars, with honest error bars nearly a full au wide.
mRadius of Proxima Centauri, 0.154 solar radii — the nearest star is only half again the size of Jupiter, glowing at 3000 K.
mRadius of Sirius B, 5634 km — a star smaller than the Earth, whose surface gravity compresses a sugar-cube's volume to a tonne of matter.
mMean distance of Saturn from the Sun, 1433.5 million km or 9.58 au — the edge of the known solar system for all of human history until 1781.
mRadius to which the Sun would have to be crushed to become a black hole, 2GM⊙/c² — a shade under three kilometres.
sEarth's rotation period relative to the fixed stars, 23 h 56 min 4.09 s — about four minutes shorter than the solar day.
sJupiter's year, 4332.589 days or 11.86 Earth years — close enough to twelve that it paced calendars across the ancient world.
sThe Martian year, 686.980 days or 1.881 Earth years — the beat that sets the 26-month rhythm of every launch window.
sMercury's year, 87.969 days — the fastest orbit of any planet, which earned it the name of the Roman gods' winged messenger.
sNeptune's year, 60 189 days or 164.8 Earth years — it completed its first full orbit since discovery on 11 July 2011.
sSaturn's year, 10 759.22 days — 29.46 Earth years, the generation-long cycle that made it antiquity's planet of old age and time.
sThe sidereal month, 27.3217 days — one true orbit of the Moon against the stars, and also exactly one rotation of the Moon itself.
sUranus's year, 30 685.4 days — 84 Earth years, so the sideways planet gives each pole a 42-year day and a 42-year night.
sVenus's year, 224.701 days — remarkable chiefly because it is shorter than the planet's own 243-day rotation.
sOne rotation of Jupiter takes 9 h 55.5 min — the fastest spin of any planet, defined by the magnetic field because the clouds disagree.
sOne true rotation of Mars, 24.6229 hours — the near-match to Earth's day that gives rover teams their slightly drifting 'sol'.
sMercury's true rotation period, 58.646 days — exactly two thirds of its year, a 3:2 resonance discovered by radar in 1965.
sOne rotation of Neptune takes 16.11 hours, clocked from Voyager 2's radio data during the single close encounter ever made.
sSaturn's rotation, conventionally 10.656 h from Voyager's radio data — a genuinely uncertain number on a planet that hides its own clock.
sOne rotation of Uranus takes 17.24 hours, retrograde by the tilt's own geometry — a Voyager radio measurement never since repeated.
sOne rotation of Venus takes 243.02 days, and it turns backwards — the slowest and the only retrograde spin among the inner planets.
sOne orbit of the Sun relative to the fixed stars, 365.2564 days — about 20 minutes longer than the tropical year of the seasons.
W/m²Total solar irradiance above the atmosphere at one au, about 1361 W/m² — the input to every climate model and solar panel estimate.
KIAU nominal effective temperature of the Sun, 5772 K — the blackbody temperature that radiates the solar luminosity from the solar radius.
kgMass of the Sun, about 333 000 Earths and 99.86 per cent of all matter in the solar system — the yardstick for every stellar mass.
m/sTypical speed of the solar wind at Earth's orbit, about 400 km/s; the slow and fast streams range from roughly 300 to 800 km/s.
m³/s²The geocentric gravitational constant GM⊕, 3.986 004 418 × 10¹⁴ m³/s², known to nine digits and used by every GPS satellite.
m³/s²The heliocentric gravitational constant GM⊙, known to ten digits from planetary radar — far better than the Sun's mass in kilograms.
m/s²Gravitational acceleration at Jupiter's 1-bar level on the equator, 24.79 m/s² — 2.5 times Earth's, before the fast spin refunds part of it.
m/s²Equatorial surface gravity on Mars, 3.71 m/s² — 38 per cent of Earth's, the figure every Mars lander design is built around.
m/s²Gravitational acceleration at Mercury's surface, 3.70 m/s² — almost exactly the same as Mars, on a planet half the diameter.
m/s²Gravitational acceleration at Neptune's 1-bar equator, 11.15 m/s² — the only planet besides Jupiter that out-pulls the Earth.
m/s²Gravitational acceleration at Saturn's 1-bar equator, 10.44 m/s² — 95 Earth masses producing barely more pull than Earth itself.
m/s²Gravitational acceleration at the lunar surface, 1.62 m/s² — one sixth of Earth's, the value the Apollo crews had to learn to walk in.
m/s²Effective gravity at Uranus's 1-bar equator, 8.69 m/s² — including the spin's centrifugal refund; gravity alone would be 8.87.
m/s²Gravitational acceleration at the surface of Venus, 8.87 m/s² — 90 per cent of Earth's, the most Earth-like gravity of any planet.
sEquinox to equinox, 365.2422 days — the year the seasons follow, and the quantity every calendar reform has tried to approximate.
mMean distance of Uranus from the Sun, 2872.5 million km or 19.2 au — the discovery that doubled the solar system overnight in 1781.
mMean distance of Venus from the Sun, 108.21 million km or 0.723 au, on the most nearly circular orbit of any planet.
