Hemisphere

THE WORD hemisphere means “half a sphere.” In geography, the term refers to half the Earth, and the enclosing boundary line of a hemisphere is a great circle. A space traveler viewing the Earth from a great distance will see only half the earth, a hemispheric perspective. This is true because the Earth is a sphere and only one side of the Earth can be seen in any one particular view.

Our only look at the entirety of the Earth’s surface is through world maps. The map view of the world is important for that reason despite the fact that every is a scaled-down replica of the Earth, 2) presents a generalized view of the Earth’s surface, and 3) is a distortion of the Earth’s surface to varying degrees.

There are an infinite number of possible hemispheric views of the Earth. When an observer looks directly at any particular point on the Earth’s surface, a hemisphere is defined. However, there are a number of hemispheres that have special importance for the discriminating viewer. For instance, if the equator is the great circle enclosing a hemisphere, the viewer will be seeing either the Northern Hemisphere or the Southern Hemisphere. In both cases, the center of the hemispheric view will be one of the poles. Viewing the Southern Hemisphere would show vast expanses of the southern oceans, interrupted by the southern extremes of South America and Africa, about half of AUSTRALIA, all of NEW ZEALAND, and a few scattered islands in the PACIFIC OCEAN. In the center of the view would be the geographical South Pole and the continent of ANTARCTICA, a true landmass covered by heavy sheets of ice.

For example, South Pole station sits on ice that measures 8,000 ft (2,438 m) in thickness. The presence of a preponderance of water in the Southern Hemisphere has a great impact of the climate in this region. Since large water bodies do not heat up and cool as quickly as comparably sized land areas, the annual changes in temperature are much lower.

The view of the Northern Hemisphere is distinctly different. At the center of the view is the geographical North Pole, a point that is impossible to permanently mark on the surface because of the constant movement of the ice on the Arctic Ocean. South of the pole are the northern regions of the great landmasses of the northern hemisphere. Particularly imposing in this regard is the longitudinal sweep of Eurasia extending over 180 degrees of longitude. RUSSIA alone boasts of having 11 time zones. The northernmost reaches of North America are also prominent in this view, its land area bracketed by the ATLANTIC and PACIFIC oceans. The climatic impact of the presence of large land areas in the Northern Hemisphere is profound. The variability in annual temperatures is extreme in this region. It is not uncommon for areas within the continents of Eurasia and North America to have average winter temperature near 0 degrees F (-17.7 degrees C) and summer averages in the 70 and 80 degrees F (21 to 26 degrees C) range, temperature ranges not found in the southern hemisphere.

Familiar to everyone are the Eastern and Western Hemispheres. The two are separated by a great circle comprising two longitude lines, both running from pole to pole in opposite directions. The longitude line designated 0 degrees passes through Greenwich, England, and serves as one of the dividers between the two hemispheres. Its counterpart passes through the Pacific Ocean 180 degrees from the prime meridian. Hence, it is identified as longitude 180 degrees. An observer standing at any location along either of these lines is on the boundary between the eastern and western hemispheres. The prime meridian is the conventional starting point for assigning degree values to longitude lines.