70 THK WORLD. 



latitude. The only difference between longitude as reckoned on 

 the earth, and right ascension as measured in the heavens, is, 

 the former is reckoned east or west from any arbitrary point, 

 Greenwich, or Washington, for example, but the latter is reckoned 

 eastward, or in the order of the signs, completely around, and 

 always from the first point of Aries, which is a determined point 

 in the sky, being the position of the vernal equinox, and which 

 turns around, apparently, with the whole celestial concave, in its 

 diurnal revolution. 



When a new comet appears, and is announced as having a R. 

 A. of 6h, and 10m, and N. D, of 2 15', the astronomer places his 

 transit telescope, or .other similar instrument, so as to point 2 15' 

 north of the imaginary celestial equator, for he knows just how 

 high above the horizon this is situated, and when his clock points 

 out 6h and 10m, he looks into the telescope and sees the newly 

 discovered object. Thus the precise position occupied by any 

 star, or planet, in the heavens, can be mapped down, using right 

 ascensions and declinations in the same manner as terrestrial 

 longitudes and latitudes. We should like to say a great deal more 

 on this subject, buj, the nature of our work forbids. 



Our ordinary clocks and watches, are adjusted to keep mean 

 solar time.. It would, at first, be supposed, that the interval from 

 noon to noon, although longer than a Siderial day, would, never- 

 theless, be an equal period, so thafif a clock was adjusted to show 

 24 hours during the interval of the sun's leaving the meridian at 

 any particular season of the year, to his return to it the next day, 

 it would always indicate an interval of 24h, for any similar revolu- 

 tion. This is not the case, and we think we can show, very 

 plainly, why it is not. The instaht when the sun is actually on 

 the meridian, is called the time of apparent noon, or 12 o'clock 

 apparent time, although, a clock regulated to keep what is called 

 mean time, or mean solar time, may then show but llh, 45m. 

 The difference between apparent time and mean solar time, is 

 called the equation of time, i. e. the correction which must be 

 applied in order to determine true time, from the time indicated 

 by the sun. It is evident that Sun-Dials will indicate apparent 

 time, and we will, therefore, devote the remainder of this chapter 



