knowledge is vital. For instance, the changing angle of the sun in the sky is respon-
sible for seasons and is dictated by latitude. Time, with all its implications for life, is
kept by the relative longitudes of locations. The world system of latitude and longi-
tude can be used to precisely specify absolute location on the planet and, so, is
essential for mapping and navigation.
Latitude and longitude is a spherical grid, and Earth’s surface is not spherical
because of its oblate shape and topographic features. However, the nonspherical
properties are ignored in the use of the grid. Latitude and longitude are based on
the classical Greek system of angular measurement. A circle is divided into 360°.
Staring clockwise from point A, one-quarter of the way around represents 90°(B),
one half is 180°(C), three-quarters is 270°(D), and a return to the start at point A is
360 °. Degrees are divided into minutes and seconds. One degree includes 60 minutes
and one minute is composed of 60 seconds. The notation for degrees, minutes, and
seconds are, respectively°,^0 , and^00. An example of a location on Earth’s surface is
37 ° 0701300 N, 97° 0402000 W. Such a notation will direct one to within a couple of meters
from any point on the planet.
The system of latitude and longitude is universally accepted. Latitude
has been certain since the time of the classical Greeks. They appreciated the
roundness of the Earth and that there was an equator. The equator is key to
the numbering of latitude because it provides a physically based “0” latitude.
The equator is in the center of the rhythmic migration of the solar declination
between the Tropic of Cancer and Tropic of Capricorn. There is 90°of angular
arc between the equator and the north end of the axis of Earth’s rotation and
between the equator and the south end of the Earth’s axis of rotation. So, lati-
tudes are numbered from 0°to 90°in the Northern Hemisphere and the Southern
Hemisphere, designated by their positions relative to the equator. Any individual
line of latitude represents a circle around the globe oriented west and east. The
equator has the largest circumference possible and other lines of latitude have
progressively shorter circumferences with increasing angular distance from the
equator.
Lines of latitude are always parallel with each other and, therefore, are also
known as parallels. Any number of parallels can be designated using degrees,
minutes, and seconds. Neglecting Earth’s oblate shape, one degree of latitude is
anywhere equivalent to approximately 111 km. Important lines of latitude usually
shown on maps or globes are the Tropic of Cancer, Tropic of Capricorn, Arctic
Circle, Antarctic Circle, and the equator (seeSeasons). The tropic lines delimit
the poleward boundary of the tropics, 23.5°north and south of the equator. The
Arctic and Antarctic circles denote the equatorward limits of polar latitudes and
are the limits for the regions in which the sun can stay above the horizon for more200 Latitude and Longitude