Page:Polar Exploration - Bruce - 1911.djvu/17

Rh standing at the North Pole the direction is always south.

These conditions apply equally to the South Pole, except that the terms north and south have in every case to be reversed. It is very important to get a proper grip of these facts if one is to have a proper conception of where the Polar Regions are, and to account for various special phenomena peculiar to these two parts of the earth.

Theoretically it is convenient to define the Polar Regions as those areas that lie round about the North Pole and round about the South Pole, within the Arctic and Antarctic Circles, which are defined by being those circles of latitude where the sun on midwinter-day does not rise and where on midsummer-day it does not set.

In contrast to the tropical regions, where the sun is always vertically overhead at some place at noon on two days (at the north and south limits on one day) every year, and always reaches in every part an altitude exceeding about 43 degrees, in the Polar Regions the sun is never more than 23½ degrees above the horizon. On account of this great obliquity of the sun's rays in the Polar Regions the sun has less heating power and the regions are colder, while in winter intense cold prevails because of the entire absence of the sun.