Seasons on Earth

All Figures from

http://www.fourmilab.ch/earthview/vplanet.html

ByDiana L. Duckworth

Rustburg High SchoolCampbell County, VA

Three Factors Cause Seasons• Tilt of Earth at 23.5 degrees latitude

– Greater tilt - more extreme seasons– Less tilt - less extreme seasons

• Revolution of Earth around Sun– Elliptical orbit– Perihelion - January– Aphelion - July

• Parallelism of Axis of Rotation– Axis always points toward Polaris

Tilt of Axis• Causes sun to be directly overhead at a

different latitude each day

• Summer Solstice @ 23.5N (~June 21)– More daylight & more intense radiation in

Northern Hemisphere

• Winter Solstice @ 23.5S (~December 21)– More daylight and more intense radiation in

Southern Hemisphere

• Equinox (twice a year) @ Equator– Days & Nights are 12 hours each everywhere

Key to Figures

• In the following illustrations, the dark area represents the 50% of the earth that is in darkness and the light areas represent the 50% of the earth that is in daylight.

• In all figures, the sun can be considered to be in the geographic center of the image.

Equinox 0Z

Equinox 3Z

Equinox 6Z

Equinox 9 Z

Equinox 12Z

Equinox 15 Z

Equinox 18Z

Equinox 21Z

Equinox 24Z

Summer Solstice 0Z

Summer Solstice 3Z

Summer Solstice 6Z

Summer Solstice 9Z

Summer Solstice 12Z

Summer Solstice 15Z

Summer Solstice 18Z

Summer Solstice 21Z

Summer Solstice 24Z

Winter Solstice 0Z

Winter Solstice 3Z

Winter Solstice 6Z

Winter Solstice 9Z

Winter Solstice 12Z

Winter Solstice 15Z

Winter Solstice 18Z

Winter Solstice 21Z

Winter Solstice 24Z

Tilt of Axis Locates Key Latitudes• Tropic of Cancer - where Sun’s rays are vertical at

Summer Solstice = tilt 23.5N

• Tropic of Capricorn - where Sun’s rays are vertical at Winter Solsltice = tilt 23.5S

• Arctic Circle - latitude of total daylight at Summer Solstice or darkness at Winter Solstice– 90 - tilt = 90N - 23.5N = 66.5N

• Antarctic Circle - latitude of total daylight at Winter Solstice or darkness at Summer Solstice– 90 - tilt = 90S - 23.5S = 66.5S

7 AM Nov 7, 2004 Sun @ 16°28’ S

Winter Solstice

view from Sun

Equinox view

from Sun

Summer Solstice

view from Sun

Summer Solstice

view from Moon

Equinoxview from Moon

Winter Solstice

view from Moon

As the World Turns

Rotation has nothing to do with seasons!

Changes with the Seasons• Maximum elevation of sun & position in

sky from Northern Hemisphere– June to December - gets lower & farther to

South – December to June - gets higher & farther to

North

• Length of Daylight– June to December - daylight gets shorter– December to June - daylight gets longer