The Universe in Numbers — Astronomical Units and Key Figures
Just how far are a light-year and an astronomical unit (AU)? Here we gather in one place the sizes, masses, speeds, and temperatures of the Earth, Sun, Moon, and our Milky Way — the reference numbers for talking about the cosmos.
Units of distance — AU, light-year, parsec
Cosmic distances are so vast that kilometers cannot cope. So astronomy uses a few big rulers. The "astronomical unit (AU)" is the average distance between the Earth and the Sun, used to measure distances within the Solar System. The "light-year" is the distance light travels in one year, the universe's standard ruler for the distances between stars and galaxies.
| Unit | Value | Notes |
|---|---|---|
| 1 astronomical unit (AU) | about 149.6 million km | Earth–Sun average distance |
| 1 light-year (ly) | about 9.46 trillion km | distance light travels in a year (≈ 63,241 AU) |
| 1 parsec (pc) | about 30.86 trillion km | ≈ 3.26 light-years (an annual parallax of 1 arcsec) |
| speed of light | about 300,000 km/s | exactly 299,792.458 km/s |
Units of time — the sidereal year and the solar year
There is more than one kind of "year." The time the Sun takes to return to the same constellation in the sky is the "sidereal year," while the time for the seasons to come full circle (equinox to equinox) is the "solar year (tropical year)." Because of the precession of the Earth's axis, the two differ by about 20 minutes.
| Unit | Value |
|---|---|
| sidereal year (relative to the stars) | 365 days 6 hours 9 minutes (365.2564 days) |
| solar year (relative to the seasons) | 365 days 5 hours 49 minutes (365.2422 days) |
| sidereal day (one rotation) | 23 hours 56 minutes 4 seconds |
Earth
The basic figures for our home planet, the Earth.
| Item | Value |
|---|---|
| radius (equatorial) | about 6,378 km |
| mass | about 5.97 × 10²⁴ kg |
| orbital speed | about 29.8 km/s (roughly 30 km per second) |
| rotation speed (equator) | about 0.46 km/s (roughly 1,670 km per hour) |
| distance from the Sun | 1 AU (about 149.6 million km) |
The Sun
Our star, the Sun, which alone accounts for 99.86% of the Solar System's mass.
| Item | Value |
|---|---|
| radius | about 696,000 km (about 109 times the Earth) |
| mass | about 1.989 × 10³⁰ kg (about 330,000 times the Earth) |
| luminosity (brightness) | about 3.828 × 10²⁶ W |
| surface temperature | about 5,500 °C (5,778 K) |
| core temperature | about 15 million °C |
The Moon
The figures for the Moon, the Earth's one and only satellite.
| Item | Value |
|---|---|
| radius | about 1,737 km (about 0.27 times the Earth) |
| mass | about 7.35 × 10²² kg (about 1/81 of the Earth) |
| average distance from the Earth | about 384,400 km |
| orbital period | about 27.3 days (sidereal month) |
The Milky Way
The scale of our galaxy, the Milky Way, to which the Sun and every star in the night sky belong.
| Item | Value |
|---|---|
| diameter | about 100,000 light-years |
| total mass | about 1.5 × 10¹² solar masses (including dark matter) |
| number of stars | about 100 billion to 400 billion |
| the Sun's position | about 26,000 light-years from the center |
| the Sun's galactic orbit | about 220 km/s · one lap about 230 million years |