RIGHT ASCENSION AND DECLINATION', 69 



will suppose, or some other time-measurer, such as a watch, o$ 

 ordinary clock, is set going, showing, also, at that instant, Oh, Om, 

 Os. Now the astronomer's clock is, like the other time-keepers, 

 divided into 24 hours, only he reckons straight forward from 1 to 

 24 hours, while in the ordinary time-piece, the hours are numbered 

 twice in a day, from 1 to 12. We ought to say, however, that the 

 astronomer begins his day at noon the 14th of April, while the 

 civil day, April 14th, began at midnight, 12 hours before, but 

 both clocks now show Oh, Om, Os. The astronomer's clock has a 

 pendulum a trifle shorter than the common clock, which makes it 

 oscillate somewhat faster, so that the gain, on the ordinary clock, 

 may be about 3m, 56s, in a day. After an interval of 24 hours, 

 by his clock, the astronomer again looks into the transit telescope 

 and sees the supposed star, or equinoctial point, which is always 

 called the first point of Aries, just on his meridian, that is, if his 

 clock is truly adjusted, but it is not yet a day, or 24 hours, by the 

 civil time, but lacks 3m, 56s. The next dtiy the clocks will be still 

 farther apart, and in about 15 days there will be 1 hour's difference, 

 the siderial clock showing Ih, when the ordinary cjock shows 

 I2h, or noon ; the latter shows the time whea the sun is on the 

 meridian, or very nearly so, but the former indicates that the first 

 point of Aries, or the equinoctial point, crossed the meridian an 

 hour before. Now the great convenience to the astronomer is this: 

 As the whole heavens appear" to revolve around the earth in a 

 siderial day, he imagines a circle traced out in the heavens, which 

 corresponds to our equator, and, commencing at the vernal equi- 

 noctial point, or first point of Aries, he divides this celestial equator, 

 into 24 equal portions, or hours, and these he subdivides into 60 

 minutes, and each minute into 60 seconds, and he calls the distance 

 of any body from this first point of Aries, measured on the celestial 

 equator, just as we measure longitude en a globe, or map, by 

 ascertaining how far east or west the place is from Greenwich, 

 measured on the terrestrial equator ; this he calls the Right As- 

 cension of that body, designated by the initials R. A., and the 

 distance of the body north or south of the equator, he calls De- 

 clination, north or south, designated thus: N. D., or S. D., 

 corresponding with our geographical terms, north and south 



