Geoscience Reference
In-Depth Information
in space and looking at a section of the Earth at the Equator.
Longitude is calculated by measuring the arc formed by two
lines projected from the Earth's center at the Equator. One line
extends to the Prime Meridian, the other to the meridian of the
location in question. The angle of the arc formed by the two
lines determines the longitude. Calculated this way, smaller
geometric arcs from the Prime Meridian have lower longi-
tude designations, and greater arcs have higher designations
(see Figure 2.7b). As with latitude, degrees of longitude can be
subdivided into minutes and seconds or be given full decimal
notations.
KEY CONCEPTS TO REMEMBER
ABOUT LONGITUdE
1.
Lines of longitude run north and south and are also
called meridians.
2.
Longitude is determined by measuring the geomet-
ric arc between two lines projected from the center of
Earth to the surface at the Equator and to the merid-
ian in question. The reference point for longitude is the
Prime Meridian (or 0°), which divides Earth into eastern
and western halves.
Figure 2.6 The Prime Meridian.
This line of longitude at the
Old Royal Observatory in Greenwich, England, is the reference
meridian for the longitude system.
As with the Equator, the Prime Meridian has a value of 0°,
and all points north and south along that meridian are at 0°
longitude. The geometric basis for determining longitude is
that a circle contains 360°. Think of the circle (the Earth) as
divided into eastern and western halves by the Prime Meridian.
The longitude grid consists of a series of meridians that extend
180° east and west of the Prime Meridian to complete the 360°
circle. Using Figure 2.7 as a guide, imagine you are floating
3.
Meridians are located east and west of the Prime Meridian
and extend 180° E and 180° W to complete a full circle.
4.
The distance between meridians is greatest at the
Equator but decreases steadily until converging at
the poles.
North Pole
You are
here
Angle of
longitude
Prime
Meridian
Prime Meridian
(a)
(b)
Figure 2.7 Calculating longitude.
(a) The longitude grid consists of a series of great circles that converge
at the poles and that are measured east and west of the Prime Meridian. (b) Longitude is calculated on the
angle of arcs measured from the center of Earth and relative to the Prime Meridian.
