Measuring distance: kilometres, au, light years and parsecs

You would not give the distance from Manchester to Sydney in millimetres. Space has the same problem, only worse, so astronomers keep a few rulers of different lengths.

Kilometres work for single worlds and their moons. The Moon is 384,400 km away.

Astronomical units, written au, work inside the Solar System. One au is the distance from the Earth to the Sun, about 150 million km. Jupiter is 5 au from the Sun and Neptune is 30. Those numbers are easy to picture, which is the point of the unit.

Light years work for the stars. A light year is how far light goes in a year, about 9.5 million million km. The nearest star beyond the Sun is 4.25 light years away.

A light year is a distance, but it carries a bonus. If a star is 4.25 light years away, the light you see tonight left it 4.25 years ago.

Each unit is tied to something that can be measured.

The astronomical unit began as the average distance between the Earth and the Sun. Since 2012 it has been a fixed number, exactly 149,597,870.7 km, so that it no longer shifts each time the Earth’s orbit is measured a little better.

The light year is the distance light covers in a year of 365.25 days at 299,792.458 km every second. That comes to 9.461 million million km, or 63,241 au.

Light time is the same idea on a smaller scale, and it is the natural unit for anyone running a spacecraft. The Moon is 1.3 light seconds away. The Sun is 8 minutes 19 seconds away. Voyager 1, the most distant thing people have made, is about one light day away, so a command sent today is answered the day after tomorrow.

The parsec is the one professional astronomers use, because it falls straight out of how star distances are measured. As the Earth goes round the Sun, a nearby star seems to shift a little against the far ones. Half of the total shift is called the star’s parallax. A star whose parallax is one arcsecond is, by definition, one parsec away, and the rule is as simple as a rule can be:

distance in parsecs = 1 ÷ parallax in arcseconds

Proxima Centauri has a parallax of 0.768 arcseconds, so it is 1 ÷ 0.768 = 1.30 parsecs away. One parsec is 3.26 light years, which gives 4.25 light years.

These angles are tiny. One arcsecond is the width of a one pound coin nearly 5 km away, and no star shifts by even that much. The Gaia spacecraft measured shifts tens of thousands of times smaller, for well over a billion stars. Beyond its reach, distances have to be worked out in less direct ways, each one checked against the one before. Astronomers call that chain the distance ladder, and parallax is its bottom rung.