Science & Units
Astronomical Distance Table
Astronomy has three distance units because it has three measurement methods, and each unit is the natural output of one of them. Radar and orbital mechanics give distances inside the solar system in astronomical units. Stellar parallax gives distances to nearby stars in parsecs, because a parallax angle in arcseconds inverts directly into parsecs. And light years are what the public understands, since they express distance as travel time for the fastest thing there is.
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The three are rigidly linked. The astronomical unit was redefined in 2012 as exactly 149,597,870.7 km, so it is now a defined constant rather than a measured one. A parsec is the distance at which one astronomical unit subtends one arcsecond, which makes it exactly 648,000/π AU, or 206,264.8 AU. A light year is the distance light travels in one Julian year of 365.25 days, which is 9,460,730,472,580.8 km.
The numbers get large enough that intuition fails. Sunlight takes 8 minutes 20 seconds to reach Earth, but 4.2 years to reach the nearest star. The Milky Way is about 100,000 light years across, so the light arriving now from its far side left before humans had writing. Andromeda, the nearest large galaxy, is 2.54 million light years away — the light you see tonight left before our species existed.
One caution about the light year: it uses the Julian year, not the calendar year. The difference is 0.07%, which sounds negligible until it is multiplied by 4.2 light years and becomes roughly three hundred million kilometres. This table uses the IAU definition, which is the one every astronomy textbook and dataset assumes.
Astronomical distance units in kilometres, miles and AU
| Unit | Kilometres | Miles | Astronomical units |
|---|---|---|---|
| Light-second | 299,792.458 | 186,282.397 | 0.002004 |
| Light-minute | 17,987,547.48 | 11,176,943.82 | 0.120239 |
| Light-hour | 1,079,252,848.8 | 670,616,629.4 | 7.214360 |
| Light-day | 25,902,068,371.2 | 16,094,799,105.2 | 173.144633 |
| Astronomical unit | 149,597,870.7 | 92,955,807.27 | 1.000000 |
| Light-year | 9,460,730,472,581 | 5,878,625,373,184 | 63,241.077084 |
| Parsec | 30,856,775,814,914 | 19,173,511,576,713 | 206,264.806247 |
One light-year is 63,241 AU and one parsec is 206,265 AU. The parsec is larger because a parallax of one arcsecond is a very small angle.
Distances inside the solar system
| Body | Kilometres | Astronomical units | Light travel time |
|---|---|---|---|
| Moon (mean) | 384,400 | 0.00257 | 1.28 s |
| Mercury | 57,909,050 | 0.38710 | 3.2 min |
| Venus | 108,208,000 | 0.72333 | 6 min |
| Earth | 149,597,871 | 1.00000 | 8.3 min |
| Mars | 227,939,200 | 1.52368 | 12.7 min |
| Jupiter | 778,570,000 | 5.20442 | 43.3 min |
| Saturn | 1,433,530,000 | 9.58256 | 1.33 h |
| Uranus | 2,872,463,000 | 19.20123 | 2.66 h |
| Neptune | 4,498,396,000 | 30.06992 | 4.17 h |
| Pluto (mean) | 5,906,380,000 | 39.48171 | 5.47 h |
Distances are mean values from the Sun; the Moon is from Earth. Light travel time is the distance divided by the speed of light, so sunlight takes 8.3 minutes and moonlight 1.3 seconds.
Distances beyond the solar system
| Object | Light years | Parsecs |
|---|---|---|
| Proxima Centauri | 4.246 | 1.3018 |
| Sirius | 8.601 | 2.6371 |
| Vega | 25.04 | 7.6773 |
| Pleiades cluster | 444 | 136.131 |
| Galactic centre | 26,700 | 8,186 |
| Large Magellanic Cloud | 163,000 | 49,976 |
| Andromeda galaxy | 2,537,000 | 777,848 |
Proxima Centauri is the nearest star at 4.246 light years. Andromeda is 2.54 million light years away, the farthest object visible to the naked eye.
Parallax to distance
| Parallax (arcseconds) | Distance (parsecs) | Distance (light years) |
|---|---|---|
| 1.0000 | 1 | 3.26 |
| 0.5000 | 2 | 6.52 |
| 0.2000 | 5 | 16.31 |
| 0.1000 | 10 | 32.62 |
| 0.0500 | 20 | 65.23 |
| 0.0200 | 50 | 163.08 |
| 0.0100 | 100 | 326.16 |
| 0.0050 | 200 | 652.31 |
| 0.0010 | 1,000 | 3,261.56 |
Distance in parsecs is simply 1 divided by the parallax in arcseconds. Gaia measures parallaxes down to about 0.00002 arcseconds, which reaches 50,000 parsecs.
How to Use These Tables
- Reading a press release: if a distance is given in parsecs, multiply by 3.2616 to get light years. 10 parsecs is 32.6 light years.
- Solar system scale: 1 AU is 149.6 million km, so Mars at 1.52 AU is 228 million km from the Sun and a signal takes 12.7 minutes each way.
- Parallax data: divide 1 by the parallax in arcseconds to get parsecs. A star with a 0.05 arcsecond parallax is 20 parsecs, or 65.2 light years, away.
- Comparing galaxies: megaparsecs are the working unit. Andromeda is 0.78 Mpc away, and the Virgo cluster is about 16 Mpc.
- Light travel time: divide kilometres by 299,792.458 to get seconds. The Moon is 1.28 light seconds away, the Sun 499 light seconds.
Common Questions
Is a light year a unit of time?
No, it is a distance: the distance light travels in one year, about 9.46 trillion kilometres. The name is the only thing that suggests time.
Why is a parsec bigger than a light year?
Because one arcsecond is a very small angle. A parsec is the distance at which 1 AU spans 1 arcsecond, and that works out to 3.26 light years, or 206,265 AU.
How long does sunlight take to reach Earth?
8 minutes 20 seconds on average, since 1 AU is 149.6 million km and light covers 299,792 km per second. At aphelion it is 8 minutes 27 seconds, at perihelion 8 minutes 13 seconds.
What is an astronomical unit exactly?
149,597,870.7 km, fixed by the IAU in 2012. Before that it was a measured quantity with an uncertainty; now it is a definition, like the metre.
How far is the nearest star?
Proxima Centauri is 4.246 light years away, which is 1.30 parsecs or about 40.2 trillion kilometres. Light from it left in early 2022.
Why do astronomers prefer parsecs?
Because distance falls out of the measurement directly: parallax in arcseconds inverted gives parsecs with no conversion factor. Light years need a multiplication by 3.26 every time.
More Science & Units
Sources
- IAU 2012 Resolution B2 — redefinition of the astronomical unit — International Astronomical Union
- IAU — SI units and astronomical units — International Astronomical Union
- NIST — speed of light in vacuum, exact value — National Institute of Standards and Technology
Filed with Science calculations.