One is allowed to look at that
distant house through one edge of the pane of glass and then
through the opposite edge; and he has to determine the change in
the direction of the distant house produced by this change of one
foot in his own position. From this he is to estimate how far off
the other mountain is. To do this, one would have to measure just
about the amount of parallax that Bessel found in his star. And
yet this star is among the few nearest to our system. The nearest
star of all, Alpha Centauri, visible only in latitudes south of
our middle ones, is perhaps half as far as Bessel's star, while
Sirius and one or two others are nearly at the same distance.
About 100 stars, all told, have had their parallax measured with a
greater or less degree of probability. The work is going on from
year to year, each successive astronomer who takes it up being
able, as a general rule, to avail himself of better instruments or
to use a better method. But, after all, the distances of even some
of the 100 stars carefully measured must still remain quite
doubtful.
Let us now return to the idea of dividing the space in which the
universe is situated into concentric spheres drawn at various
distances around our system as a centre.
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