ASTRONOMY. INTRODUCTION. defined. ASTRONOMY is the science which treats of the heavenly Astronomy bodies, describes their appearances, determines their magnitudes, and discovers the laws which govern their motions. tronomy. When we merely state facts and describe appearances as The divi they exist in the heavens, we call it Descriptive Astronomy. sions of asWhen we compute magnitudes, determine distances, record observations, and make any computations whatever, we call it Practical Astronomy. The investigation of the laws which govern the celestial motions, and the explanation of the causes which bring about the known results, is called Physical Astronomy. astronomy. When the mariner makes use of the index of the heavens, Nautical to determine his position on the earth, such observations, and their corresponding computations, are called Nautical Astronomy. my united. By nautical astronomy we determine positions on the Geography earth, and subsequently, the magnitude of the earth; and and astrono thus, we perceive, that Geography and Astronomy must be linked together; and no one can fully understand the former science, without the aid of the latter. Astronomy is the most ancient of all the sciences, for, in the earliest age, the people could not have avoided observing the successive returns of day and night, and summer and winter. They could not fail to perceive that short days corresponded to winter, and long days to summer; and it was thus, probably, that the attentions of men were first drawn to the study of astronomy. The anti quity of as tronomy. INTRODUC. not science. In this work, we shall not take facts unless they are within Facts alone the sphere of our own observations. We shall not peremptorily state that the earth is 7912 miles in diameter; that the moon is about 240,000 miles from the earth, and the sun 95,000,000 of miles; for such facts, alone, and of themselves, do not constitute knowledge, though often mistaken for knowledge. We shall direct the mind of the reader, step by step, through the observations and through the investigations, so that he can decide for himself that the earth must be of such a magnitude, and is thus far from the other heavenly bodies; and that will be knowledge of the most essential kind. The foun All astronomical knowledge has its foundation in observadation of tion; and the first object of this book shall be to point out astronomical knowledge. what observations must be taken, and what deductions must Conven and defini tions. be made therefrom; but the great book which the pupil must study, if he would meet with success, is the one which spreads out its pages on the blue arch above; and he must place but secondary dependence on any book that is merely the work of human art. As we disapprove of the practice of throwing to the reader astounding astronomical facts, whether he can digest them or not, and as we are to take the inductive method, and to lead the student by the hand, we must commence on the supposition that the reader is entirely unacquainted even with the common astronomical facts, and now for the first time seriously brings his mind to the study of the subject; but we shall suppose some maturity of mind, and some preparation, by the acquisition of at least respectable mathematical knowledge. Every science has its technicalities and conventional terms; tional terms and astronomy is by no means an exception to the general rule; and as it will prepare the way for a clearer understanding of our subject, we now give a short list of some of the technical terms, which must be used in our composition. Horizon. Every person, wherever he may be, conceives himself to be in the center of a circle; and the circumference of that circle is where the earth and sky apparently meet. That circle is called the horizon. Altitude. The perpendicular hight from the horizon, INTRODUC. measured by degrees of a circle. Meridian. An imaginary line, north and south from any point or place, whether it is conceived to run along the earth or through the heavens. If the meridian is conceived to divide both the earth and the heavens, it is then considered as a plane, and is spoken of as the plane of the meridian. Poles. The points where all meridians come together: poles of the earth- the extremities of the earth's axis. the horizon. Zenith. The zenith of any place, is the point directly Poles of overhead; and the Nadir is directly opposite to the zenith, or under our feet. The zenith and nadir are the poles to the horizon. Verticals. All lines passing from the zenith, perpendicu- Prime ver. lar to the horizon, are called Verticals, or Vertical Circles. tical. The one passing at right angles to the meridian, and striking the horizon at the east and west points, is called the Prime Vertical. Azimuth. The angular position of a body from the meridian, measured on the circle of the horizon, is called its Azimuth. The angular position, measured from its prime vertical, is Amplitude called its Amplitude. The sum of the azimuth and amplitude must always make 90 degrees. Equator.The Earth's Equator is a great circle, east and west, and equidistant from the poles, dividing the earth into two hemispheres, a northern, and a southern. The Celestial Equator is the plane of the earth's equator conceived to extend into the heavens. Celestial equator. When the sun, or any other heavenly body, meets the celestial equator, it is said to be in the Equinox, and the tial. equatorial line in the heavens is called the Equinoctial. Latitude. -The latitude of any place on the earth, is its distance from the equator, measured in degrees on the meridian, either north or south. If the measure is toward the north, it is north latitude; if toward the south, south latitude. Equinoc |