ON PRACTICAL ASTRONOMr, 545, 



respect to the earth's' centre, and to the place of the spectator, which is 

 equal to the angle subtended at the centre of the luminary by the seniidia- 

 meter of the earth passing through the place of observation; since all cal- 

 culations of the geocentric places of the heavenly bodies are referred to the 

 earth's centre. This angle, which is to be added to the apparent altitude, 

 amounts sometimes, in the case; of the moon, when near the horizon, to more 

 than a degree; the refraction, which is in a contrary direction, and is to be 

 subtracted from the altitude, being at the horizon about 33 minutes. (Plate 

 XXXV. Fig. 513.) 



The most important applications of practical astronomy are in the de- 

 termination of the latitudes and longitudes of places on the earth's surface. 

 The latitude, which is the angular distance of the place from the equator, or 

 the angle formed by the plane of its horizon with the earth's axis, is easily 

 ascertained by finding the meridian altitude of a body, of which the decli- 

 nation is known; since, by deducting or adding the declination, we have at 

 once the elevation of the equinoctial, or of the plane of the equator, above the 

 horizon, and subtracting this from a right angle, we find the elevation of the 

 pole, or the latitude. (Plate XXXV. Fig. 512.) 



It is also common to determine the latitude of a place by means of two 

 altitudes observed at different times in the same day, noticing accurately the 

 interval of time that elapses between the observations. This method has 

 a great advantage in cloudy weather, when it is, not possible to insure an 

 observation of a meridian altitude. 



The longitude of a place, or the relative position of its meridian, is by -no 

 means so readily determined. For this purpose it becomes necessary to as- 

 certain the time that elapses between the passages of a given point in the 

 heavens over its meridian and some other meridian wliich serves as a standard 

 of comparison. Thus, if the sun arrives three hours later at the meridian of 

 any place than at the meridian of London, that place must necessarily be 

 45 degrees west of London, or in 45* west longitude: and if we know, when 

 it is noon at the given place, that it is three o'clock in the afternoon at 

 Greenwich, we maybe certain that we are in some part of a meridian 45° west 

 i)i' that of Greenwich. Had we perfect timekeepers, we might easily adjust them 



