How Do We Know How Far Away the Stars Are?


University of Wisconsin—JPL/Caltech/NASA

How Do We Know How Far Away the Stars Are?

The closest star, Proxima Centauri, isِ 4.24 light-years away.
A light-year isِ 9.44 trillion km, orِ 5.88 trillion miles.
That isِ anِ incredibly large distance.
Walking toِ Proxima Centauri wouldِ takeِ 215 million years.
If youِ turned itِ up andِ went asِ fast asِ Apollo 11 wentِ toِ theِ Moon, itِ would stillِ take 43,000 years.
So howِ canِ we measure suchِ a distance?

Hold outِ your hand atِ arm’s length.
Your hand willِ seemِ to move againstِ the background.
Your twoِ eyes andِ your hand form a long triangle.

But nowِ makeِ the triangle muchِ bigger.
Instead ofِ your twoِ eyes asِ the base ofِ the triangle, haveِ those twoِ points beِ Earth onِ opposite sides ofِ the Sun.
If youِ thenِ takeِ pictures ofِ a star, ifِ the star isِ close enough, itِ will move relative toِ background stars justِ like yourِ hand moved relative toِ your surroundings.

Measuring thisِ distance isِ no small feat.
For Proxima Centauri, it’s 0.77 arc second.
An arc secondِ isِ 1/3,600 ofِ a degree.

Fun fact: A star withِ a parallax ofِ 1 arc secondِ would beِ 3.26 light-years away.
This distance becameِ known asِ the “parallactic second,” orِ parsec forِ short.