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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.
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