Light crosses the 384,400 kilometers between the Moon and your eye in about one and a quarter seconds. Sunlight needs 8 minutes and 20 seconds. Nothing with mass can be pushed to that speed and no signal has ever been found to beat it, which makes light a dependable yardstick. A light-year is simply the distance a beam of light covers in one year in empty space: about 9.46 trillion kilometers, or 5.88 trillion miles.

The word “year” causes most of the confusion. A light-year is a length, in the same way that “a three-hour drive” describes how far away Raleigh is. Astronomers use it because ordinary units collapse under the size of the numbers. The nearest star beyond the Sun, Proxima Centauri, is about 40 trillion kilometers away. Saying 4.24 light-years is easier, and it also tells you something useful: the light you see from Proxima tonight left it a little more than four years ago.

For most of history light was assumed to arrive instantly. In 1676 the Danish astronomer Ole Rømer, working in Paris, noticed that the eclipses of Jupiter’s moon Io ran late when Earth was on the far side of its orbit from Jupiter and early when it was near. He concluded that light takes time to cross the extra distance. His figures implied a speed roughly a quarter too low, but the principle was right.

Later measurements closed in on the modern value. James Bradley used the slight tilt of starlight caused by Earth’s motion in 1728. Albert Michelson spent half a century refining mirror-and-rotating-prism experiments in the United States. By the 1970s laser measurements were so precise that the meter itself became the weak link, and in 1983 the meter was redefined as the distance light travels in 1/299,792,458 of a second. The speed of light is now exact because the ruler is built from it.

Because light takes time to arrive, every view of the sky is a view of the past. You see the Moon as it was 1.3 seconds ago and the Sun as it was eight minutes ago. Sirius, the brightest star in the night sky, appears as it was 8.6 years ago. The Andromeda Galaxy, the farthest thing most people can see without a telescope, shows light that left about 2.5 million years ago, before our species existed.

This is the working method of cosmology. Telescopes such as Hubble and Webb do not need a time machine to study the young universe. They only need to collect light that has been traveling for most of the 13.8 billion years since the Big Bang.

Nobody can stretch a tape to a star. Distances are built up in steps, each one calibrating the next, and astronomers call the result the cosmic distance ladder.

Three units cover almost everything. The astronomical unit (AU) is the average Earth–Sun distance, about 149.6 million kilometers, and it suits the solar system. The light-year suits the stars. Professional papers usually prefer the parsec, the distance at which a star shows a parallax of one arcsecond. One parsec is 3.26 light-years.

The universe is 13.8 billion years old, so it is natural to assume we can see 13.8 billion light-years in every direction. The real figure is about 46.5 billion light-years. Space has been expanding the entire time the light was in flight, so the places that emitted the oldest light we see are now far more distant than they were when the light set out. No rule is broken: nothing moves through space faster than light. It is space itself that has grown.

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