THEORETICAL ASTRONOMY. 97 
II]. Turoreticat ASTRONOMY. 
The Circle and Ellipse. 
18. In the study of Astronomy a knowledge of the circle and ellipse is 
absolutely necessary. Ifa straight line, CD (pl. 6, fig. 3), make a complete 
revolution in the same plane around one extremity, C, the other end, Bh 
will describe the circumference, DHAGFD, in which each point is equally 
distant from the centre,C. The plane surface inclosed by this circumfer- 
ence is called the circle. Every straight line drawn from the centre, C, to 
the circumference is a radius; and every straight line connecting two 
points of a circumference and passing through the centre is a diameter 
(AB). All radii are equal to each other, so also are all diameters. Any 
straight line connecting two points of the circumference, not passing 
through the centre, is called a chord (KF). It divides the circle into two 
unequal parts, EAHDBF and EGF, of which the latter, as the smaller, is 
called the segment. The diameter, AB, on the contrary, divides the circle 
into two equal parts, AHDB and AEGFSB, called semicircles. The angle. - 
DCB, formed by two radii, DC and CB, at the centre of the circle, is called 
a central angle; the surface, BCD, inclosed between a central angle and the 
are of the circumference inclosed between the radii, is called a sector. 
Finally, a straight line outside of the circle and touching the circumference 
in only one point, is called a tangent, JK. 
An ellipse, ADBEA (pl. 6, fig. 4), is a complete curve, possessing the 
peculiarity that, if two straight lines be drawn from certain points, SS’, in 
its area, called the foci, to any point in the circumference, their sum will 
be equal to the sum of similar lines drawn to any other points. Thus 
SP +S’P=S8p+NS8’p. This same sum will also be always equal to the 
length of the greater axis, AB. The lines SP and S’P (or also Sp and Sp) 
are called the radii vectores of the point P (or p). The straight line, DC, 
passing through the centre, C, at right angles to the major axis, AB, is 
called the minor axis. CS or CS’ is called the eccentricity of the ellipse. 
Should this ellipse represent the orbit of a planet in one of whose foci the 
sun is situated, then if the planet be situated at P or p, the line SP or Sp 
will be the radius vector of the planet. 
Parallaz ; Horizontal Parallax and Parallax in Altitude; Parallax 
of a Place. 
19. Fig. 5 will serve for the explanation of what astronomers mean by 
the word parallaz. Let ABD represent the meridian circle of a place of 
observation, A; C the centre of the earth ; let HJ be a part of the infinitely 
distant sphere of the heavens; finally, let the moon be at M. The line 
HCD represents the true horizon, aAa’ the apparent horizon of the place, 
A. If the moon be supposed to stand in the horizon at M, it will be 
ICONOGRAPHIC ENCYCLOPADIA.—VOL. I. 7 97 
