How far is a light-year? Measuring distances in space

A light-year is the distance light travels in one year: about 9.46 trillion kilometres (9.4607 × 10¹² km), or about 5.88 trillion miles. It is a unit of distance, not time. The nearest star beyond the Sun, Proxima Centauri, is about 4.25 light-years away, roughly 40 trillion km. The light-year converter switches between light-years, parsecs, astronomical units, kilometres and miles, and shows how long the trip would take.

What a light-year is

The International Astronomical Union defines the light-year using the speed of light in a vacuum, exactly 299,792,458 m/s, and a Julian year of exactly 365.25 days (31,557,600 seconds). Multiply them and you get exactly 9,460,730,472,580.8 km. Using a fixed year rather than a calendar year means the unit never changes. Numbers this large are much easier to handle in scientific notation, which Scientific notation explained, with e-notation and engineering notation covers, and the scientific notation converter will expand or compress them for you.

Making the number meaningful

Smaller light units help build intuition. Light takes about 1.28 seconds to reach the Moon and about 8.3 minutes to reach us from the Sun, so the Sun is about 8.3 light-minutes away. A light-year is about 63,000 times the Earth–Sun distance. The converter also lists travel times at real speeds: Voyager 1, one of the fastest objects leaving the Solar System at about 17 km/s, would need roughly 75,000 years to cover the distance to Proxima Centauri, though it is not heading that way. A car at 100 km/h would take tens of millions of years.

Parsecs and astronomical units

Professional astronomers more often use two other units:

• Astronomical unit (AU): defined as exactly 149,597,870,700 m, about the average Earth–Sun distance. It suits the Solar System, where planets lie between about 0.4 and 30 AU from the Sun.
• Parsec (pc): the distance at which 1 AU appears to span one arcsecond of angle. It comes straight from the parallax method of measuring star distances, where a nearby star appears to shift against the background as Earth orbits the Sun. One parsec is about 3.26 light-years, or about 206,265 AU. Kiloparsecs and megaparsecs measure galaxies.

So Proxima Centauri at 4.25 light-years is about 1.30 parsecs. The two units sit comfortably side by side; popular writing favours light-years, research papers favour parsecs.

Parallax is also why the parsec exists at all. Even the nearest star shifts by less than one arcsecond over a year, an angle smaller than a coin seen from a few kilometres away, so the measurement needs precise instruments. Space telescopes such as ESA’s Gaia measure these tiny shifts for vast numbers of stars, and the longer-range methods astronomers use for distant galaxies are calibrated, step by step, from those nearby parallax distances. That chain is often called the cosmic distance ladder, and parallax is its first rung.

How far things are

• Proxima Centauri, the nearest star to the Sun: about 4.25 light-years.
• Sirius, the brightest star in the night sky: about 8.6 light-years.
• The centre of the Milky Way: about 26,000 light-years.
• The Andromeda Galaxy, the nearest large galaxy: about 2.5 million light-years.

Distances to stars come with measurement uncertainty, and for very distant galaxies astronomers use several different definitions of distance, because the universe has expanded while the light was travelling. I use the standard published values in the converter presets and round them for display. For a sense of scale closer to home, the orbital period calculator shows how distance from the Sun sets the length of a planet's year.

Looking back in time

Because light takes time to travel, distance in light-years doubles as a lookback time. The light from Proxima Centauri left it about four years ago; the light from Andromeda that reaches your eye tonight set off about 2.5 million years ago. Telescopes looking at the most distant galaxies are seeing them as they were billions of years ago. That is the one sense in which a light-year relates to time, but the unit itself is always a distance. Sound, by comparison, is nearly 900,000 times slower, as The speed of sound: how fast it travels and what changes it shows.