Elements of astronomy ..with explanatory notes, and questions for examination . struct an Ellipse. Stick two pins intoa piece of paper, at a short distance from each other, asat F and Fl, and pass over them a loop of thread, placea pencil in the loop, and keeping the thread tight, a tri-angle will be formed like FMF1, the pencil being at the pencil completely round F and Fl, its pointwill mark out an ellipse. For since in making the circuit,the length of the loop does not change, neither the dis-tance between F and F1, it necessarily follows that thesum of the distances from the pins
Elements of astronomy ..with explanatory notes, and questions for examination . struct an Ellipse. Stick two pins intoa piece of paper, at a short distance from each other, asat F and Fl, and pass over them a loop of thread, placea pencil in the loop, and keeping the thread tight, a tri-angle will be formed like FMF1, the pencil being at the pencil completely round F and Fl, its pointwill mark out an ellipse. For since in making the circuit,the length of the loop does not change, neither the dis-tance between F and F1, it necessarily follows that thesum of the distances from the pins to the pencil; viz.,F*M, FM, &c, is invariable. 18. Eccentricity. Ellipses differ among the foci are near the centre of the ellipse, the ellipseapproaches the form of a circle; but if the foci departwidely from it the length of the conjugate axis is smallin proportion to that of the major axis, and the ellipseis said to be very The distance from the centre to either focus ; viz., FC,or FlC, is termed the eccentricity of the ellipse. In FIG. Figs. 9 and 10, two ellipses are exhibited which differgreatly in their eccentricity ; one being almost a circle,and the other very oval. 1. Eccentric, from ex out of\ and centrum centre, (Latin) out of thecentre. Construct nn ellipse 1 What is meant by the term eccentricity 1 22 ASTRONOMY. 19. A Sphere. A sphere is a solid, bounded by acurved surface, every point of which is equally distantfrom a point vjithin, called the centre; every line pass-ing through this centre, and limited by the surface, is adiameter. The half of this line is a radius of the FIG. 11.
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