1. MAP AND CHART

2. CHARTING THE GREAT WINDS

3. RIDING THE WIND

4. THE MAP CARTOUCHE

5. THE GULF STREAM

6. CHART MAKING FOR NAVIGATORS

7. HURRICANES

8. PROFILES

9. CHARTING NEPTUNE’S REALM

10. SURFACE CURRENTS

11. DENSITY CURRENTS

12. CURRENT AND CLIMATE

13. HUMAN INTERACTION

14. DEFINING THE EARTH

15. LATITUDE

16. LONGITUDE

17. COMPASS DEVIATION

Osher Map Library
University of Southern Maine

An exhibition at the Osher Map Library and Smith Center for Cartographic Education, University of Southern Maine, Portland, 4 April 2000 to 11 January 2001

Donald S. Johnson, guest curator

A. Learning Objectives

B. Background Information

C. Teacher Activities

D. Materials Required

E. Presentation of Lesson

F. Glossary

Apparent surface speed: the speed at which a point on the surface of the globe seems to be moving eastward. At the Equator, the spot is covering the distance equal to the circumference of the Equator in one day; 25,000 miles each day. Other points on the surface seem to be moving at a slower speed since, although they make one revolution in one day, their distance traveled is less. In the diagram below, four points have been marked: Equator, 30°N, 60°N, and the North Pole.

To calculate the apparent speed, it is necessary to determine the circumference of the circle transcribed by each point as it rotates parallel to the Equator. If we assume that the diameter of the Earth at the Equator is eight thousand miles (actually 7,928 miles) with a circumference of 25,000 miles (actually 24,893 miles), that produces a radius at the equator of four thousand miles. Using the formula:

cosine DO = AC/AB (adjacent divided by hypotenuse)
 

The radius of each of the plotted points can be determined as follows:

30°N 60°N
cos30° = AC/AB cos60° = AC/AB
(cos30°)(AB) = AC (cos60°)(AB) = AC
(0.866)(4000) = 3464 (0.5)(4000) = 2000

Circumference becomes:
(2)(r)(p) (2)(r)(p)
(2)(3464)(3.14) = 21,942 (2)(2000)(3.14) = 12,560

Dividing these circumferences by 1,000 to get workable numbers, the Equator is traveling eastward at 25 units, at 30°N the point is traveling eastward at 22 units, and at 60°N the point is now moving eastward at twelve units. At the pole there is no apparent speed.
 

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