Chapter XI.. (4) _find The_ =north= _by Means of Other

(4) Find the =North= by means of other stars than the Pole Star (which is the North Star) in case that star is obscured by clouds.

(a) This can be done by finding the constellation of Cassiopeia, see Chapter I; and also by Pegasus; Auriga and Orion, as explained in Chapter II.

(5) Have a general knowledge of the positions and movements of the Earth, Sun, Moon, and the Planets; and of tides, eclipses, meteors and comets.

(a) By reading the chapters on the Sun, Planets, Earth and Moon and

(b) Other things in the sky carefully, you will be able to pass the requirements named in No. 5.

(6) Plot on at least two nights per month for six months the positions of all naked eye planets visible between sundown and one hour thereafter. The plot of each planet shall contain at least three fixed stars with their names or designations; colors of planets and stars are to be recorded as observed.

(a) How to plot the position of a planet is fully explained in the chapter on Planets, but you should also read the one on The Stars of the Zodiac, and have a good star map (App. O).

APPENDIX B.

=Figures.=--When explaining the positions and forms of things, it is often necessary to use certain terms and figures, that is to say, lines which are either real or imaginary, but which can be drawn on paper. See Fig. 185.

(1) A straight line is, of course, a line which runs uniformly in the same direction, and which is regular and without curves. A straight line is the shortest distance between two points. (2) When we say that lines are parallel, we mean that they lie so that every part of each is equally spaced from the other. (3) A line is horizontal when it is parallel with the level surface of the Earth under it. (4) A line is perpendicular to the surface of the Earth when it is plumb, that is, in a line with the center of the Earth. (5) A vertical line generally means a plumb line. (6) A right angle is formed when a vertical, or a perpendicular, line meets a horizontal line. (7) A circle is a curved line, all points of which are equally distant from its center. Circular means round like a circle. (8) By diameter is meant a straight line drawn from one side, or half of a circle, to the opposite side through its center. (9) The radius of a circle is a straight line drawn from the center of a circle, or a ball, to its circumference. (10) A ring is a disk or object having a circular hole cut in its center. (11) An arc of a circle is a part of a circle. (12) A quarter circle is, of course, the one-fourth part of a circle. (13) A tangent is a line which touches a curve but does not cut it. (14) An ellipse is an oval figure, drawn on a plane surface. (15) The equator is a circle which divides the Earth or other ball into equal parts, and is 90 degrees from the north and south poles. (16) The ecliptic is a circle round the Sky in whose plane lies the orbit of the Earth.