june decembermarch september
TRANSCRIPT
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
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.
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
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47O
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.
•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).
•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
•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.
•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.
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.
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.