100

PTOLEMY versus COPERNICUS .

explain, that they sometimes appear to he travelling amongstthe stars from east to west; at other times they seem to standstill; then once more they change their course, and apparentlymove from west to east. Their apparent movements areoccasionally very slow, at other times much more rapid.We are in no wise surprised that they long ago received the

<o

Fig. XXIV.—Path of Mercury relatively to the Earth from 1708 to 1715, after Cassini.

title of Planets,* or wanderers in the heavens; they cer-tainly migiit well have been termed very erratic travellersindeed. So peculiar are the paths which they appear totraverse when watched by an observer upon the Earth , that wethink it may be well, before we proceed any further in ourdiscussion, to ask the attention of our readers to some diagrams,

* From the Greek nkavr)Tt]s, a w anderer.