LONGITUDE. 81 



be faster than the dial till the time of the autumnal equinox, then 

 slower till the winter solstice, and again faster till the vernal equi- 

 nox. The earth's orbit is not a circle, but if the line of apsides A 

 B, see figure on page 78 corresponded with the line VI-XVIII, in 

 direction, then the clock and dial would agree at the time of winter 

 and summer solstice, i. e. December 23, and June 21st, but it 

 does not, for we have seen that the earth is in perigee December 

 31st, and in apogee July 1st, hence, in forming a table to show the 

 equation of time, i, c, the correction that must be applied to the 

 dial, or apparent solar time, in order to obtain true solar, or what 

 is called mean time, which is the time in ordinary use, we must 

 compound the two inequalities, for sometimes when the dial 

 would be fastest, on account of the unequal motion of the sun in* 

 his apparent orbit, it would be slowest from the effect of the incli- 

 nation of the plane of the ecliptic, to the plane of the equator, 

 thus, April 15th, the dial will be slower than the clock, from the 

 inequality of the sun's motion, about 7m, 23s, and at the same 

 time it will be faster, from the obliquity of the ecliptic, about the 

 .same amount, hence they are really together on that day. The 

 tables of the equation of time, are thus constructed. We have 

 now explained, somewhat at length, the method of obtaining true 

 time, from the time indicated by the sun, for it is of the utmost 

 importance to the astronomer, and the navigator, to be able, on 

 all occasions, to determine the local time. 



It must be evident, that inasmuch as the earth is round, the sun 

 will appear, as the earth turns on its axis, to rise and come to the 

 meridian successively at every point upon its surface. If, therefore, 

 some particular spot, Greenwich for example, is chosen, whose 

 meridian shall be the one from which the time, or longitude, is 

 reckoned, then if we know what time it is at that meridian, when 

 the sun happens to be on the meridian at another place, we can, 

 at once, by taking the difference between the times, viz : noon at 

 that place, and, perhaps 4 o'clock P. M., at Greenwich, determine 

 that it is 4h, west of the meridian of Greenwich, or, allowing 15 

 to the hour, 60 west. The meridian of Greenwich, where the 

 Royal Observatory is located, is generally acknowledged as the 

 first meridian, and longitude is reckoned east or west from it. In 



