} Telescopic appearances of Mars, Telescopic appearances of Jupiter, I. The Comet of 1811, - 299 310 Telescopic appearance of the Moon, 325 AMERICAN GRAMMAR OF THE ELEMENTS OF ASTRONOMY INTRODUCTION. Definitions and Fundamental Principles. 1. ASTRONOMY is a mixed mathematical science, which treats of the heavenly bodies, their motions, periods, eclipses, magnitudes, distances, and other phenomena. The determination of their magnitudes, distances, and the orbits which they describe, is called plane or pure Astronomy; and the investigations of the causes of their motions, is called physical Astronomy. 2. All bodies are necessarily extended, and therefore are found existing under figure or shape, which is the boundary of extension. 3. Extension has three dimensions; length, breadth, and thickness. 4. A line is length without breadth. The ex-tremities of a line are called points. A point therefore has no extension. 5. A straight line, or right line, is the shortest distance from one point to another. 6. Every line which is neither a straight line nor composed of straight lines, is called a curve line 7. A surface is that which has length and breadth without thickness. 8. A plane is a surface, in which if any two points be joined by a straight line, the whole of that line will be in the surface. 9. Every surface which is neither a plane nor composed of planes, is called a cuore surface.. 10. A solid is that which unites the three dimensions of extension. 11. The inclination of two lines to each other is called an angle. 12. When two straight lines, lying in the same plane, may be produced both ways indefinitely, without meeting, they are parallel. 13. When a plane surface is bounded by an uniform curve line, such that all straight lines drawn to it from a certain point in the plane, are equal, the surface is called a circle. A circle is usually described with a pair of compasses; one point of which is fixed, whilst the other is turned round to the place where the motion first began. The fixed point is called the centre of the circle; and the curve line described by the other point is called the circumference. 14. The term circle also often implies the circumference, and not the circular surface; and half the circumference of a circle is usually called a semicircle. 15. Any portion of the circumference of a circle is called an arc; and one-fourth of the circumference of a circle is usually called a quadrant. 16. The circumference of every circle is divided into 360 equal parts, called degrees; and of which the symbol is 12° or 70, if 12 or 7 be their number. Each degree is also divided into 60 equal parts, called minutes; and of which the symbol is 14', or 9', if 14 or 9 be their number: and, finally, each 1 minute is divided into 60 equal parts, called seconds; and of which the symbol is 7" or 30", if 7 or 30 be their number. read 51 degrees 25 A B minutes 42 seconds plus six-seventh of one second. 17. An angle subtended by the fourth part of the circumference of a circle, or by an arc of 90 degrees, is called a right angle. Thus, the angle ACD is a right angle, if the arc AD sub tending it, contains 90 degrees. 18. An angle subtended by an arc less than 90 degrees, is called an acute angle. Thus, the angle ACB, subtended by the arc AB, which is less than 90 degrees, is an acute angle. 19. An angle subtended by an arc greater than 90 degrees, is called an obtuse angle. Thus, the angle FCD, which is subtended by the arc FD greater than 90 degrees, is called an obtuse angle. 20. It is proper to observe, that in most of the French scientific treatises, that have of late years been published, the circumference of every circle is divided first into 400 equal parts or degrees; then each degree into 100 equal parts or minutes; and, finally, each minute into 100 equal parts or seconds. So that a French degree is less than an American, in the proportion of 90 to 100; a French minute less than an American, in the proportion of 90×60 to 100×100; and a French second less than an American, in the proportion of 90×60× 60 to 100×100×100. Hence, if n be the number of French degrees, the corresponding number of American equals -, which form points to an easy arithmetical operation for finding the number of degrees in the American scale from the number in the French scale, since from the pro posed number we must subtract the same, after the decimal point has been removed one place to the left. EXAMPLES. 1. What number of degrees, minutes, and seconds, in the American scale, correspond to 100 degrees in the French scale ? 100 10 90° Answer. 2. What number of degrees, minutes, &c. in the American scale, correspond to 91° 25' in the French scale ? 91.25 9.125 82.125 6 7.50 6 30.0 Ans. 82° 7' 30". 3. What number of degrees, minutes, &c. in the American scale, correspond to 35° 0735, to 180°, to 200°, and to 360°, in the French scale? Ans. 31° 33′58′′, 162°, 180°, and 324°. |