The radius of the earth's orbit, a line
more than ninety millions of miles in length, not only vanishes
from sight before we reach the distance of the great mass of
stars, but becomes such a mere point that when magnified by the
powerful instruments of modern times the most delicate appliances
fail to make it measurable. Here the solar motion comes to our
help. This motion, by which, as I have said, we are carried
unceasingly through space, is made evident by a motion of most of
the stars in the opposite direction, just as passing through a
country on a railway we see the houses on the right and on the
left being left behind us. It is clear enough that the apparent
motion will be more rapid the nearer the object. We may therefore
form some idea of the distance of the stars when we know the
amount of the motion. It is found that in the great mass of stars
of the sixth magnitude, the smallest visible to the naked eye, the
motion is about three seconds per century. As a measure thus
stated does not convey an accurate conception of magnitude to one
not practised in the subject, I would say that in the heavens, to
the ordinary eye, a pair of stars will appear single unless they
are separated by a distance of 150 or 200 seconds.
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