Chapter 3
The Earth as a Rotating Planet
OVERVIEW
This is the first chapter of Part 2 which deals with the way solar radiation drives energy and matter flows in the atmosphere and oceans and how these flows are linked to weather and climate.
This chapter introduces you to some basic ideas about the Earth, its rotation, and revolution.
 The Earth is shaped as an oblate ellipsoid because the Earth's rotation causes it to bulge
slightly at the equator.
 The Earth rotates in an eastward direction.
 The Earth's rotation has three important environmental effects:
1. It imposes a daily, or diurnal, cycle of daylight, air temperature, air humidity, and air motion.
2. It produces the Coriolis effect which deflects the flow of fluids (air and water) to the left
in the southern hemisphere and to the right in the northern hemisphere.
3. Tides result from the moon’s gravitational pull on the side of the Earth closest to the
moon, creating a rise and fall of ocean water as the Earth rotates.
 The Geographic Grid provides a system for locating features on the Earth’s surface using
parallels of latitude and meridians of longitude.
 Latitude is the angular distance of a point north or south of the equator. It increases from a
minimum of 0° at the equator to a maximum of 90° at the north and the south poles. Lines of
latitude are parallel to each other and describe circles that decrease in circumference away
from the equator.
 Longitude is the angular distance of a point east or west of the prime meridian at Greenwich,
England. It increases to the west and the east away from the prime meridian (0°) to a maximum of 180°. Lines of longitude are farthest apart at the equator and converge at the poles.
All circles described by meridians of longitude are the same circumference.
 A map projection is a system for changing the curved/spherical geographic grid to a flat grid.
 Map scale relates distance on a map to distance on the Earth’s surface.
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 The polar projection produces a map with true shapes of small areas.
 The Mercator projection shows true compass direction on any straight line on the map and is
useful for showing the flow of winds and ocean currents as well as lines of equal air temperature and pressure.
 The Goode projection is an equal area projection useful for depicting geographical features
that occupy surface areas.
 The standard time system is based on twenty four time zones that keep time according to
standard meridians that are spaced 15° apart and represent a time difference of one hour.
 The international date line is located near 180° longitude. Crossing this line in a westward
direction requires the calendar to be advanced by one day.
 The Earth revolves counterclockwise around the sun every 365¼ days in an elliptical orbit.
 The Earth is closest to the sun at perihelion (~ January 3) and farthest from the sun at aphelion (~ July 4).
 The Earth’s axis of rotation is tilted 23½° away from the perpendicular and its north pole always points towards Polaris (the north star).
 The axial tilt and the revolution of the Earth around the sun combine to produce the progression of the seasons.
 At an equinox, everywhere on Earth experiences a 12-hour day and a 12-hour night.
 At a solstice, polar regions experience either a 24-hour day or a 24-hour night.
 The maximum solar radiation is received at the subsolar point which crosses the equator
twice in the course of a year as it moves between the Tropic of Cancer to the Tropic of Capricorn.
 Two important facts about the Sun-Earth energy flow system are that:
1. half of the Earth is always receiving solar energy.
2.
Not all places on the Earth’s surface receive the same amount of energy.
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KEY TERMS
oblate ellipsoid
diurnal
coriolis effect
geographic grid
latitude
longitude
global positioning system
map projections
map scale
polar projection
Mercator projection
Goode projection
standard time zones
international date line
daylight saving time
Geographic Information System
revolution
perihelion
aphelion
solstice
equinox
subsolar point
plane of the elliptic
Tropic of Cancer
Tropic of Capricorn
Arctic circle
Antarctic circle
equator
standard meridian
declination
cartography
solar day
STUDY QUESTIONS
1. List the three environmental effects of the Earth’s rotation.
2. How do we know the Earth is round?
3. Describe how you would determine the position of latitude 40° south.
4. What is the global positioning system and how does it work?
5. Describe the appearance and use of the polar projection.
6. How is the Mercator projection useful to sailors? How is it useful to geographers?
7. What useful properties does the Goode projection have?
8. What problems do geographers encounter when using each of these projections?
9. Describe a geographic information system.
10. Describe how the standard time zones were established.
11. What is perihelion? What is aphelion?
12. What is the impact of the tilt of the Earth’s axis on environmental conditions at the surface?
13. How does the sun’s declination vary throughout the year?
CHAPTER QUIZ
Multiple Choice Questions
1. North-south lines of the geographic grid are:
a) parallels
b) meridians
c) lines of latitude
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2. If the time in New York is 9 a.m., the time in Los Angeles is:
a) the same
b) earlier
c) later
3. The subsolar point is 23½° north latitude at the:
a) vernal equinox
b) summer solstice
c) autumnal equinox
d) winter solstice
4. The Earth is closest to the sun at:
a) perihelion
b) equinox
c) aphelion
d) the summer solstice
5. Which of the following is not one of the spatial objects used by a geographic information system?
a) point
b) line
c) polygon
d) attribute
True/False Questions
In the standard time zone system, time zones are usually 15° apart. (T/F)
At the summer solstice, the area north of the Arctic Circle experiences continuous night. (T/F)
A straight line drawn anywhere on a polar projection is a line of constant compass direction.
(T/F)
4. The global positioning system uses over 50 satellites that continuously orbit the Earth. (T/F)
1.
2.
3.
Short Answer Questions
1. What is the significance of the Tropic of Cancer and the Tropic of Capricorn?
2. How does day length vary throughout the year for someone living at 40° N latitude?
3. What time change would you make to your watch if you crossed the International Date Line
traveling west?
4. How does map projection affect map properties?
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Short Essay Questions (1 - 2 paragraphs)
Describe how the tilt of the Earth’s axis, combined with its revolution around the sun, affects
the seasons where you live.
2. Describe how the spherical shape of the Earth is translated into the geographic grid.
1.
Internet Resources
1. The Geography Department of Hunter College (City University of New York) provides basic
information and links to other resources on map projections:
<http://everest.hunter.cuny.edu/mp/.>
2. A discussion of the concepts of longitude and latitude provided by the Hammond World
Atlas Corporation. This page also provides access to an interactive demonstration showing
how map projections vary in their representation of the Earth:
<http://www.hammondmap.com/latlong.html>
3. A map showing the time zones of the world and a time zone convertor:
<http://www.worldtimezone.com/index24.html>
4. A site discussing equinox and solstice festivals in pre-Christian and Christian European society: <http://www.equinox-and-solstice.com/.>
5. The exact dates and times of perihelion, aphelion, equinox, and solstice for 1992-2005 provided by the US Naval Observatory: <http://aa.usno.navy.mil/data/docs/EarthSeasons.html>
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