Representing this motion on such a scale that the distance of our
planet from the sun shall be one inch, we find that the nearest
star, on the same scale, will be more than four miles away, and
scarcely one out of a million will be at a less distance than ten
miles. It is only by the most wonderful perfection both in the
heliometer, the instrument principally used for these measures,
and in methods of observation, that any displacement at all can be
seen even among the nearest stars. The parallaxes of perhaps a
hundred stars have been determined, with greater or less
precision, and a few hundred more may be near enough for
measurement. All the others are immeasurably distant; and it is
only by statistical methods based on their proper motions and
their probable near approach to equality in distribution that any
idea can be gained of their distances.
To form a conception of the stellar system, we must have a unit of
measure not only exceeding any terrestrial standard, but even any
distance in the solar system. For purely astronomical purposes the
most convenient unit is the distance corresponding to a parallax
of 1", which is a little more than 200,000 times the sun's
distance. But for the purposes of all but the professional
astronomer the most convenient unit will be the light-year--that
is, the distance through which light would travel in one year.
Pages:
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80