Page:The Moon (Pickering).djvu/13

 PREFACE

T IS INTENDED in the present volume to give an account of some of the more recent advances in our knowledge of the Moon, leaving to the text-books a statement of the information that was earlier acquired, and with which most people are already more or less familiar. Some of these topics are lightly touched upon, however, in the second chapter, where some reference to them seemed necessary.

After the perusal of this volume the question may perhaps be asked how it is that anything new could have been discovered in regard to so conspicuous an object as the Moon—an object that must have been constantly observed by the generations of astronomers already passed, many of whom were provided with far larger and more costly instruments than those used by the author.

The answer is a comparatively simple one. For many researches in astronomy, such as the study of faint stars, the separation and measurement of close doubles, the study of their spectra, etc., a telescope furnished with a large objective or lens is the paramount consideration. On the other hand, for planetary and lunar research the all-important consideration is a "steady" atmosphere. Such an atmosphere is found only in low latitudes, far from the great anti-cyclones of more northern regions. The earlier astronomers located their observatories either in high northern or high southern latitudes, thereby avoiding the very regions where the best results would have been obtained.

The Arequipa station of the Harvard Observatory, where most of the Harvard discoveries on the Moon, on Mars and on Jupiter's satellites were made, is furnished with a telescope of only a little more than twelve inches aperture. It has, however, the enormous advantage that it is situated within but sixteen degrees of the equator.

The "seeing"—that is, the atmospheric definition—there is so good that many of the more obvious facts in the solar system not previously known were discovered. Of the obvious facts still unknown that were left, one—the canals in the Moon—was detected in Jamaica, a station similarly located, within eighteen degrees of the v