dred 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.