Geography For Dummies. Jerry T. Mitchell
Читать онлайн книгу.
line called the axis. Look on a globe and you find two fixed points, halfway around Earth from each other, where the axis intersects Earth’s surface: the North Pole and the South Pole. So, Hipparchus drew a line that ran all the way around the globe and was always an equal distance (hence, equator) from the two Poles. The result is a latitudinal “starting line” from which all others could be placed on the globe.
The prime meridian
The longitudinal “starting line” is called the prime meridian, which signifies its importance as the line from which all other lines of longitude are numbered. Locating this line proved more problematic than locating the equator. Quite simply, no logical equivalent of the equator exists with respect to longitude. Thus, while the equator came into general use as the latitudinal starting line, mapmakers were perfectly free to draw the longitudinal starting line anywhere they pleased. And that is what they did.
Typically, mapmakers drew the prime meridian right through their country’s capital city. By the late 1800s, lack of a universal prime meridian had become a real pain in the compass. International trade and commerce were growing. Countries were claiming territory that would become colonial empires. But one country’s world maps did not agree with another’s, and the international climate made it increasingly advisable that they do so. I have a map hanging in my living room to prove the point! There are different longitude coordinates at the top of the map compared to those at the bottom. Both were given to aid a map reader more used to a coordinate system beginning with a different meridian.
As a result of this growing confusion, in 1884 the International Meridian Conference was convened in Washington, D.C. to promote the adoption of a common prime meridian. Out of that was born an agreement to adopt the British system of longitude as the world standard. Thus, the global grid’s prime meridian passes right through the Royal Greenwich Observatory (which is in the London suburb of Greenwich) as well as parts of western Europe and Africa, and the Atlantic Ocean. The British system was chosen largely because in 1884 Britain was the world’s major military and economic power, and also had a fine tradition of mapmaking.
Getting Lined Up
With the starting lines in place, one can now contemplate putting all of the other lines of latitude and longitude on a globe. In doing that, Hipparchus used the notion that 360 degrees (°) are in a circle (But why 360? See the sidebar “Why is Earth 360° round?”). Accordingly, he drew lines of latitude such that each and every one is separated by one degree of arc from the next. He then did the same with longitude. This is why lines of latitude and longitude are referred to as degrees.
Latitude
Lines of latitude run across the map (east-west) and are called parallels because each line of latitude is parallel to every other line of latitude.
The equator (Latitude 0°) divides the world into the Northern Hemisphere and the Southern Hemisphere.WHY IS EARTH 360° ROUND?The ancient Sumerians believed there were 360 days in a year. Like other civilizations way back when, the Sumerians equated their gods with celestial objects. Not surprisingly, the sun god was especially important. Because it took the Earth 360 days to travel around the sun (or so they believed), the Sumerians figured the number 360 had extra-special significance. As a result, they developed a system of mathematics based on multiples of 6 and 60. Nowadays, we would call it base-6 mathematics or (get ready for this) a sexagesimal system. In any event, 6 times 60 equals 360.The ancient Egyptians adopted the ancient Sumerians’ numerical ideas, and eventually discovered the error concerning the length of the year. But by then, however, the number 360 had achieved such acceptance and status that the Egyptians decided not to mess with it. Accordingly, they kept the 360-day year but, being fun-loving people, added an annual 5-day holiday.The ancient Greeks, like the ancient Egyptians, were adept at adopting things and ideas from civilizations more ancient than they. So, when Hipparchus, in about 140 B.C., began fiddling with the notion of dividing a circle (and Earth) into degrees, he chose the number 360.
Starting from the equator, each successive line (degree) of latitude is numbered consecutively both to the north and to the south as far as the North Pole (Latitude 90° North) and South Pole (Latitude 90° South).
Except for the equator, each line of latitude is identified by a number between 0 and 90 and by the word North or South (or the abbreviations N or S) to indicate its location north or south of the equator. Thus, the line that is 20 degrees north of the equator is referred to as Latitude 20° North (or 20° North Latitude). It would be misleading and incomplete to just call this line “Latitude 20” because another line of latitude south of the equator could also be called “Latitude 20.”
Only one line of latitude — the equator — is a great circle, a line that divides Earth in half.
WHAT’S WRONG WITH THIS MAP?
(© John Wiley & Sons Inc.)
The answer to the headline is this: Nothing much, really. You could say the map is upside down, and you would be right to a point. After all, nowadays maps commonly have north at the top. But considered as a planet in the multi-dimensional vastness of space, Earth has no “right side up.” Thus, no compelling scientific reason exists as to why you can’t make a map with south toward the top — other than that it would look strange and confusing to most people. Indeed, in olden times maps were oriented every which way. Chinese maps tended to have south at the top, and early Christian maps had east on top with Jerusalem at the center.
Suffice to say, there are a number of competing ideas about how north got on top. The discovery of magnetic north? The preference of ninth-century Frankish emperor Charlemagne, who as powerful king would have held a bit of sway? The widespread use of the Mercator projection by navigators (see Chapter 4 to find out more about this)?
Regardless of how it came about, north at the top tends to be the default, but there are exceptions. If you have a bit of free time, go online and search for Chile’s Directorate General of Civil Aviation. Their logo includes a map that bucks convention.
Longitude
Lines of longitude run from the North Pole to the South Pole (top to bottom of the map) and are called meridians.
As opposed to latitude, no two lines of longitude are parallel to each other. Rather, successive lines of longitude are about 70 miles apart at the equator, but from there they slowly converge until they come together at the two poles (see Figure 3-2).
The prime meridian (Longitude 0°) divides the world into the Eastern Hemisphere and the Western Hemisphere.
Starting from the prime meridian, every line (degree) of longitude is numbered consecutively to the east and to the west half way around the world. Because Earth is 360 degrees around, 180 degrees of longitude lie east and west of the prime meridian.
Every line of longitude (except the prime meridian and the 180-degree line) is identified by a number from 1 to 179, and by the words East or West (or the letters E or W) to indicate its location east or west of the prime meridian. Thus, the line that is 20 degrees east of the prime meridian is referred to as Longitude 20° East. It would be misleading to call this