shape of earth

Visualizing Physical Geography
Copyright © 2008 John Wiley and Sons Publishers Inc.
Chapter 1
The Earth as a Rotating Planet
by Alan Strahler and Zeeya Merali

Chapter Overview
The Shape of the Earth
The Earth’s Rotation
The Geographic Grid
Map Projections
Global Time
The Earth’s Revolution Around the Sun

The Earth is round

The Earth is not a perfect sphere
• Equatorial diameter slightly greater than polar diameter
• Earth is an oblate ellipsoid–slightly flattened
• The geoid exaggerates small departures from spherical

Earth rotates on its axis:
•Counterclockwise at North Pole
•Left to right (eastward) at Equator
•One rotation is a solar day (24 hours)
Axis: an imaginary
straight line through the
center of the Earth
around which the Earth
rotates
Poles: the two points on
the Earth’s surface
where the axis of
rotation emerges

Environmental Effects of the Earth’s Rotation:
•Day and night
•Fluctuating air temperature
•Coriolis Effect
•Tides

The Geographic Grid
Parallels and Meridians
Geographic grid: network of parallels and meridians used to fix location
on the Earth
Parallel: east-west circle on
the Earth’s surface, lying on a
plane parallel to the equator
Meridian: north-south line on the
Earth’s surface, connecting the
poles

The Geographic Grid
Parallels and Meridians
Equator: Parallel of latitude lying midway between the Earth’s poles; it is
designated latitude 0º
• Longest parallel of latitude
• Midway between poles
• Fundamental reference line for measuring position
Latitude: arc of a
meridian between the
equator and a given
point on the globe
Longitude: arc of a
parallel between
the prime meridian
and a given point
on the globe

The Geographic Grid
Latitude and Longitude
Latitude is measured north and
south of the equator, up to 90º
Longitude is measured east and
west of the Prime Meridian—
meridian that passes through
Greenwich, England—up to 180º

Map Projections
Many types of maps, for many different purposes

Map Projections
Map Scale
Scale fraction: a ratio
that tells how to convert
distance on the map to
true distance on the
Earth
1:50,000
1 unit of map
distance=50,000 units
of distance on the
Earth

Map Projections
Map projection: a system of parallels and meridians
representing the Earth’s curved surface drawn on a flat surface
• Curved surface cannot be projected onto a flat sheet without distortion
• Each map projection has a specific purpose
• Each projection has advantages and drawbacks

Map Projections
Mercator projection: map projection with horizontal parallels and
vertical meridians
• Invented 1569 by Gerardus Mercator
• Used for navigation
• Scale increases from equator to poles
• Straight line on the map is a line of constant compass bearing
• Straight line on a Mercator projection not the shortest distance between two points

Map Projections
Goode Projection: Equal-area map projection often used
to display information such as climate or soil type
• Shows true sizes of regions on Earth’s surface
• Distorts shapes of places, especially high latitudes and at edges of map

Map Projections
Polar projection: map projection centered on Earth’s North or
South Pole
• Used for maps of polar regions
• Scale increases outward
• Shows only one hemisphere

Global Time
Standard Time: time system based on the local time of a
standard meridian and applied to belts of longitude extending
roughly 7 ½ º on either side of the meridian
Time zones: zones
or belts within which
standard time is
applied

Global Time
World Time Zones

Global Time
International Date Line
•180th
meridian
•Moving west across the date line: advance calendar one day
•Moving east across date line: set calendar back one day
•No change in clock time when crossing date line

Global Time
Daylight Saving Time: clocks set ahead one hour
Not observed in all locations

shape of earth

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