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 and far stars, the approximate distance
of a few of them has been determined. The nearest one is the brightest
star in the Centaur, never visible in our northern latitudes, which
has a parallax of about one second. The next nearest is No. 61 in
the Swan, or 61 Cygni, having a parallax of 0".34. Approximate
measurements have been made on Sirius, Capella, the Pole Star,
etc., about eighteen in all. The distances are immense: only the
swiftest agents can traverse them. If our earth were suddenly to
dissolve its allegiance to the king of day, and attempt a flight
to the North Star, and should maintain its flight of one thousand
miles a minute, it would flyaway toward Polaris for thousands upon
thousands of years, till a million years had passed away, before
it reached that northern dome of the distant sky, and gave its
new allegiance to another sun. The sun it had left behind it would
gradually diminish till it was small as Arcturus, then small as