78 ASTRONOMY. 
equator ; at the equator being 0°, and at either of its poles 90°. The part 
CQ’ of the equator ACQ, lying between the vernal equinox C, and the 
declination circle NS’Q’ of the star 8’, is called the right ascension of this 
star; it is reckoned from the vernal equinox C, around the equator from 
west to east, varying from 0° to 360° (or from 0 to 24 hours), and is 
expressed in these degrees (or hours). By means, then, of the right ascen- 
sion and declination of a star, we fix its position in the heavens for a long 
interval of time, with respect to the equator. 
Draw from the pole, P, of the ecliptic, ECK, through the star 8’, an are 
PS’K’, cutting the ecliptic at right angles, then the great circle of which 
PS’K’ is only the quadrant, is called the circle of latitude of the star S’, 
and the are K’S’, the latitude of this star. This is north when the star is 
above the ecliptic, as in the figure, and south when it is below. The 
latitude is estimated in degrees from the ecliptic; at the ecliptic it is 0°, 
at either pole 90°. The part CK’, of the ecliptic ECK, lying between the 
vernal equinox C, and the circle of latitude PS’K’ of the star S’, is called 
the longitude of the star. It is estimated on the ecliptic from west to east, 
and commences with the vernal equinox, expressed in degrees from 0° to 
360°, or in terms of the 12 signs of the zodiac. The latitude and longitude 
of a star completely determine its position on the celestial sphere with 
respect to the ecliptic. 
The arc HN of our figure represents the height of the pole N above the 
horizon HRT, that is, the altitude of the pole; and the are TQ the height 
of the equator ARQ (on the meridian), above this horizon HRT, or the 
altitude of the equator. The altitude of pole and equator for the same 
place of observation, are together equal to 90°. The spherical angle Q’NQ, 
having the north pole, N, for its vertex, or the corresponding arc QQ’ of the 
equator, is called the hour angle of the star 8S’. 
3. The following is a very satisfactory proof among many well known 
ones, of the spherical shape of the earth. Suppose an observer (jig. 8) 
stationed at a particular point, 8, from which a ship sails off in a straight line. 
At a short distance the whole of the vessel will be visible to the water-line ; 
with increasing distance the ship decreases in apparent height, but is visible 
to the water’s edge. After reaching the horizon at B, there is not only a 
still further decrease in apparent size, but a disappearance of part of the 
vessel itself, beginning with the hull. At C only the sails and masts are 
visible ; the appearance presented is represented by c. From a higher point 
T, however, whose horizon passes through D, the hull of the ship will be 
again visible. The distance still increasing, the lower sails seem just to 
sink into the water, as at d, and finally to disappear entirely. The dis- 
tinctness with which the summits of the masts are observed, just before 
their disappearance, must carry home the conviction, that but for the 
intervening segment, ABCDE, of the sea, the actual distance, SE, is not so 
great as to prevent an equally perfect view of the whole. 
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