Lesson 4: Introduction to Earth

Solar Altitude

*Angle of the sun above the horizon *SA=90*-Arc Distance *Arc Distance=Distance between subsolar point and latitude.

Changes in patterns of sunlight around the earth

The vertical rays of the sun strike earth only between the Tropic of Cancer and tropic of Capricorn. *After the March equinox, the vertical rays of the sun migrate north from the equator, striking the Tropic of Cancer on the June solstice. *After the June solstice, the vertical rays migrate south, striking the equator again on the September equinox and reaching the Tropic of Capricorn on the December solstice. *Following the December solstice, the vertical rays migrate northward, reaching the equator once again on the March equinox.

March Equinox

*Equator faces sun directly *Occurs on or about March 20 *Neither pole tilts toward sun *All regions on earth experience 12 hours of daylight and 12 hours of darkness. *Vernal equinox in the Northern Hemisphere and the autumnal equinox in the Southern Hemisphere. (first day of spring and first day of fall respectively.)

Day length at the equator

Only at the equator is day length constant throughout the year; virtually 12 hours of daylight every day of the year.

Earth's Rotation on its Axis

Looking down at the North Pole, Earth rotates counterclockwise, from West to East. A complete rotation requires 24 hours.

Declination of the sun (subsolar point)

The latitude receiving the vertical rays of the sun (rays striking the surface at a right angle).

September Equinox

*Equator faces sun directly *Occurs on or about September 22 *Neither pole tilts toward sun *All regions on earth experience 12 hours of daylight and 12 hours of darkness *Also called the Autumnal equinox in the Northern Hemisphere and the Vernal equinox in the Southern Hemisphere. (First day of fall and first day of spring in their respective hemispheres.)

December Solstice

*Northern hemisphere tilts away from sun and has shortest day and longest night. *Occurs on or about December 21 *Southern hemisphere tilts toward sun and has longest day and shortest night. *The vertical rays of the Sun now strike 23.5 degrees south at the Tropic of Capricorn. *Areas north of the Arctic Circle are in continuous darkness, whereas areas south of the Antarctic circle are in daylight for 24 hours. *Conditions in the December and June Solstice are very similar; the conditions in each hemisphere are simply reversed.

June Solstice

*Occurs on or about June 21, Earth reaches the position in its orbit where the North Pole is oriented most directly toward the sun. On this day, the vertical rays of the sun strike the Tropic of Cancer, 23.5 degrees north of the equator. *The Tropic of Cancer marks the northernmost latitude reached by the vertical rays of the Sun during the year.

Circle of Illumination

*The great circle that separates daylight from darkness. *On the June solstice the circle of illumination bisects the equator, so on this day the equator receives equal day and night. *As we move north of the equator, the portion of each parallel in daylight increases, day length increase as we go north, and day length decreases as we move south of the equator.

What causes the annual change of seasons?

During the year, the changing relationship of Earth to the sun results in variations in day length and in the angle at which the sun's rays strike the surface of the earth.