68 TH 



5t A B C D, represent the earth's annual orbit, showing the 

 in four different positions, and let a be the situation of some 

 particular meridian, that of Greenwich, for example. Now, on 

 the supposition that the earth does not rotate on its axis at all, 

 suppose it moving- in its orbit, in the order of the letters ; it is not 

 difficult to see that the effect will be the same, as though the earth, 

 remaining at rest in its orbit, had turned once on its axis during 

 the year, but in a contrary direction to its present diurnal mo- 

 tion. Thus, while at A, the sun would be on the meridian 1 

 a, but at B, one fourth of a year after, the sun would set in the 

 east, and at C, half a year afterwards, it would be midnight at the 

 same meridian, a~ At D the sun would just begin to rise in the 

 west, and finally at A would come to the meridian again. It will 

 now be understood, that although the earth does turn on its axis, 

 during its yearly circuit, yet this day as really occurs as if the 

 earth had not the diurnal revolution, hence the number of rotations, 

 measured by the sun's coming to the meridian, will be less than 

 the number as announced by a star, by one day, and therefore the 

 Siderial day must be shorter than a Solar day, by the proportional 

 part of a revolution, which is thus divided up among, and added 

 to the 365 Solar days of the year. Upon the supposition that the 

 mean Solar day is just 24 hours in length, the Siderial day will be, 

 the one-three hundred and sixty-fifth and one-fourth, of 24 hours, 

 shorter, i. e. 3m, 5"6s, very nearly,- and a star, in consequence, 

 will come to the meridian 3m, 56s, sooner than the sun, each 

 day, or will gain so much on the sun daily. 



We have more than once intimated that the time elapsed be- 

 tween a star's leaving the meridian, to its return to it again, viz : 

 23h, 56m, 4.,01s, is the precise measure of a rotation of the earth, 

 and for this reason astronomers prefer to regulate their time keepers 

 to show what is called Siderial time. Now, suppose to-day to be 

 the 14th of April, which is near the time of vernal equinox, the 

 precise point where the ecliptic intersects the equator, we will 

 imagine to be shown by a bright star. By means of his transit 

 instrument, the astronomer ascertains exactly when this star is on 

 his meridian, and just then sets his clock going, the hands showing 

 at the time Oh, Om, Os, and at the same time the town-clock, we 



