Читать книгу Geography For Dummies - Jerry T. Mitchell - Страница 59

The system of latitude lines has the following characteristics:

 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.