In other words, the distance of these
stars is immeasurably great. The actual amount of light emitted by
each is certainly thousands and probably tens of thousands of
times that of the sun.
Notwithstanding the difficulties that surround the subject, we can
at least say something of the distance of a considerable number of
the stars. Two methods are available for our estimate--measures of
parallax and determination of proper motions.
The problem of stellar parallax, simple though it is in its
conception, is the most delicate and difficult of all which the
practical astronomer has to encounter. An idea of it may be gained
by supposing a minute object on a mountain-top, we know not how
many miles away, to be visible through a telescope. The observer
is allowed to change the position of his instrument by two inches,
but no more. He is required to determine the change in the
direction of the object produced by this minute displacement with
accuracy enough to determine the distance of the mountain. This is
quite analogous to the determination of the change in the
direction in which we see a star as the earth, moving through its
vast circuit, passes from one extremity of its orbit to the other.
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