11.40 light-years away
61 Cygni
Which star's distance was measured first, and how do you measure one at all?
For all of history before 1838, nobody knew how far away any star was. This is the one that broke the deadlock, and every distance on this site descends from the method used on it.
The oldest hard problem in astronomy
If the Earth goes round the Sun, nearby stars should shift against distant ones as we view them from opposite sides of the orbit — the same way a finger held at arm's length jumps when you swap eyes. That shift is parallax, and its absence was for centuries the strongest argument against the Earth moving at all.
The shift is real. It is just extremely small, because the stars are extremely far away. For 61 Cygni it is about a third of an arcsecond: roughly the angle a one-euro coin makes at fifteen kilometres.
Why this star, and what came of it
Bessel chose 61 Cygni because it moves quickly across the sky, which was the best available clue that a star was close. In 1838 he published a parallax that put it about eleven light-years away — within a few per cent of the modern figure, measured with a telescope and a great deal of patience.
It was the first time anyone had a true distance to anything beyond the solar system, and it converted the stars from points of light into places. Everything on this site that says 'light-years' rests on the technique he demonstrated here.