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Astronomical Distance Converter

Converters · Added

Space is measured in units chosen for the scale being described: kilometres for orbits, astronomical units for the solar system, light-years for the neighbourhood and parsecs for anything astronomers write papers about. Four of the units here are exact by definition rather than measured, which is worth knowing before quoting a figure to more digits than it deserves.

Result

1 au in light-year

0.0000158125074 ly

1 au = 0.0000158125074 ly

The same value in every unit

Metre (m)
149,597,871,000
Kilometre (km)
149,597,871
Mile (mi)
92,955,807.3
Light-second (ls)
499.004784
Light-minute (lmin)
8.3167464
Light-hour (lh)
0.13861244
Light-day (ld)
0.00577551833
Light-year (ly)
0.0000158125074
Astronomical unit (au)
1
Lunar distance (mean) (LD)
389.173413
Earth radius (equatorial) (R⊕)
23,454.7911
Solar radius (nominal) (R☉)
215.032156
Parsec (pc)
0.00000484813681
Kiloparsec (kpc)
4.8481e-9
Megaparsec (Mpc)
4.8481e-12

The light-second, light-year, astronomical unit and parsec are exact by definition. The lunar distance is a mean — the Moon's actual distance varies by about 10% over a month — and the two radii are nominal values, not measurements you should carry to nine digits.

How to use the astronomical distance converter

  1. 1Enter the distance and pick the unit it is in.
  2. 2Pick the unit you want. Every other unit in the family appears in the table underneath.
  3. 3Use the light travel time units to turn a distance into how long light takes to cross it.
  4. 4Treat the lunar distance and the two radii as nominal values rather than exact ones — the note under the result says which is which.

Examples

The nearest star

Input
4.2465 light-years
Result
1.3020 parsecs, or 268,553 astronomical units

Proxima Centauri. A parsec is about 3.26 light-years, which is why astronomers and popular science quote different numbers for the same star.

How far away the Sun is

Input
1 astronomical unit
Result
149,597,870.7 km, and 499 light-seconds

Sunlight leaving now arrives in about eight minutes and twenty seconds.

The scale of the Moon

Input
1 lunar distance
Result
384,399 km, or about 1.28 light-seconds

The mean value — the Moon's actual distance swings about 10% between perigee and apogee.

About the astronomical distance converter

Why each unit exists

Kilometres work up to about the Moon and then stop being readable — the distance to Neptune is 4,500,000,000 km, and nobody can hold that many zeros. The astronomical unit fixes that inside the solar system by making Earth's orbit the yardstick, so Jupiter is a little over five and Neptune is thirty. Distances become numbers you can reason about.

Beyond the solar system the AU fails the same way, and the light-year takes over: Proxima Centauri is 268,000 AU, which is unreadable, or 4.25 light-years, which is not. The parsec then exists for a different reason again — not readability, but because it is what a parallax measurement directly produces. Divide one by the measured angle in arcseconds and the answer is in parsecs with no conversion at all.

That is why popular science says light-years and professional astronomy says parsecs. They are not competing conventions; they come out of different activities. Papers on distant galaxies use megaparsecs for the same reason, since a galaxy at 50 Mpc would otherwise be 163 million light-years.

The measurement ladder underneath the units

None of these distances was measured directly. Parallax works out to a few thousand parsecs with modern space astrometry and no further, so beyond that astronomers use standard candles — objects of known intrinsic brightness, such as Cepheid variables and type Ia supernovae — each calibrated against the rung below it. This is the cosmic distance ladder, and every rung inherits the uncertainty of the ones under it.

The practical consequence for anyone converting units is that precision in the conversion is not precision in the distance. Turning 50 megaparsecs into light-years to nine significant figures is arithmetically exact and physically meaningless, because the 50 itself is good to a few percent at best. The exactness of the definitions is about reproducibility between astronomers, not about how well anybody knows where anything is.

Frequently asked questions

What exactly is a parsec?
The distance at which one astronomical unit subtends an angle of one arcsecond — a parallax of one second, which is where the name comes from. It follows arithmetically from the AU: 648,000 divided by π astronomical units, or about 3.26 light-years. It exists because it is what the measurement actually produces. Astronomers measure a star's parallax angle as the Earth moves around the Sun, and the distance in parsecs is simply one divided by that angle in arcseconds.
Which of these units are exact?
The light-second, light-year, astronomical unit and parsec, all four. The metre is defined by fixing the speed of light at 299,792,458 m/s, so a light-second is exactly that many metres. The light-year uses the Julian year of exactly 365.25 days. The IAU fixed the astronomical unit at exactly 149,597,870,700 m in 2012, ending its previous life as a measured property of Earth's orbit. The parsec follows from the AU by definition. Everything else here is a mean or a nominal value.
Is a light-year a unit of time?
No, it is a distance — the distance light travels in a year — and the confusion is common enough that the pun is a running joke in science fiction. What makes it useful is the double meaning it carries: a star four light-years away is also four years in the past as you look at it, because that is how long the light has been travelling. The light travel time units here make that explicit at smaller scales, where light-seconds and light-minutes are how spacecraft communication delays are quoted.
Why is the lunar distance not exact?
Because the Moon's orbit is an ellipse and the distance changes continuously — from about 356,500 km at perigee to about 406,700 km at apogee, a swing of roughly 10%. The 384,399 km used here is the conventional mean. It is a useful yardstick for describing how close an asteroid passed, which is the main context it appears in, but it is a convention rather than a measurement of anything at a particular moment.
What about the solar and Earth radii?
The solar radius is the IAU 2015 nominal value of 695,700 km, adopted as a fixed conversion constant precisely so that papers stop disagreeing about it — the Sun has no sharp edge, so any radius is a definition of where you stop counting. The Earth radius here is the WGS 84 equatorial radius of 6,378,137 m, which is exact as a datum but is not the polar radius or the mean radius, both of which are smaller.

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