parsec

How astronomers turned Earth's orbit into a ruler.

This is the Sun. Earth sits one astronomical unit away — 150 million kilometers.
Six months later, Earth has swung to the far side of its orbit. Watch the gold sightline as it moves.
Seen from the two ends of the orbit, the nearby star lines up against different points of the background. It appears to shift.
That shift is parallax. Half of it is the parallax angle, p.
When p is exactly one arcsecond, the star is one parsec away. Parallax, arcsecond: par-sec.
Real parallaxes are tiny. Proxima Centauri, the nearest star, shifts by 0.77″. This diagram exaggerates the angle about two hundred thousand times.

The arithmetic is one division.

d = 1 / pdistance in parsecs = 1 ÷ parallax in arcseconds

A star that shifts by half an arcsecond is two parsecs away. A tenth of an arcsecond, ten parsecs. Measure the wobble, invert it, done. This single trick anchored every rung of the cosmic distance ladder above it.

An arcsecond is 1/3600 of a degree — roughly a coin seen from four kilometers away.

one parsec, in light-years3.26
in astronomical units206,265
in kilometers30.9 trillion

It scales better than the light-year.

Astronomers work in parsecs, kiloparsecs, megaparsecs. The unit climbs cleanly from the neighborhood to the whole observable universe.

bar length ∝ log of distance