The Outline of Science, Vol. 1 | Page 8

from one
another. If a shell were shot in a straight line from one side of
Neptune's orbit to the other it would take five hundred years to
complete its journey. Yet this distance, the greatest in the Solar System
as now known (excepting the far swing of some of the comets), is
insignificant compared to the distances of the stars. One of the nearest
stars to the earth that we know of is Alpha Centauri, estimated to be
some twenty-five million millions of miles away. Sirius, the brightest
star in the firmament, is double this distance from the earth.
We must imagine the colony of planets to which we belong as a
compact little family swimming in an immense void. At distances
which would take our shell, not hundreds, but millions of years to
traverse, we reach the stars--or rather, a star, for the distances between
stars are as great as the distance between the nearest of them and our
Sun. The Earth, the planet on which we live, is a mighty globe bounded
by a crust of rock many miles in thickness; the great volumes of water
which we call our oceans lie in the deeper hollows of the crust. Above
the surface an ocean of invisible gas, the atmosphere, rises to a height
of about three hundred miles, getting thinner and thinner as it ascends.
[Illustration: LAPLACE

One of the greatest mathematical astronomers of all time and the
originator of the nebular theory.]
[Illustration: Photo: Royal Astronomical Society.
PROFESSOR J. C. ADAMS
who, anticipating the great French mathematician, Le Verrier,
discovered the planet Neptune by calculations based on the
irregularities of the orbit of Uranus. One of the most dramatic
discoveries in the history of Science.]
[Illustration: Photo: Elliott & Fry, Ltd.
PROFESSOR EDDINGTON
Professor of Astronomy at Cambridge. The most famous of the English
disciples of Einstein.]
[Illustration: FIG. 1.--DIAGRAMS OF THE SOLAR SYSTEM
THE COMPARATIVE DISTANCES OF THE PLANETS
(Drawn approximately to scale)
The isolation of the Solar System is very great. On the above scale the
nearest star (at a distance of 25 trillions of miles) would be over one
half mile away. The hours, days, and years are the measures of time as
we use them; that is: Jupiter's "Day" (one rotation of the planet) is made
in ten of our hours; Mercury's "Year" (one revolution of the planet
around the Sun) is eighty-eight of our days. Mercury's "Day" and
"Year" are the same. This planet turns always the same side to the Sun.]
[Illustration: THE COMPARATIVE SIZES OF THE SUN AND THE
PLANETS (Drawn approximately to scale)
On this scale the Sun would be 17-1/2 inches in diameter; it is far
greater than all the planets put together. Jupiter, in turn, is greater than
all the other planets put together.]

Except when the winds rise to a high speed, we seem to live in a very
tranquil world. At night, when the glare of the sun passes out of our
atmosphere, the stars and planets seem to move across the heavens with
a stately and solemn slowness. It was one of the first discoveries of
modern astronomy that this movement is only apparent. The apparent
creeping of the stars across the heavens at night is accounted for by the
fact that the earth turns upon its axis once in every twenty-four hours.
When we remember the size of the earth we see that this implies a
prodigious speed.
In addition to this the earth revolves round the sun at a speed of more
than a thousand miles a minute. Its path round the sun, year in year out,
measures about 580,000,000 miles. The earth is held closely to this
path by the gravitational pull of the sun, which has a mass 333,432
times that of the earth. If at any moment the sun ceased to exert this
pull the earth would instantly fly off into space straight in the direction
in which it was moving at the time, that is to say, at a tangent. This
tendency to fly off at a tangent is continuous. It is the balance between
it and the sun's pull which keeps the earth to her almost circular orbit.
In the same way the seven other planets are held to their orbits.
Circling round the earth, in the same way as the earth circles round the
sun, is our moon. Sometimes the moon passes directly between us and
the sun, and cuts off the light from us. We then have a total or partial
eclipse of the sun. At other times the earth passes directly between the
sun and the moon, and causes an eclipse of the moon. The great ball of
the earth naturally trails a mighty shadow across space, and the moon is
"eclipsed" when it passes