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 PAGE Chart of Mars _Frontispiece_ Diagram showing causes of day and night on portions of Mercury 35 Regions of day and night on Mercury 38 Venus's atmosphere seen as a ring of light 56 View of Jupiter _facing_ 168 Three views of Saturn _facing_ 186 Diagram showing the moon's path through space 217 The lunar Alps, Apennines, and Caucasus _facing_ 222 The moon at first and last quarter _facing_ 226 Phases and rotation of the moon 250 Charts showing the zodiacal constellations: 1. From right ascension 0 hours to 4 hours 259 2. " " 4 " " 8 " 261 3. " " 8 " " 2 " 263 4. " " 12 " " 16 " 265 5. " " 16 " " 20 " 267 6. " " 20 " " 24 " 269 OTHER WORLDS CHAPTER I INTRODUCTORY