June

December

March

September

http://www.nasa.gov/vision/earth/features/blue_marble.html

Why is it hot in the tropics and cold at the poles?

What Causes Earth’s Seasons?• Earth’s axis is tilted 23.5o

• It always points in the same direction-- (toward Polaris, the North Star)

• We orbit our Sun once a year (revolution)

http://www.lpi.usra.edu/education/skytellers/seasons/about.shtml

Polaris Polaris

Polaris

Polaris

The tilt of Earth’s axis causes one hemisphere (northern vs. southern) to receive more DIRECT sunlight

Direct light causes more heating.

Solar Illumination

1 KW/m2 1 KW/m2

1 m2 2 m2

In winter a bundle of light is spread over a bigger area than in summer due to the tilt of the North Pole away from the Sun.

The Earth is curved and this causes the different angles of incidence.

Direct vs Indirect Rays

Direct Rays

Indirect Rays

Indirect Rays

E W NWNE SWSE

So what So what isis the ALTITUDE of the Sun at the ALTITUDE of the Sun at NOON throughout the year?NOON throughout the year?

23.5O

23.5O

Summer Solstice

Winter Solstice

Equinox

Total annual change:

sunrise sunsetQuickTime™ and a

decompressorare needed to see this picture.

47O

QuickTime™ and a decompressor

are needed to see this picture.

Earth’s Orbit and the Seasons

Seasons on the Earth

QuickTime™ and a decompressor

are needed to see this picture.

Important Latitudes!!

Equinoxes and Solstices The Summer Solstice (June 21): The date when the

Sun is directly striking its most northern latitude (the Tropic of Cancer).

The Vernal (Spring) Equinox (March 21): The date when the Sun crosses the equator when going from south of the equator to north of the equator.

QuickTime™ and a decompressor

are needed to see this picture.

QuickTime™ and a decompressor

are needed to see this picture.

The Autumnal (Fall) Equuinox (September 23): When the Sun crosses the equator when going from north to south.

The Winter Solstice (December 21): The date when the Sun’s vertical ray is striking its furthest southern latitude (the Tropic of Capricorn).

QuickTime™ and a decompressor

are needed to see this picture.QuickTime™ and a

decompressorare needed to see this picture.

The Seasons

•Summer solstice:–The Sun will be directly overhead at

the Tropic of Cancer (23.5oN).

–The Sun does not set for people within the Arctic circle (67.5oN).

–The Sun does not rise for people within the Antarctic Circle (23.5oS).

Earth on June 21

•Winter Solstice:–The Sun will appear directly overhead to someone at the Tropic of Capricorn (23.5o).

–The Sun does not set for people within the Antarctic Circle

–The Sun does not rise for people within the Arctic Circle

Earth on December 21

•Autumnal equinox:–The Sun will appear directly

overhead to someone on the equator (0o).

–The North Pole begins 6 solid months of night.

–The South Pole begins 6 solid months of day.

QuickTime™ and a decompressor

are needed to see this picture.

•Vernal equinox:–The Sun will appear directly

overhead to someone at the equator (0o).

–The South Pole begins 6 solid months of day.

–The North Pole begins 6 solid months of night.

The Seasons as Viewed From North Pole

Earth’s orbit is Earth’s orbit is almostalmost a a perfect circle…….perfect circle…….

• 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

Why doesn’t distance matter?• Variation of Earth-Sun distance is small —

about 3%; this small variation is overwhelmed by the effects of axis tilt.

QuickTime™ and a decompressor

are needed to see this picture.

QuickTime™ and a decompressor

are needed to see this picture.

July

Daylight Hours Across the GlobeTime is indicated as number of hours (h)

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

10.5 h 11 h 12 h 12.5 h 13 h 14 h 14.5 h 14 h 12.5 h 12.5 h 11 h 11.5 h

14.5 h 13.5 h 13.5 h 12.5 h 11 h 11 h 11 h 11.5 h 11.5 h 12.5 h 14 h 14.5 h

12 h 12 h 12 h 12 h 12 h 12 h 12 h 12 h 12 h 12 h 12 h 12 h

17.5 h 15.5 h 13.5 h 11.5 h 10 h 8.5 h 8.5 h 9.5 h 11.5 h 13 h 15 h 16.5 h

5 h 7 h 10 h 13.5 h 17.5 h 21.5 h 22 h 18 h 15.5 h 11.5 h 8.5 h 5.5 h

12 h 12 h 12 h 12 h 13 h 13 h 12 h 12 h 12 h 12 h 12 h 12 h

15 h 14 h 13.5 h 12.5 h 11.5 h 10 h 10.5 h 11 h 11.5 h 12.5 h 14 h 14 h

9 h 10 h 11 h 13.5 h 15 h 17 h 16.5 h 15.5 h 13.5 h 11.5 h 10.5 h 9.5 h

24 h 24 h 19 h 14.5 h 0 h 0 h 0 h 0 h 7 h 15 h 24 h 24 h

Miami

Brisbane

Nairobi

Punta Arenas

Nome

Singapore

Cape Town

Seattle

Vostok

Seasonal changes are more extreme at high latitudes

Path of the Sun on the summer solstice at the Arctic Circle

Analemma: If we plotted the position of the sun at noon for an entire year, it would make this shape!!

Sun’s altitude also changes with seasons

Sun’s position at noon in summer:

higher altitude means more direct sunlight.

Sun’s position at noon in winter: lower altitude means less direct sunlight.

Effects of the Seasons

• Changes in solar intensity• Changes in solar radiation• Changes in day length• Changes in temperature

All of these changes are most extremeat high latitudes and minimized at the equator.