SPHERICAL ASTRONOMY. 85 
The Seasons ; Daily and Yearly Motion of the Sun. 
9. The illustration on pl. 9, figs. 1, 2, 3, serves to elucidate the theory of 
seasons, and of the daily and annual motion of the sun. The following may 
be taken in connexion with what is said further on (sect. 26) respecting 
the annual motion of the earth round the sun. 
The visible horizon is that circle in whose centre we are supposed to 
stand, and which bounds our vision, and upon which the heavens appear to 
rest. The sun is said to rise in the morning when it appears above the 
horizon, as at C, pl. 9, fig. 2, and to se¢ in the evening when sinking below 
the horizon, as at C’. The sun appears each day to describe a greater or 
less circular arc, CMC’, above the horizon, and this arc is constantly 
inclined towards the horizon. Noon of a place is that time of day when 
the sun at M has arrived at its greatest height above the horizon. ‘The 
vertical of a place (Paris in fig. 2) is the straight ine VT, parallel to the 
direction of the plumb line. The line RSO is the meridian line of Paris, 
or the direction of a shadow at time of noon at Paris; the four points, O, 
G, R, L, are the four cardinal points of the horizon. The sun appears to 
traverse a part, CMC’, of a circle by day, completed at night in C’/NC. 
beneath the horizon, and the entire apparent circuit of the sun about the 
earth lasts 24 hours, or an entire day. A careful examination will, how- 
ever, disclose the fact that the points of rising and setting of the sun, as 
also those in which he cuts the meridian, vary from day to day, with respect 
to the horizon of one and the same place. Thus, on 21st December, CMC’ 
is the day circle of the sun; GQL that of the 20th March, greater than the 
preceding, and KUA that of the 21st of June, which is greatest of all. 
Hence the sun appears to remain stationary for a time, and then to return 
towards the south, describing anew the arc GQL to the 23d of September, 
and CMC’ on the 21st of December, as before. Here he appears stationary 
for a time, and then returns to the north as before. These ares described 
by the sun are all parallel to each other: the greatest of them, KUAK (to- 
wards the north) is called the tropic of Cancer, the least, CMC’NC (to the 
south) the tropic of Capricorn. The two periods of the year in which the 
sun describes the tropics are called the solstices. The circle described on 
the 23d September and the 20th March is called the equator, and the two 
periods when the sun describes the equator are called the eqguinoxes. Con- 
sequently the sun seems to move north for six months in the year, viz. 
from December 21st to 21st June, and south for six months, from 21st June 
to 21st December. 
There is still another motion observable in the case of the sun, viz. a 
daily progression eastward of about a degree, while the fixed stars retain 
their relative positions unchanged. Since now 1° in space answers to four 
minutes in time, the sun will return to the same point of the heavens four 
minutes later each day, which in 90 days will amount to six hours. The 
imaginary straight line, PF, passing through the centre, 8, of the horizon, 
and about which the sun appears to describe his circles, is called the azis 
of the heavens. Since all parallel circles of the sun are inclined to the 
85 
