size—Its beauty in the sky—Remarkable circularity of its orbit—Probable absence of seasons and stable conditions of temperature and weather on Venus—Its dense and abundant atmosphere—Seeing the atmosphere of Venus from the earth—Is the real face of the planet hidden under an atmospheric veil?—Conditions of habitability—All planetary life need not be of the terrestrial type—The limit fixed by destructive temperature—Importance of air and water in the problem—Reasons why Venus may be a more agreeable abode than the earth—Splendor of our globe as seen from Venus—What astronomers on Venus might learn about the earth—A serious question raised—Does Venus, like Mercury, rotate but once in the course of a revolution about the sun?—Reasons for and against that view

CHAPTER IV

MARS, A WORLD MORE ADVANCED THAN OURS 85

Resemblances between Mars and the earth—Its seasons and its white polar caps—Peculiar surface markings—Schiaparelli's discovery of the canals—His description of their appearance and of their duplication—Influence of the seasons on the aspect of the canals—What are the canals?—Mr. Lowell's observations—The theory of irrigation—How the inhabitants of Mars are supposed to have taken advantage of the annual accession of water supplied by the melting of the polar caps—Wonderful details shown in charts of Mars—Curious effects that may follow from the small force of gravity on Mars—Imaginary giants—Reasons for thinking that Mars may be, in an evolutionary sense, older than the earth—Speculations about interplanetary signals from Mars, and their origin—Mars's atmosphere—The question of water—The problem of temperature—Eccentricities of Mars's moons

CHAPTER V

THE ASTEROIDS, A FAMILY OF DWARF WORLDS 129

Only four asteroids large enough to be measured—Remarkable differences in their brightness irrespective of size—Their widely scattered and intermixed orbits—Eccentric orbit of Eros—the nearest celestial body to the earth except the moon—Its existence recorded by photography before it was discovered—Its great and rapid fluctuations in light, and the curious hypotheses based upon them—Is it a fragment of an exploded planet?—The startling theory of Olbers as to the origin of the asteroids revived—Curious results of the slight force of gravity on an asteroid—An imaginary visit to a world only twelve miles in diameter

CHAPTER VI

JUPITER, THE GREATEST OF KNOWN WORLDS 160

Jupiter compared with our globe—His swift rotation on his axis—Remarkable lack of density—The force of gravity on Jupiter—Wonderful clouds—Strange phenomena of the great belts—Brilliant display of colors—The great red spot and the many theories it has given rise to—Curious facts about the varying rates of rotation of the huge planet's surface—The theory of a hidden world in Jupiter—When Jupiter was a companion star to the sun—The miracle of world-making before our eyes—Are Jupiter's satellites habitable?—Magnificent spectacles in the Jovian system

CHAPTER VII

SATURN, A PRODIGY AMONG PLANETS 185

The wonder of the great rings—Saturn's great distance and long year—The least dense of all the planets—It would float in water—What kind of a world is it?—Sir Humphry Davy's imaginary inhabitants of Saturn—Facts about the rings, which are a phenomenon unparalleled in the visible universe—The surprising nature of the rings, as revealed by mathematics and the spectroscope—The question of their origin and ultimate fate—Dr. Dick's idea of their habitability—Swedenborg's curious description of the appearance of the rings from Saturn—Is Saturn a globe of vapor, or of dust?—The nine satellites and "Roche's limit"—The play of spectacular shadows in the Saturnian system—Uranus and Neptune—Is there a yet undiscovered planet greater than Jupiter?

CHAPTER VIII

THE MOON, CHILD OF THE EARTH AND THE SUN 212

The moon a favorite subject for intellectual speculation—Its nearness to the earth graphically illustrated—Ideas of the ancients—Galileo's discoveries—What first raised a serious question as to its habitability—Singularity of the moon's motions—Appearance of its surface to the naked eye and with the telescope—The "seas" and the wonderful mountains and craters—A terrible abyss described—Tycho's mysterious rays—Difference between lunar and terrestrial volcanoes—Mountain-ringed valleys—Gigantic cracks in the lunar globe—Slight force of gravity of the moon and some interesting deductions—The moon a world of giantism—What kind of atmospheric gases can the moon contain—The question of water and of former oceans—The great volcanic cataclysm in the moon's history—Evidence of volcanic and other changes now occurring—Is there vegetation on the moon?—Lunar day and night—The earth as seen from the moon—Discoveries yet to be made

CHAPTER IX

HOW TO FIND THE PLANETS 256

It is easy to make acquaintance with the planets and to follow them among the stars—The first step a knowledge of the constellations—How this is to be acquired—How to use the Nautical Almanac in connection with the charts in this book—The visibility of Mercury and Venus—The oppositions of Mars, Jupiter, and Saturn

INDEX 277

LIST OF ILLUSTRATIONS