On the left a star is shining, and one after another coloured waves spread out from it — pink, cyan, violet — each with its own delay, so the circles never coincide. Above them two nebulae smoulder, slowly changing shape, and all around stars twinkle with periods chosen to be incommensurable. Along the scale at the bottom a cyan marker runs from the star and reaches exactly the one-light-year mark, where a pink line awaits it: that is the measurement itself — the path of light in a year, laid off on a ruler. The letters in the middle are the same in every locale; the name in each language stands beneath the plate.
Non-SI unit · astronomy · length
Light-year, ly
A ruler whose divisions are marked out by light
Unit of length · the path of light in vacuum in one Julian year · 9 460 730 472 580 800 m
The word was coined not by astronomers but by popularisers, and professionals frowned at first: it seemed absurd to them to measure distance by time, and by a year at that, which every calendar reckons differently. Clarity won all the same, because the phrase “light takes four years to reach the nearest star” is understood by anyone who has ever waited for a letter, while a number in millions of millions of kilometres says nothing to the imagination.
The advantage of this unit is that, together with the distance, it tells you the age of the picture. Looking at Proxima we see it as it was four and a quarter years ago, and looking at the Andromeda nebula — as it was when not a single human of the modern kind had yet appeared on Earth. In this sense a telescope is not so much a magnifying glass as a machine that shows the past, and it shows different parts of the sky in different epochs at once.
The exact value of the unit comes not from a measurement but from two agreements: the speed of light in vacuum is fixed as an exact number of metres per second, and the year used for this purpose is the Julian one, exactly thirty-one million five hundred and fifty-seven thousand six hundred seconds. Multiply one by the other and you get the length of the light-year to the last digit, with no uncertainty at all. Uncertainty appears later, when this ruler is laid against the real sky.
The international symbol is two lower-case Latin letters, ly, without dots and without a space between them; in running text people write “light-year”, but in tables and chart labels ly is the norm, because it reads the same in any country. Multiples are formed with the usual prefixes: kly, Mly, Gly — a thousand, a million and a billion light-years.
A light-year is a distance, and the word “year” in it changes nothing: saying “we flew for twelve light-years” is as absurd as saying “we walked for twelve kilometres per hour”. Nor should it be confused with the parsec, which is a little over three and a quarter light-years, or with a year as a span of time: the definition uses the Julian year of exactly three hundred and sixty-five and a quarter days, not the tropical or the calendar year.
01 · Definition
the path of light in a year · the age of the picture · travel time for a shipA light-year is the distance that light travels in vacuum in one Julian year, that is, in three hundred and sixty-five and a quarter days of eighty-six thousand four hundred seconds each. The definition contains two quantities, both fixed exactly, and so their product is exact too: there is no standard, no measurement and no corrections here — only the multiplication of two numbers adopted by agreement.
The words “in vacuum” in the definition are no ornament. In air, in glass and in interstellar gas light travels more slowly, and if the unit were defined by a real beam, its length would depend on which part of the sky the beam had crossed. Astronomical vacuum is nearly perfect, but the correction for matter is still noticeable for radio waves, and so the definition builds in the speed in vacuum specifically.
The second subtlety is which year is taken. The tropical year is about eleven minutes shorter than the Julian one, the calendar year keeps changing its length, and the sidereal year is longer; in the end the difference comes to millionths, which is not negligible for astronomy. The International Astronomical Union chose the Julian year precisely for its constancy: it is tied neither to the Earth’s rotation nor to its revolution around the Sun, and so does not change with time.
Finally, the quantity has a property that neither the metre nor the parsec has: it tells you at once how late we are. The distance in light-years is the age of the image, and every map of the sky turns out to be a map of different epochs stitched into one picture. Professionals put up with the unit for the sake of this property, though in papers they prefer the parsec: it is derived from a directly measured angle rather than from time.
Whom we see and whom we can reach
In the upper half of the scene an object shines, and a wave travels from it to us: while it is in transit we see not the star but its past, and the label by the eyepiece shows the year in which the light set out. The distance slider moves the object from the edge of the Solar System to neighbouring galaxies, and the epoch we observe changes with it. In the lower half a ship flies to the same object, and it has two clocks: the violet one stays on Earth, the pink one runs on board. The second slider speeds the ship up, and the closer it gets to the speed of light, the further the readings diverge — until a journey that takes centuries as seen from Earth shrinks to a few years for the crew. Note that the ship cannot overtake light with any setting: the pink marker always stays behind the cyan one.
02 · Conversion
distances · light in transit · near-light-speed flightOne distance in six forms
The first tab converts the distance you enter into other units — from parsecs and astronomical units to light-days and ordinary kilometres. The second shows the light of which epochs reaches us from well-known objects. The third works out a flight: how many years it takes by the Earth’s clock and how many by the ship’s at different speeds.
A rule of thumb: a parsec is a little over three light-years, a light-year is a little over sixty-three thousand astronomical units, and light takes just over eight minutes to reach us from the Sun.
