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Page 23
[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 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
could be discerned by the naked eye, until at last it would finally
fade out in utter darkness long before the new sun was reached.
Light can traverse the distance around our earth eight times in
one second. It comes in eight minutes from the sun, but it takes
three and a quarter years to come from Alpha [Page 73] Centauri,
seven and a quarter years from 61 Cygni, and forty-five years from
the Polar Star.
Sometimes it happens that men steer along a lee shore, dependent
for direction on Polaris, that light-house in the sky. Sometimes it
has happened that men have traversed great swamps by night when that
star was the light-housse of freedom. In either case the exigency
of life and liberty was provided for forty-five years before by a
Providence that is divine.
We do not attempt to name in miles these enormous distances; we
must seek another yard-stick. Our astronomical unit and standard of
measurement is the distance of the earth from the sun--92,500,000
miles. This is the golden reed with which we measure the celestial
city. Thus, by laying down our astronomical unit 226,000 times, we
measure to Alpha Centauri, more than twenty millions of millions
of miles. Doubtless other suns are as far from Alpha Centauri and
each other as that is from ours.
Stars are not near or far according to their brightness. 61 Cygni is
a telescopic star, while Sirius, the brightest star in the heavens,
is twice as far away from us. One star differs from another star
in intrinsic glory.
The highest testimonies to the accuracy of these celestial observations
are found in the perfect predictions of eclipses, transits of planets
over the sun, occultation of stars by the moon, and those statements
of the Nautical Almanac that enable the sailor to know exactly
where he is on the pathless ocean by the telling of the stars:
"On the trackless ocean this book is the mariner's trusted friend
and counsellor; daily and nightly its revelations bring safety
to ships in all parts of the [Page 74] world. It is something more
than a mere book; it is an ever-present manifestation of the order
and harmony of the universe."
Another example of this wonderful accuracy is found in tracing
the asteroids. Within 200,000,000 or 300,000,000 miles from the
sun, the one hundred and ninety-two minute bodies that have been
already discovered move in paths very nearly the same--indeed two of
them traverse the same orbit, being one hundred and eighty degrees
apart;--they look alike, yet the eye of man in a few observations
so determines the curve of each orbit, that one is never mistaken
for another. But astronomy has higher uses than fixing time,
establishing landmarks, and guiding the sailor. It greatly quickens
and enlarges thought, excites a desire to know, leads to the utmost
exactness, and ministers to adoration and love of the Maker of
the innumerable suns.
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