Geography Reference
In-Depth Information
Winter
December 21 is the approximate date of the winter solstice. The vertical rays then strike the Tropic of
Capricorn, marking thefirstdayofwinter.Alsoonthat date, theNorthernHemisphere experiences its
longest period of darkness and shortest period of night. The vertical rays then “turn northward.” For
about the next three months, nighttime periods continue to exceed daytime periods in the Northern
Hemisphere, but by a difference that diminishes daily. Finally, on or about March 21, the vertical rays
are back at the equator, marking the spring equinox. Day and night are again equal and the seasonal
cycle is complete.
Special lines of latitude revisited
Before leaving this section, discussion is in order of those other two “special lines of latitude” — the
Arctic and Antarctic Circles. Each demarcates parts of the world where something peculiar happens.
Specifically, every location north of the Arctic Circle and south of the Antarctic Circle experiences at
least one continuous 24-hour period of daylight, and at least one continuous 24-hour period of dark-
ness during each year. Moreover, the farther north and south one goes with respect to those two lines,
the greater is the number of days of complete daylight and darkness. The extreme cases occur at the
two poles, where the year is divided into one six-month-long period of daylight followed by one six-
month-long period of darkness.
To help understand this, look back at Figure 9-3. The North Pole is 90 degrees' worth of lat-
itude from the equator. On the first day of fall, the sun is directly overhead at the equator,
but will appear to an observer at the North Pole to be on the horizon (90° from overhead).
Everyday for the next six months, the vertical rays strike Earth below the equator. From the
perspective of the North Pole, the sun is below the horizon all the while, so darkness ensues.
Every other latitude between the North Pole and the Arctic Circle, Latitude 66 1/2° North, also
experiences at least one continuous 24-hour period of darkness. The significance of Latitude
66 1/2° North is that it's exactly 90° from the Tropic of Capricorn, which marks the farthest
southerly point that feels the sun's vertical rays.
The opposite occurs during spring and summer, when the North Pole has continuous daylight. As the
summer solstice approaches, more and more latitudes south of the North Pole experience similar con-
ditions. Finally, on the summer solstice, the experience of a continuous day of sunlight reaches its
most southerly locale — the Arctic Circle.
