PART I. FIG. PAGE DANTE STUDYING. _From a fresco by Luca Signorelli at Orvieto_ _Frontispiece_ 1. The Sun’s Path in the Sky at different Seasons 11 2. The Moon at Sunset, soon after New, and three days old 14 3. Path of Mars among the Stars, 1909 16 4. The Star Sphere 19 5. Synodic and Sidereal Periods 21 STAR MAP 34 THE MOON-GOD OF UR. _From a Cylinder-seal in the British Museum, date about_ B.C. 2400. _By permission of the Trustees of the British Museum_ 36 6. The triple Star-sign of the Babylonians 37 7, 8, 9. The young Moon and the Pleiades after sunset, as seen in Babylon, B.C. 1000 42 10. The Scorpion. _From a Babylonian boundary stone of about_ B.C. 1120, _set up in the reign of Nebuchadnezzar I. Now in the British Museum_ 43 11. The Goat with Fishes’ Scales. _From a Babylonian boundary stone_ 43 THE BOAT OF THE SUN TRAVELLING OVER THE SKY. _From an ancient Egyptian papyrus. Reproduced from Flammarion’s Astronomical Myths_ 46 12. The Universe according to Anaximander 59 13. The Universe of Leucippus 63 14. The Universe of Democritus 64 15. The Universe of Pythagoras 71 16. The System of Philolaus: night on Earth 73 17. do. twelve hours later: day on Earth 73 18. Earth and Sun according to Heraclides 74 19. The Spheres of the Sun in the system of Eudoxus 90 20. Method of Aristarchus for finding the Distance of the Sun 108 21. Pekin Observatory 114 22. A Pekin Astrolabe of the thirteenth century A.D. _From photographs taken in 1888 and published in the Bulletin de la Société belge d’Astronomie_ 116 23. The Moveable Eccentric 119 24. The Movement of Spica 124 25. The Sun and the Equinox 128 26. The Movement of the Earth’s Axis which is the true cause of Precession. _From Young’s Manual of Astronomy_, 1902 131 27. Precession of the Equinoxes 133 28. Path of Mars, 1909 139 29.