the value 0".349.

Later determinations for alpha2 Centauri, by Gill,[2] make its parallax
0".75--This is the nearest known fixed star; and its light takes 4 1/3
years to reach us. The light year is taken as the unit of measurement
in the starry heavens, as the earth's mean distance is "the
astronomical unit" for the solar system.[3] The proper motions and
parallaxes combined tell us the velocity of the motion of these stars
across the line of sight: alpha Centauri 14.4 miles a second=4.2
astronomical units a year; 61 Cygni 37.9 miles a second=11.2
astronomical units a year. These successes led to renewed zeal, and
now the distances of many stars are known more or less accurately.

Several of the brightest stars, which might be expected to be the
nearest, have not shown a parallax amounting to a twentieth of a
second of arc. Among these are Canopus, alpha Orionis, alpha Cygni, beta
Centauri, and gamma Cassiopeia. Oudemans has published a list of
parallaxes observed.[4]

_Proper Motion._--In 1718 Halley[5] detected the proper motions
of Arcturus and Sirius. In 1738 J. Cassinis[6] showed that the former
had moved five minutes of arc since Tycho Brahe fixed its position. In
1792 Piazzi noted the motion of 61 Cygni as given above. For a long
time the greatest observed proper motion was that of a small star 1830
Groombridge, nearly 7" a year; but others have since been found
reaching as much as 10".

Now the spectroscope enables the motion of stars to be detected at a
single observation, but only that part of the motion that is in the
line of sight. For a complete knowledge of a star's motion the proper