186 PHYSICS. © 
at noon, and at this time, or a few hours later, the heat is generally the 
greatest. Nevertheless, the height of the noon-day sun is neither the same 
at the same place throughout the year, nor the same at all places on the 
same day. It is only between the tropics that the sun at noon stands 
vertically over head, and this only once in the year at each tropic, and 
twice at the equator. The deviation from perpendicularity during the 
rest of the year is, however, so slight, that this region (between 23° 28’ N., 
and 23° 28'S. lat.) is naturally the warmest on the earth, deserving the 
name of torrid zone. In those countries of the earth lying about the poles 
and within the polar circles, the sun is above the horizon for a day, or 
even for days, weeks, or months (six months at the poles), during a certain 
portion of the year, and below it for equal lengths of time during another 
portion, the length of these uninterrupted days or nights increasing towards 
either pole. Nevertheless, as the sun in these regions can never ascend 
far above the horizon, his rays must always fall very obliquely in the two 
frigid zones. The rest of the earth’s surface lying between the tropics 
or polar circles, or extending from 23° 28’ to 66° 32’ north and south 
latitude, forms the two temperate zones, north and south. These include 
more than one half, or .52 of the entire surface of the earth; the two 
frigid zones about a twelfth, or over .08; and the torrid zone 
almost .40. 
The two temperate zones have four seasons (spring, summer, autumn, 
and winter), these commencing at different times in the two zones. For 
the north temperate zone spring commences March 21, when day and night 
are everywhere equal; summer on June 21, when the days are longest; 
autumn on September 23, when day and night are again equal; and winter 
on December 21. when the days are shortest. The seasons of the south 
temperate zone are precisely the reverse of these just mentioned, summer 
and spring of one answering to winter and autumn of the other. Irom 
March 21st to September 238d, the sun is north of the equator, and 
traverses from west to east the six signs, Aries, Taurus, Gemini, Cancer, 
Leo, Virgo; in the rest of the year he passes through the remaining signs, 
Libra, Scorpio, Sagittarius, Capricornus, Aquarius, and Pisces. He is in 
the equator March 21st and September 23d, at which time places on the 
equator have him in the zenith. (See pl. 23, fig. 1, where the dotted circle 
indicates the equator; the circle intersecting it in two points, and marked 
with the signs of the ecliptic, the ecliptic ; and the irregular curved line, the 
temperature in the course of the year. This figure is known as Howard’s 
diagram.) 
As a general rule, the further from the equator the greater is the 
difference between the summer and winter temperature; even in the 
vicinity of the polar circles the summer may be very hot. This depends 
upon the influence exerted by the unequal length of days. This difference 
is very slight at the tropics, where the inequality of days is inconsiderable. 
Thus at the equator the days and nights are equal throughout; at a 
latitude of 84° the longest day is twelve and a half hours; at 162° it is 
thirteen hours, or 4wo hours longer than the shortest day; and at the 
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