parsec
The universe has a ruler.
Begin with
One orbit.
Earth. The Sun. A baseline 150 million kilometers long.
Then wait
Six months.
Earth reaches the other side.
The star appears
to move.
We did.
Two viewpoints. One tiny change in perspective.
That apparent motion is
Parallax.
1″
When the parallax angle p is one arcsecond, the distance is one parsec.
A name that contains its own definition.
The arithmetic is one division
d=1p
Measure the angle. Invert it. Done.
1 pc
A stellar neighborhood.
1 kpc
The Milky Way begins to take shape.
1 Mpc
Galaxies become neighbors.
1 Gpc
The universe becomes structure.
14 Gpc
The present-day radius of the observable universe.
A unit of distance.
Not a unit of time.
parsec.net
The definition
One angle. One division.
A parsec is the distance at which one astronomical unit subtends an angle of one arcsecond. In practical astronomy, distance in parsecs is the reciprocal of parallax in arcseconds: d = 1 / p.
The word was proposed in 1913 by astronomer Herbert Hall Turner, combining parallax and arcsecond. Proxima Centauri has a parallax angle of about 0.77 arcseconds, placing it about 1.3 parsecs away.