The Earth and the Moon

Abstract

The earth’s position and that of the moon lie outside of Venus in the orbit which Ptolemy and Tycho assigned to the sun. Taking the moon with it as a companion, the earth completes its periodic course around the sun in a year (the tropical year which consists of 365 days, 5 hours, 48 minutes, and 45 seconds, and the sidereal year of 365 days, 6 hours, 9 minutes, and 26 seconds, with a time difference of 20 minutes, 41 seconds). Furthermore, the earth completes its full orbit every day in twenty-four hours, turning about its own axis from west to east and repeating this revolution 365 times in a year. During the same interval the moon turns around the earth twelve times. But how and by reason of what innate capacity does the moon accomplish this? And what causes this rotary movement? Why does the earth turn about its own axis and at the same time turn about the sun annually? Why does not the moon turn about its own axis like the earth rather than complete its orbit as an epicycle about the earth in 29 days? We shall discuss these and other questions in Book V.