A compendium of astronomy; containing the elements of the science, familiarly explained and illustrated, with the latest discoveriesAdapted to the use of schools and academies, and of the general reader . -tentive inspection with the naked eye. Instruments formeasuring angles were first used in the Alexandrianschool, about 300 years before the Christian era. 85. Wherever we are situated on the earth we appearto be in the center of a vast sphere, on the concave sur-face of which all celestial objects are inscribed. If wetake any two points on the surface of the sphere, as twostars for example,
A compendium of astronomy; containing the elements of the science, familiarly explained and illustrated, with the latest discoveriesAdapted to the use of schools and academies, and of the general reader . -tentive inspection with the naked eye. Instruments formeasuring angles were first used in the Alexandrianschool, about 300 years before the Christian era. 85. Wherever we are situated on the earth we appearto be in the center of a vast sphere, on the concave sur-face of which all celestial objects are inscribed. If wetake any two points on the surface of the sphere, as twostars for example, and imagine straight lines to be drawnto them from the eye, the angle included between these 83. If the common year begins on a certain day of the week,how will it end ? How is it with leap year ? How does anyday of the month compare in the preceding and following yearwith respect to the day of the week 1 How is this in leap year 1 84. How did the most ancient nations acquire their knowl-edge of the heavenly bodies 1 When were astronomical in-struments first introduced 1 52 THE EARTH. lines will be measured by the arc of the sky containedbetween the two points. Thus if HBD, (Fig. 10,) rep-Fig. resents the concave surface of the sphere, A, B, twopoints on it, as two stars, and CA, CB, straight linesdrawn from the spectator to those points, then the angu-lar distance between them is measured by the arc AB,or the angle ACB. But this angle may be measured ona much smaller circle, having the same center, as EFG,since the arc EF will have the same number of degreesas the arc AB. The simplest mode of taking an anglebetween two stars, is by means of an arm opening at ajoint like the blade of a penknife, the end of the armmoving like CE upon the graduated circle KEG. The common surveyors compass affords a simple ex-ample of angular measurement. Here the needle lies ina north and south line, while the circular rim of thecompass, when the instrument is level, corresponds tothe horizo
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Keywords: ., bookcentury1800, bookdecade1850, booksubjectastronomy, bookyear18
