arlier star-places, suggested
a different criterion of distance. It is impossible to escape from the
conclusion that the apparently swiftest-moving stars are, _on the
whole_, also the nearest to us, however numerous the individual
exceptions to the rule. Now, as early as 1792,[69] Piazzi had noted as
an indication of relative vicinity to the earth, the unusually large
proper motion (5.2" annually) of a double star of the fifth magnitude in
the constellation of the Swan. Still more emphatically in 1812[70]
Bessel drew the attention of astronomers to the fact, and 61 Cygni
became known as the "flying star." The _seeming_ rate of its flight,
indeed, is of so leisurely a kind, that in a thousand years it will have
shifted its place by less than 3-1/2 lunar diameters, and that a quarter
of a million would be required to carry it round the entire circuit of
the visible heavens. Nevertheless, it has few rivals in rapidity of
movement, the apparent displacement of the vast majority of stars being,
by comparison, almost insensible.

This interesting, though inconspicuous object, then, was chosen by
Bessel to be put to the question with his heliometer, while Struve made
a similar and somewhat earlier trial with the bright gem of the Lyre,
whose Arabic title of the "Falling Eagle" survives as a time-worn
remnant in "Vega." Both astronomers agreed to use the "differential"
method, for which their instruments and the vicinity to their selected
stars of minute, physically detached companions offered special
facilities. In the last month of 1838 Bessel made known the result of
one year's observations, showing for 61 Cygni a parallax of about a
third of a second (0.3136").[71] He then had his heliometer taken down
and repaired, after which he resumed the inquiry, and finally terminated
a series of 402 measures in March 1840.[72] The resulting parallax of
0.3483" (corresponding to a distance about 600,000 times that of the
earth from the sun), seemed to be ascertained beyond the possibility of
cavil, and is memorable as the first _published_ instance of the
fathom-line, so industriously thrown into celestial space, having really
and indubitably _touched bottom_. It was confirmed in 1842-43 with
curious exactness by C. A. F. Peters at Pulkowa; but later researches
showed that it required increase to nearly half a second.[73]

Struve's measurements inspired less confidence. They extended over three
years (1835-38), but were comparatively