110 | ASTRONOMY. 
tion of temperature for the northern and southern hemispheres, in proportion 
as these are turned more or less to the sun. Pl. 9, fig. 1, shows that the 
earth (revolving in the direction of the arrows), wherever in her orbit she’ 
may happen to be, always has her axis constantly parallel to itself and at the 
same angle of inclination to the plane of her orbit. The earth is found at A, 
on Dec. 21, in the beginning of winter, the shortest day of the year for our 
northern hemisphere. The sun is then at his. greatest distance south, 
describes the tropic of Capricorn, and at noon stands in the zenith to all 
inhabitants of the earth whose latitude is 23° 27’ south. The north pole. 
nevertheless, lies in the middle of its six morths’ night. The boundary of 
the earth’s shadow will fall in the north polar circle, whose inhabitants will 
then have a night of 24 hours. On the other hand, this will be summer to 
the southern hemisphere, and its inhabitants will have the longest days. 
This point, A (fig. 1), of the earth’s orbit is the winter solstice ; the earth, 
which now, seen from the sun, stands in 0° of Cancer, will traverse the part 
AB of its orbit in 89 days, from Dec. 21 to March 20, thus marking the 
duration of the winter. 
B is the position of the earth on the first day of spring (autumn for the 
southern hemisphere): the sun then describes the equator (see the direction 
of the equinoctial line), and as the shaded portion of the earth divides the 
parallel circles into two equal parts, the days and nights will at this time be 
equal all over the world. At the north pole the long day of six months is 
just commencing. The earth, standing in 0° of Libra, now traverses the 
‘part BC of its path in 93 days, from March 20th to June 21, marking the 
duration of spring. 
C is the position of the earth on June 21, the first day of summer (of 
winter in the southern hemisphere); the sun then describes the tropic of 
Cancer, and the north pole lies in the middle of its day of six months. The 
‘inhabitants of the north polar, or arctic circle, see the sun for 24 hours, and 
all other dwellers on the northern hemisphere have longer days than nights. 
The south pole lies in the middle of its night of six months, and the inhabit- 
ants of the southern hemisphere have longer nights than days. C is the 
summer solstice, and the summer lasts 94 days, that is, the time from June 
21st to Sept. 23, during which the earth passes from C to D. The earth 
itself, on the 21st June, as seen from the sun, stands in 0° of Capricornus. 
Finally, on the 23d Sept., the first day of autumn (spring for the southern 
hemisphere), the earth is at D, the sun standing again in the equator, the 
days and nights are again equal all over the earth. The sun now becomes 
invisible to the north pole, and visible to the south pole, the autumnal equi- 
nox occurring on Sept. 23d, the vernal on March 20. Autumn lasts 89 
days, that is, the interval from Sept. 23d to Dec. 21, in which time the 
earth traverses the distance DA of its orbit. 
From the preceding explanation it is evident that the four seasons are not 
of equal duration, for spring and summer together embrace 187 days, while 
autumn and winter last only for 178 days. There is thus a difference of 
nine days between the times occupied by the earth in traversing BC, CD, 
and DA, AB. This difference of nine days is a consequence partly of the: 
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