ond in the centre, would all shrink to a mere
point. Not quite to a point from the nearest stars, or we should
never be able to measure the distance of any of them. Professor Airy
says that our orbit, seen from the nearest star, would be the same
as a circle six-tenths of an inch in diameter seen at the distance
of a mile: it would all be hidden by a thread one-twenty-fifth of an
inch in diameter, held six hundred and fifty feet from the eye. If a
straight line could be drawn from a star, Sirius in the east to the
star Vega in the west, touching our [Page 71] earth's orbit on one
side, as T R A (Fig. 28), and a line were to be drawn six months
later from the same stars, touching our earth's orbit on the other
side, as R B T, such a line would not diverge sufficiently from a
straight line for us to detect its divergence. Numerous vain
attempts had been made, up to the year 1835, to detect and measure
the angle of parallax by which we could rescue some one or more of
the stars from the inconceivable depths of space, and ascertain
their distance from us. We are ever impelled to triumph over what is
declared to be unconquerable. There are peaks in the Alps no man has
ever climbed. They are assaulted every year by men zealous of more
worlds to conquer. So these greater heights of the heavens have been
assaulted, till some ambitious spirits have outsoared even
imagination by the certainties of mathematics.

[Illustration: Fig. 28.]

It is obvious that if one star were three times as far from us as
another, the nearer one would seem to be displaced by our movement
in our orbit three times as much as the other; so, by comparing one
star with another, we reach a ground of judgment. The ascertainment
of longitude at sea by means of the moon affords a good illustration.
Along the track where the moon sails, nine bright stars, four planets,
and the sun have been selected. The nautical almanacs give the
distance of the moon from these successive stars every hour in
the night for three years in advance. The sailor can measure the
distance at any time by his sextant. Looking from the world at
D (Fig. 29), the distance of the moon and [Page 72] star is A E,
which is given in the almanac. Looking from C, the distance is only
B E, which enables even the uneducated sailor to find the distance,
C D, on the earth, or his distance from Greenwich.

[Illustration: Fig. 29.--Mode of Ascertaining Longitude.]

So, by comparisons of the near