50 August 2014 sky & telescope
OBSERVING
Celestial Calendar
High overhead in late summer, near Deneb, is an un-
assuming pair of orange dwarf stars just visible to the
naked eye at magnitudes 5.2 and 6.0. Humble they may
appear, but they’ve held a grand place in astronomical
history since 1838. That year they became the fi rst stars to
have their parallax and distance measured, proving once
and for all that stars are suns extremely far away.
Why did such an insignifi cant-looking pair of stars
win this honor? Friedrich Wilhelm Bessel, an astronomer
at Königsberg Observatory in Germany, took an educated
guess that 61 Cygni was especially close to us because of
its fast proper motion across the sky. In 1804 the Italian
star cataloger Giuseppe Piazzi had discovered that the
pair was crossing the starry background at 5 arcsecondsThe Flight of 61 Cygni
Watch the proper motion of this famous double orange dwarf.
Two of the four closest K dwarfs in the sky glow against the rich Cygnus Milky Way. They form a fi ne double star for small telescopes,
and if you pay attention over the years, you can watch their fast common proper motion away from a faint background star. S&T’s
Dennis di Cicco took this image on May 11th. An enlargement is below.0.1°