The inequality in the length of the days in different parts of the year,
occasioned by the inclination of the Earth’s axis, is of itself
sufficient to produce a marked variation in temperature.
During the day the Earth receives from the Sun more heat than it
radiates into space; while during the night it radiates more than it
receives. Hence a succession of long days and short nights results in an
accumulation of heat, raising the average temperature and producing
summer; while long nights and short days result in a temperature below
the average, producing winter.
Again, the heating power of the Sun in each hemisphere is greatest at
the period of the longest days, because of its greater altitude in the
heavens; and least at the period of shortest days. Thus long days and a
high sun operate together to produce the high temperature of summer;
while long nights and a low sun cause the low temperature of winter.
The following table gives the length of the longest day, excluding the
time of twilight, and of the shortest night, in the different latitudes,
with the difference of duration in hours and minutes, thus exhibiting
more clearly the above law.