. Elements of astronomy: accompanied with numerous illustrations, a colored representation of the solar, stellar, and nebular spectra, and celestial charts of the northern and the southern hemisphere. MATHEMATICAL DEFINITIONS. 21 33. A Semicircle is one-half, a Quadrant one-fourth,and a Sextant one-sixth, of a circle. 34. An angle is measured by the number of degrees inthe arc that subtends it. A right angle is subtended byone-fourth of the circumference (see C D E in Fig. 8),and is therefore an angle of 90 degrees. Fig. 9. • 35. An Ellipse is a curve every point of which is at suchdistances f
. Elements of astronomy: accompanied with numerous illustrations, a colored representation of the solar, stellar, and nebular spectra, and celestial charts of the northern and the southern hemisphere. MATHEMATICAL DEFINITIONS. 21 33. A Semicircle is one-half, a Quadrant one-fourth,and a Sextant one-sixth, of a circle. 34. An angle is measured by the number of degrees inthe arc that subtends it. A right angle is subtended byone-fourth of the circumference (see C D E in Fig. 8),and is therefore an angle of 90 degrees. Fig. 9. • 35. An Ellipse is a curve every point of which is at suchdistances from two points with-in, called its foci, that the sumof these distances is in each casethe same. AJB Gis an ellipse;D and jBare its foci. A D + A E- BJD +BE= CD + CM An ellipse may be described by fastening the ends of a piece of threadat any two points (the -foci) whose distance from each other is less thanthe length of the thread, and then drawing a line around these pointswith a pencil placed against the thread, and kept stretched out as far asthe thread will allow. The process is shown in Fig.
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Keywords: ., bookcentury1800, bookdecade1870, booksubjectastronomy, bookyear18
