Constants library

Astronomical Unit exact

\mathrm{au} = 1.495978707 \times 10^{11}\ \text{m}

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.

Solar Mass measured

M_\odot = 1.98841 \times 10^{30}\ \text{kg}

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.

Nominal Solar Radius exact

\mathcal{R}^{\mathrm{N}}_\odot = 695,700,000\ \text{m}

mIAU nominal solar radius, exactly 6.957 × 10⁸ m — a fixed convention, since a gaseous Sun has no true surface to measure.

Nominal Solar Luminosity exact

\mathcal{L}^{\mathrm{N}}_\odot = 3.828 \times 10^{26}\ \text{W}

WIAU nominal solar luminosity, exactly 3.828 × 10²⁶ W — the conventional unit in which every other star's power output is quoted.

Solar Effective Temperature exact

\mathcal{T}^{\mathrm{N}}_{\mathrm{eff}\odot} = 5772\ \text{K}

KIAU nominal effective temperature of the Sun, 5772 K — the blackbody temperature that radiates the solar luminosity from the solar radius.

Solar Constant (Total Solar Irradiance) measured

S_0 = 1361\ \text{W/m}^{2}

W/m²Total solar irradiance above the atmosphere at one au, about 1361 W/m² — the input to every climate model and solar panel estimate.

Standard Gravitational Parameter of the Sun measured

GM_\odot = 1.32712440041 \times 10^{20}\ \text{m}^{3}\text{/s}^{2}

m³/s²The heliocentric gravitational constant GM⊙, known to ten digits from planetary radar — far better than the Sun's mass in kilograms.

Solar Wind Speed (typical)

v_{\mathrm{sw}} = 400,000\ \text{m/s}

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.

Mass of the Earth measured

M_\oplus = 5.9722 \times 10^{24}\ \text{kg}

kgMass of the Earth, 5.9722 × 10²⁴ kg — the unit in which rocky exoplanets are weighed, and limited in precision only by G.

Earth Mean Orbital Speed

v_\oplus = 29,780\ \text{m/s}

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.

Earth Orbit Semi-Major Axis

a_{\oplus,\mathrm{orb}} = 1.495982612 \times 10^{11}\ \text{m}

mEarth's actual mean orbital distance, 1.000 002 61 au — close to the astronomical unit but a measured quantity, not the definition.

Earth Orbital Eccentricity

e_\oplus = 0.0167086\ \text{—}

—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.

Mass of the Moon measured

M_{\mathrm{Moon}} = 7.3459 \times 10^{22}\ \text{kg}

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.

Mean Radius of the Moon measured

R_{\mathrm{Moon}} = 1,737,400\ \text{m}

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.

Mean Earth–Moon Distance

a_{\mathrm{Moon}} = 384,400,000\ \text{m}

mSemi-major axis of the lunar orbit, 384 400 km centre to centre; the actual distance ranges from 356 500 to 406 700 km.

Surface Gravity of the Moon measured

g_{\mathrm{Moon}} = 1.62\ \text{m/s}^{2}

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.

Escape Velocity of the Moon

v_{\mathrm{esc},\mathrm{Moon}} = 2380\ \text{m/s}

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.

Mass of Mars measured

M_{\mathrm{Mars}} = 6.4171 \times 10^{23}\ \text{kg}

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.

Mean Radius of Mars measured

R_{\mathrm{Mars}} = 3,389,500\ \text{m}

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.

Surface Gravity of Mars measured

g_{\mathrm{Mars}} = 3.71\ \text{m/s}^{2}

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.

Escape Velocity of Mars

v_{\mathrm{esc},\mathrm{Mars}} = 5030\ \text{m/s}

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.

Mass of Mercury measured

M_{\mathrm{Mercury}} = 3.3011 \times 10^{23}\ \text{kg}

kgMass of Mercury, 3.301 × 10²³ kg — the smallest planet, yet the second densest, with an iron core filling most of its volume.

Mass of Venus measured

M_{\mathrm{Venus}} = 4.8675 \times 10^{24}\ \text{kg}

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.

Mass of Jupiter measured

M_{\mathrm{J}} = 1.8982 \times 10^{27}\ \text{kg}

kgMass of Jupiter, 1.898 × 10²⁷ kg — 318 Earths, and more than twice all the other planets combined; the unit for weighing exoplanets.

Equatorial Radius of Jupiter measured

R_{\mathrm{J}} = 71,492,000\ \text{m}

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.

Mass of Saturn measured

M_{\mathrm{Saturn}} = 5.6834 \times 10^{26}\ \text{kg}

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.

Mass of Uranus measured

M_{\mathrm{Uranus}} = 8.681 \times 10^{25}\ \text{kg}

kgMass of Uranus, 8.681 × 10²⁵ kg — 14.5 Earths of hydrogen, helium and icy volatiles, tipped on its side at 98 degrees.

Mass of Neptune measured

M_{\mathrm{Neptune}} = 1.02413 \times 10^{26}\ \text{kg}

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.