Page 59 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

Chapter 1 essentials of Geography 23
We noted that latitude is determined easily by sight- ing the Sun or the North Star as a pointer. In contrast, a method of accurately determining longitude, especially at sea, remained a major difficulty in navigation until after 1760. The key to measuring the longitude of a place lies in accurately knowing time and required the invention of a clock without a pendulum. On the MasteringGeography website, read “The Timely Search for Longitude,” about an important invention that solved the problem of time and longitude.
Great Circles and Small Circles
Great circles and small circles are important navi- gational concepts that help summarize latitude and longitude (Figure 1.17). A great circle is any circle of Earth’s circumference whose centre coincides with the centre of Earth. An infinite number of great circles can be drawn on Earth. Every meridian is one-half of a great circle that passes through the poles. On flat maps, airline and shipping routes appear to arch their way across oceans and landmasses. These are great cir- cle routes, tracing the shortest distances between two points on Earth (see Figure 1.24).
In contrast to meridians, only one parallel is a great circle—the equatorial parallel. All other parallels di- minish in length toward the poles and, along with any other non–great circles that one might draw, constitute small circles. These circles have centres that do not coin- cide with Earth’s centre.
Figure 1.18 combines latitude and parallels with longitude and meridians to illustrate Earth’s complete coordinate grid system. Note the red dot that marks our measurement of 49° N and 60° E, a location in western Kazakhstan. Next time you look at a world globe, follow the parallel and meridian that converge on your location.
CRiTiCAlthinking 1.3 Where Are You?
Select a location (for example, your campus, home, or work- place or a city) and determine its latitude and longitude— both in degrees, minutes, and seconds and as decimal degrees. Describe the resources you used to gather this geographic information, such as an atlas, website, Google earthTM, or GPS measurement. Consult Figure 1.14 to find the approximate lengths of the latitude and longitude degrees at that location. •
Meridians and Global Time
A worldwide time system is necessary to coordinate inter- national trade, airline schedules, business and agricul- tural activities, and daily life. Our time system is based on longitude, the prime meridian, and the fact that Earth ro- tates on its own axis, revolving 360° every 24 hours, or 15° per hour (360° ÷ 24 = 15°).
Each pair of meridians forms a great circle.
All other parallels form small circles.
Great circle
(b) A plane intersecting the globe along a great circle divides the globe into equal halves and passes through its centre.
Small circle
(c) A plane that intersects the globe along a small circle splits the globe into unequal sections—this plane does not pass through the centre of the globe.
(a)
The equatorial parallel is a great circle.
▲Figure 1.17 Great circles and small circles.
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