A TREATISE ON ASTRONOMY; DESCRIPTIVE, PHYSICAL, AND PRACTICAL. DESIGNED FOR SCHOOLS, COLLEGES, AND PRIVATE STUDENTS BY H. N. ROBINSON, A. M., FORMERLY PROFESSOR OF MATHEMATICS IN THE U. S. NAVY; AUTHOR OF A TREATISE ALBANY: ERASTUS H. PEASE & CO., 82 STATE STREET CINCINNATI: JACOB ERNST, NO. 183 MAIN STREET. Entered, according to act of Congress, in the year 1849, BY HORATIO N. ROBINSON, In the Clerk's Office of the District Court of the United States, for the District of Ohio. PREFACE. To give at once a clear explanation of the design and intended character of this work, it is important to state that its author, in early life, imbibed quite a passion for astronomy, and, of course, he naturally sought the aid of books; but, in this field of research, he was really astonished to find how little substantial aid he could procure from that source, and not even to this day have his desires been gratified. Then, as now, books of great worth and high merit were to be found, but they did not meet the wants of a learner; the substantially good were too voluminous and mathematically abstruse to be much used by the humble pupil, and the less mathematical were too superficial and trifling to give satisfaction to the real aspirant after astronomical knowledge. Of the less mathematical and more elaborate works on astronomy there are two classes-the pure and valuable, like the writings of Biot and Herschel; but, excellent as these are, they are not adapted to the purposes of instruction; and every effort to make class books of them has substantially failed. From the other class, which consists of essays and popular lectures, little substantial knowledge can be gathered, for they do not teach astronomy; as a general thing, they only glorify it; they may excite our wonder concerning the immensity or grandeur of the heavens, but they give us no additional power to investigate the science. Another class of more brief and valuable productions were, and are always to be found, in which most of the important facts are recorded; such as the distances, magnitudes, and motions of the heavenly bodies; but how these facts became known is rarely explained: this is what the true searcher after science will always demand, and this book is designed expressly to meet that demand. In the first part of the book we suppose the reader entirely unacquainted with the subject; but we suppose him competent to the task-to be, at least, sixteen years of age-to have a good knowledge of proportion, some knowledge of algebra, geometry, and trigonometry-and then, and not until then, can the study be pursued with any degree of success worth mentioning. Such a person, and with such acquirements as PREFACE. PREFACE. we have here designated, we believe, can take this book and learn astronomy in comparatively a short time; for the chief design of the work is, to teach whoever desires to learn: and it matters not where the learner may be, in a college, academy, school, or a solitary student at home, and alone in the pursuit. The book is designed for two classes of students-the well prepared in the mathematics, and the less prepared; the former are expected to read the text notes, the latter should omit them. With the text notes, we conceive it, or rather designed it to be, a very suitable book to give sound elementary instruction in astronomy; but we do not offer the work as complete on practical astronomy; for whoever becomes a practical astronomer will, of course, seek the aid of complete and elaborate sets of tables, such as would be improper to insert in a school book. We have inserted tables only for the purpose of carrying out a sound theoretical plan of instruction, and, therefore, we have given as few as possible, and those few in a very contracted form. The epochs for the sun and moon may be extended forward or backward, to any extent, by any one who understands the theory. The chapters on comets, variable stars, &c., are compilations, and are printed in smaller type; and the works to which we are most indebted, are Herschel's Astronomy and the Cambridge Astronomy, originally the work of M. Biot. Other parts of the work, we believe, will be admitted as mainly original, by all who take pains to examine it. The chief merits claimed for this book are, brevity, clearness of illustration, anticipating the difficulties of the pupil, and removing them, and bringing out all the essential points of the science. Some originality is claimed, also, in several of our illustrations, particularly that of showing the rationale of tides rising on the opposite sides of the earth from the moon; and in the general treatment of eclipses; but it is for others to determine how much merit should be awarded for such originalities; we have, however, used greater conciseness and perspicuity in general computations than is to be found in most of the books on this subject; and this last remark will apply to the whole work. CONTENTS. Page. INTRODUCTION.-Definition of terms, &c.,...... |