466 



NOTES. 



of a body in an ellipse and in a circle whose diameter A P is the major axis of 

 the ellipse. 



NOTE 49, p. 11. Apsides. The points P and A, fig. 10, at the extremities of 

 the major axis of an orbit. P is commonly called the perihelion, a Greek term 

 signifying round the sun; and the point A is called the aphelion, a Greek term 

 signifying at a distance from the sun. 



NOTE 50, p. 11. Ninety degrees. A circle is divided into 360 equal parts, or 

 degrees ; each degree into 60 equal parts, called minutes ; and each minute 

 into 60 equal parts, called seconds. It is usual to write these quantities thus, 

 15 16' 10", which means fifteen degrees, sixteen minutes, and ten seconds. It 

 is clear that an arc m n, fig. 4, measures the angle m C n ; hence we may say , an 

 arc of so many degrees, or an angle of so many degrees ; for, if there be ten 

 degrees in the angle m C n, there will be ten degrees in the arc m n. It is evi- 

 dent that there are 90 in a right angle, mCd, or quadrant, since it is the 

 fourth part of 360. 



NOTE 51, p. 11. Quadratures. A celestial body is said to be in quadrature 

 when it is 90 degrees distant from the sun. For example, in fig. 14, if d be the 

 sun, S the earth, and p the moon, then the moon is said to be in quadrature 

 when she is in either of the points Q or D, because the angles Q S d and D S d, 

 which measure her apparent distance from the sun, are right angles. 



NOTE 52, p. 11. Excentricity. Deviation from circular form. In fig. 6, C S 

 is the excentricity of the orbit P Q A D. The less C S, the more nearly does the 

 orbit or ellipse approach the circular form ; and, when C S is zero, the ellipse 

 becomes a circle. 



NOTE 53, p. 11. Inclination Fig. 12. 



of an orbit. Let S, fig. 12, be 

 the centre of the sun P N A n, 

 the orbit of a planet moving 

 from west to east in the direc- 

 tion N p. Let E N m e n be the 

 shadow or projection of the or- T 

 bit on the plane of the ecliptic, 

 then NSn is the intersection 

 of these two planes, for the or- 

 bit rises above the plane of the 

 ecliptic towards N#, and sinks 



below it at N P. The angle p N TO, which these two planes make with one 

 another, is the inclination of the orbit P Np A to the plane of the ecliptic. 



NOTE 54, p. 11. Latitude of a planet. The angle p S m, fig. 12, or the height 

 of the planet p above the ecliptic E N m. In this case the latitude is north. 

 Thus, celestial latitude is the angular distance of a celestial body from the 

 plane of the ecliptic, whereas terrestrial latitude is the angular distance of a 

 place on the surface of the earth from the equator. 



NOTE 55, p. 12. Nodes. The two points N and n, fig. 12, in which the orbit 