The unit itself is exact, but distances expressed in it are far from always so: to the neighbouring stars they are known to within fractions of a percent, to bright supergiants with a spread of tens of percent, and to distant galaxies the distances also depend on the chosen model of expansion. A number of light-years in a reference book almost always carries an uncertainty that popular captions leave out.
| Quantity | Value | Note |
|---|---|---|
| {k} | {v} | {note} |
{text}
04 · Measuring instruments
laser ranging · astrometric satellite · Cepheid · supernovaA ladder in which every rung stands on the one below
The only case where the ruler is honest
Laser pulses are still fired at the corner reflectors left on the Moon, and the returning photons are caught — one arriving for every few shots. The distance comes straight from the flight time, that is, it is measured literally in light-seconds, and the accuracy reaches centimetres. The method works within the Solar System — further out there is nobody to set up a reflector, and the ruler gives way to a chain of indirect techniques.
Swaying instead of a ruler
The nearest stars are measured geometrically: in half a year the observer, together with the Earth, moves by twice the distance to the Sun, and a nearby star shifts slightly relative to distant ones. The angle of this shift is a fraction of an arcsecond, and its reciprocal gives the distance in parsecs at once, which are then converted into light-years. A modern astrometric satellite has measured more than a billion stars this way, pushing the horizon of the direct method out to several thousand light-years.
A star that reports its own brightness
Cepheid variables pulsate, and the period of their pulsation is rigidly linked to their true luminosity: learning the period with a stopwatch, an astronomer learns how bright the star really is, and comparing that with how faint it looks, obtains the distance. It was in this way, in the nineteen-twenties, that the Andromeda nebula was found to lie far beyond our Galaxy, and the size of the world grew many times over overnight.
Where the light-year stops being convenient
At great distances the flashes of supernovae of a particular kind, whose luminosity is nearly the same, come into play, along with the redshift of lines in the spectrum. But here the unit begins to have trouble: while the light was travelling, space was expanding, and “distance” splits into several different quantities — how long the light was in flight, and how far away the object is now. That is why cosmology prefers to speak of redshift and leaves light-years to popular captions.
05 · Writing rules
ly without dots · multiple prefixes · it is a lengthYear in the name, metres in substance
Almost all mistakes with this unit come from its name: the word “year” drags time along with it, and into the text creep turns of phrase in which distance is measured in hours and speed in kilometres.
The first entry substitutes distance for duration and is therefore meaningless, like “twelve kilograms of road”. The second and fourth break the form of the symbol: two lower-case letters, without dots and without a space. The third takes the calendar year instead of the Julian one and thereby changes the value of the unit in the sixth digit. The fifth confuses distance with age: the light travelled for four years, while the star has existed for billions of years. The sixth is that very original mistake from which all the others grow, and it turns up, unfortunately, even in the captions of quite serious illustrations.
06 · Neighbouring units
parsec · astronomical unit · light-secondThree neighbouring units share one and the same range of distances, and they differ in where the measure is taken from: for the parsec it is an angle, for the astronomical unit the Earth’s orbit, and for the light-year and the light-second the travel time.
The unit of professional papers: it is derived straight from a measured angle rather than from time, and so no outside agreement about the length of a year has to be plugged into it.
The measure of the Solar System’s internal affairs: the distance from the Earth to the Sun, fixed as an exact number of metres. Beyond the orbit of Neptune it is already awkward to use.
The younger sister of the same breed, handy for communications: light takes a second and a quarter to reach the Moon, and a conversation with it can no longer be held without noticeable pauses.
Among the Simetrium data sheets, next door stands the parsec, and at the root of both units lies the same speed of light, which also measures the signal travel time in terrestrial communication lines.
07 · Historical section
archive · 1676 → 1838 → 1851 → 1984How the lateness of light became a ruler
{title}
{text}
The height of the bars shows how far it was possible to reach by direct geometric measurement in different epochs, and the white marker travels the whole way. Note that each leap is linked not to a new theory but to a new instrument: first the heliometer, then the photographic plate, then a satellite placed beyond the atmosphere.
An exact unit for inexact distances
The oddity of this unit is that it is known absolutely exactly, while everything measured with it is known only approximately. The exactness came for free: the speed of light stopped being measured the moment the metre was defined through it, and the Julian year was agreed to equal exactly thirty-one and a half million seconds, and the product of two exact numbers can be nothing but a third exact number.
Uncertainty appears at the next step, when the ruler is laid against the sky. Distances to the nearest stars are known to hundredths of a percent, but already for bright supergiants like Betelgeuse the spread of estimates reaches tens of percent, because the very surface of such stars is blurred and the parallax has to be cleaned out of a trembling image. At galactic distances a chain of indirect methods comes in, and each of its rungs adds its own uncertainty to the one inherited from the rung below.
Hence the rule of good form: a number of light-years should be given together with its uncertainty and the method by which it was obtained, because “two and a half thousand light-years” from parallax and the same number from redshift are quantities of different nature. The unit remains blameless in all this: it is exactly what it was defined to be, and all uncertainty comes not from the ruler but from what is measured with it.
Catalogue of quantities
astronomy and distancesSI base units
the metre and the second are highlighted — the light-year is built from themWhere we look and what we see
the year the light set out is counted back from the current year| Object | Distance | In parsecs | Light in transit | Light set out | What was happening then |
|---|---|---|---|---|---|
| {name} | {ly} | {pc} | {wait} | {when} | {era} |
The last column is the main argument for this unit: it turns distance into a date and so makes observation resemble reading letters sent in different centuries. It is worth remembering, however, that for distant objects these dates are nominal: while the light was travelling, space had time to expand, and “how many years it flew” no longer coincides with “how far the object is now”.