Topics of Discussion

• The physical effects of rotation
• The geographical use of rotation
• Latitude and longitude
• The geographic grid
• Global time
• The physical effects of revolution
• The causes of seasons

Rotation

• Earth rotates on its imaginary axis like a top.
• The earth rotates from west to east.
• Each rotation defines a solar day - 24 hours.

What are the physical affects of earth’s rotation?

1. Daily Cycle of Light and Heat
• Night and Day - Plants, Animals, and People
Photosynthesis
• Daily change in temperature
2. Earth’s Shape
Is the earth a perfect sphere?
• Earth is an oblate spheroid
• Earth bulges at the equator
Polar diameter =7900 miles
Equatorial diameter = 7927 miles
3. Coriolis Effect
• The earth rotates at different speeds at different latitudes.
• Any moving solid, liquid, or gas is turned to the RIGHT of its intended path in the Northern Hemisphere.
• Winds
• Ocean currents
• Missiles
• Any moving solid, liquid, or gas is turned to the LEFT of its intended path in the Southern Hemisphere
4. Tides
The gravitational pull of the moon and the sun produces a rhythmic rise and fall of the oceans as the earth rotates.

The Geographical Use of Rotation

• Earth's rotation provides a reference system for location.
• The earth rotates around two fixed points:
North Pole
South Pole

Rotation and Latitude

• Halfway between the poles, the equator divides the earth into hemispheres.
• Lines north and south of the equator are parallels.
• Parallels are lines of latitude.

Latitude is measured at an angle from the center of the earth, north and south of the equator.

Meridians or Longitude

• Lines drawn from pole to pole are called meridians or lines of longitude.
• Longitude measures points east and west of the Prime Meridian.
• The prime meridian passes through Greenwich, England.

Geographic Grid

• Together, latitude and longitude form a geographic grid.
• Latitude and longitude can be used to define precise locations on earth.

Absolute Location

The absolute location of San Francisco is latitude 37º 45"N and longitude 122º 26"W.

Global Time

• What time is it on earth?
• Global time is Solar time
• Solar noon - the sun’s highest point in the sky
• Standard time is a device to improve global timekeeping.
• The world is divided into 24 time zones.
• Each time zone is based upon standard meridians at 15° intervals from the prime meridian.

Global Time - US Time Zones

• Can you name the time zones?
• When traveling west, you put your watch back.
• When traveling east, you put your watch ahead.

International Date Line

• The International Date Line is the 180º meridian.
• Crossing the date line requires adjusting your calendar as well as your watch.
• You lose a day going west and gain a day going east.

Earth’s Revolution

• Earth’s orbit around the sun takes 365-1/4 days.
• The earth traces an elliptical path from west to east.

Is Earth’s Revolution the Cause of Seasons?
Is earth’s changing distance from the sun the cause of seasons?

• The earth is closest to the sun on January 3.
• The earth is farthest from the sun on July 4.

Cause of Earth’s Seasons

• The tilt of earth’s axis as it orbits the sun
• The earth’s axis is tilted 23-1/2º from the vertical or 66-1/2º from the plane of the ecliptic
• Plane of the ecliptic - the plane of earth’s orbit around the sun

Parallelism
Seasons also occur because earth's tilted axis is parallel to itself as it orbits the sun.

• June 21 - the northern hemisphere tilts toward the sun
• December 22 - the southern hemisphere tilts toward the sun

The Four Seasons
As the earth orbits the sun, different places receive the sun’s more direct rays.

• Solstices
• On June 21, the Tropic of Cancer tilts toward the sun (summer solstice)
• On December 22, Tropic of Capricorn tilts toward the sun (winter solstice)
• Equinox
• On March 21 and September 23, the noonday sun is overhead at the equator
• Vernal equinox and autumnal equinox
• Equinox means equal days and equal nights.

Why is Winter colder than Summer?
Two factors make winter colder than summer:

• The angle of the sun’s rays determines the heating power
• The length of daylight

Seasons and the Angle of the Sun
The angle at which the sun’s rays reach the earth’s surface varies with latitude, season and the time of day.

• At high latitudes, the angle of the sun’s rays are low or below the horizon much of the year.
• At middle latitudes, the angle of the sun’s rays varies from higher in summer to lower in winter.
• At low latitudes, the angle of the sun’s rays are high throughout much of the year.

Seasons and the Length of Daylight
As the earth changes its position relative to the sun the length of daylight also changes.

• In the tropics, day and night are nearly equal in length throughout the year.
• In the middle latitudes, the length of day and night varies greatly from winter to summer.
• At high latitudes, summer days and winter nights are longer than in the middle latitudes.

The Arctic and Antarctic Circles

• The Arctic Circle (66½ºN) is the farthest point from the North Pole that experiences at least one day when the sun never sets or never rises.
• The Antarctic Circle (66½ºS) is the farthest point from the South Pole that experiences at least one day when the sun never sets or never rises.

Summary

• Rotation has physical effects that include:
• The earth’s oblate spheroidal shape
• Day and night
• Coriolis effect
• Tides
• Locations on earth are defined by lines of latitude and longitude that form a geographic grid.
• For convenience, we use standard time which divides the world into 24 hourly time zones and an International Date Line.
• Earth’s rotation causes differences in time and dates.
• Seasons are caused by:
• Earth’s revolution
• The tilt of the earth’s axis
• Parallelism

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