What Causes the Seasons?

What about seasons? Why do we have them?

Earth’s distance from the sun varies throughout the year – doesn’t that cause the seasons?

Earth is closest to our Sun (91 million miles) in winter—January 3

Earth is farthest from our Sun (94 million miles) in summer –July 4

Tilt!Because of the tilt of Earth’s axis, the amount of solar radiation received by Northern and Southern Hemispheres changes seasonally

Northern Hemisphere has summer when it tilts toward the sun, winter when it tilts away

Southern Hemisphere has summer when it tilts toward the sun, winter when it tilts away

The earth is tilted 23.50

• Earth’s axis is tilted 23.50 – it always points in the same direction (Polaris, the North Star) as we orbit our Sun once a year

• This tilt causes the hemispheres to receive different amounts of sunlight throughout the year

Radiation is more intense near the equator compared to the poles. For this reason, it’s warmer near the equator than at the poles.

North Pole

Earth

Equator

South Pole

North Pole

Earth

When the north pole tilts toward thesun, it gets more radiation – more warmth

during the summer

SUMMER (Northern Hemisphere)

South Pole

WINTER (Southern Hemisphere)

When the north pole tilts toward thesun, the south pole tilts away

So when it’s summer in the north, it’s winter in the south

Equator

Planets that are tilted on an axis can experience seasons.

Equinoxes/Solstices• Vernal Equinox – March 21/22 (1st day of

spring) = amount of daylight and darkness

• Summer Solstice – June 21/22 (1st day of summer) longest daylight, shortest night

• Autumnal Equinox – Sept 22/23 (1st day of fall) = daylight and darkness

• Winter Solstice – Dec 21/22 (1st day of winter) shortest period of daylight, longest night

Seasons—a summary• caused by the tilt of the earths axis

• As earth revolves around the sun its axis always points in the same direction

• Each hemisphere leans toward the sun during different parts of the orbit

• The seasons N and S of the equator are always opposite of each other

The Earths Rotation Axis

http://highered.mcgraw-hill.com/sites/0072482621/student_view0/animations.html#

http://www.physicalgeography.net/fundamentals/6hsummer.htmlhttp://www.physicalgeography.net/fundamentals/6hwinter.htmlhttp://www.physicalgeography.net/fundamentals/6hequinox.html