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.