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Page 42
The attractive influence of the sun also gives us a tide four-tenths
as great as that of the moon. When these two influences of the sun
and moon combine, as they do, in conjunction--when both bodies
are on one side of the earth; or in opposition, sun and moon being
on opposite sides of the earth--we have spring or increased tides.
When the moon is in its first or third quarter, _i. e._, when a
line from the moon to the earth makes a right angle with one from
the sun to the earth, these influences antagonize one another,
and we have the neap or low tides.
It is easy to see that if, when the moon was drawing its usual
tide, the sun drew four-tenths of the water in a tide at right
angles with it, the moon's tide must be by so much lower. Because
of the inertia of the water [Page 148] it does not yield instantly
to the moon's influence, and the crest of the tide is some hours
behind the advancing moon.
The amount of tide in various places is affected by almost innumerable
influences, as distance of moon at its apogee or perigee; its position
north, south, or at the equator; distance of earth from sun at
perihelion and aphelion; the position of islands; the trend of
continents, etc. All eastern shores have far greater tides than
western. As the earth rolls to the east it leaves the tide-crest
under the moon to impinge on eastern shores, hence the tides of
from seventy-five to one hundred feet in the Bay of Fundy. Lakes and
most seas are too small to have perceptible tides. The spring-tides
in the Mediterranean Sea are only about three inches.
This constant ebb and flow of the great sea is a grand provision for
its purification. Even the wind is sent to the sea to be cleansed.
The sea washes every shore, purifies every cove, bay, and river
twice every twenty-four hours. All putrescible matter liable to
breed a pestilence is carried far from shore and sunk under fathoms
of the never-stagnant sea. The distant moon lends its mighty power
to carry the burdens of commerce. She takes all the loads that
can be floated on her flowing tides, and cheerfully carries them
in opposite directions in successive journeys.
It must be conceded that the profoundest study has not mastered
the whole philosophy of tides. There are certain facts which are
apparent, but for an explanation of their true theory such men as
Laplace, Newton, and Airy have labored in vain. There are plenty
of other worlds still to conquer.
[Page 150]
[Illustration: Fig. 57.--Lunar Day.]
[Page 151]
THE MOON.
New moon, [Symbol]; first quarter, [Symbol]; full moon, [Symbol];
last quarter, [Symbol].
EXTREME DISTANCE FROM THE EARTH, 259,600 MILES; LEAST, 221,000
MILES; MEAN, 240,000 MILES. DIAMETER, 2164.6 MILES [2153, LOCKYER].
REVOLUTION ABOUT THE EARTH, 29-1/2 DAYS. AXIAL REVOLUTION, SAME
TIME.
When the astronomer Herschel was observing the southern sky from
the Cape of Good Hope, the most clever hoax was perpetrated that
ever was palmed upon a credulous public. Some new and wonderful
instruments were carefully described as having been used by that
astronomer, whereby he was enabled to bring the moon so close that
he could see thereon trees, houses, animals, and men-like human
beings. He could even discern their movements, and gestures that
indicated a peaceful race. The extent of the hoax will be perceived
when it is stated that no telescope that we are now able to make
reveals the moon more clearly than it would appear to the naked
eye if it was one hundred or one hundred and fifty miles away.
The distance at which a man can be seen by the unaided eye varies
according to circumstances of position, background, light, and
eye, but it is much inside of five miles.
Since, however, the moon is our nearest neighbor, a member of our
own family in fact, it is a most interesting object of study.
A glance at its familiar face reveals its unequal illumination.
All ages and races have seen a man in the moon. All lovers have
sworn by its constancy, and only part of them have kept their oaths.
Every twenty-nine or thirty days we see a silver crescent in the
west, and are glad if it comes over the right shoulder--so [Page
152] much tribute does habit pay to superstition. The next night it
is thirteen degrees farther east from the sun. We note the stars it
occults, or passes by, and leaves behind as it broadens its disk,
till it rises full-orbed in the east when the sun sinks in the west.
It is easy to see that the moon goes around the earth from west to
east. Afterward it rises later and smaller each night, till at
length, lost from sight, it rises about the same time as the sun,
and soon becomes the welcome crescent new moon again.
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