The Reason for the Seasons
Learning Objectives
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What causes seasons? Is the Earth closest to the Sun
in (northern) summer? When does summer occur south
of the equator? Why are days longer in the summer?
How do the ecliptic and celestial equator define the
Earth’s axial tilt? How many degrees is the Earth’s tilt?
What are the solstices and equinoxes? Their dates?
The total hours of daylight on those dates?
Can you draw a diagram to find the Sun’s angle from
zenith as it crosses your meridian on the solstices and
the equinoxes at any latitude?
What latitudes define the tropics and the arctic? Why?
Note the [xtra] beside lectures 1-8 on the schedule.
These may be helpful to learn hard concepts
What Causes the Seasons?
! The Earth is closer to
the Sun in the
summer? Right?
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No. No. NoNoNo. NO.
The Earth’s orbit is not
perfectly circular, but it
is actually closest to
the Sun in January
Also, summer in the Earth’s Northern
Hemisphere is winter in the Earth’s Southern
Hemisphere and vice versa
What Causes the Seasons?
So what does cause the seasons?
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It’s the tilt of the Earth’s rotation axis
Note, the direction of the tilt doesn’t change
over the year
Sun's Daily Paths
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In the summer, the
Sun is north of the
celestial equator.
Laramie gets
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Long days
 Sun high in the sky
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In the winter, the
Sun is south of the
celestial equator
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Short days
 Sun low in the sky
Approximate Solstices & Equinoxes
in Earth’s Northern Hemisphere
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Winter Solstice – December 21
 night
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Vernal (Spring) Equinox – March 21
 12
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hours of daylight, 12 hours of night
Summer Solstice – June 21
 day
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longer than day in the Northern Hemisphere
longer than night in the Northern Hemisphere
Autumnal (Fall) Equinox – September 21
 12
hours of daylight, 12 hours of night
Approximate Solstices & Equinoxes
in Earth’s Southern Hemisphere
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Winter Solstice – June 21
 night
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Vernal (Spring) Equinox – September 21
 12
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hours of daylight, 12 hours of night
Summer Solstice – December 21
 day
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longer than day in the Southern Hemisphere
longer than night in the Southern Hemisphere
Autumnal (Fall) Equinox – March 21
 12
hours of daylight, 12 hours of night
Poll Everywhere: Thoughts
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The tilt of the Earth to its orbital plane (i.e. the
angle between the celestial equator and the
ecliptic) is ~23.5o (this is formally called
Earth’s axial tilt)
The celestial equator is at 0o declination
So, the Sun moves from +23.5o declination
(23.5oN, the Summer Solstice in the Northern
Hemisphere) to -23.5o (23.5oS, the Winter
Solstice in the Northern Hemisphere)
Laramie is near 40o geographic latitude, so
around 40o declination is at zenith
Important bad drawing
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Latitude, seasons, how high does the
Sun get as it crosses your meridian
Practice Question
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The Earth is tilted ~23.5o to the ecliptic, so
the Sun moves from +23.5o to -23.5o
declination. Laramie is near 40o latitude. On
June 21, in Laramie, when the Sun crosses
the meridian (is at its highest point), how far
is the Sun from zenith?
16.5o
23.5o
40o
63.5o
The Tropics
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Over the year, the Sun varies from 23.5º north
to 23.5º south of the celestial equator
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If you live between 23.5ºN and 23.5ºS latitude,
the Sun can reach zenith – directly overhead
 the Sun never reaches zenith in Laramie, 40ºN
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Between 23.5ºN and 23.5ºS latitude is called
the tropics
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23.5º N – Tropic of Cancer
 23.5º S – Tropic of Capricorn
The Arctic and Antarctic
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North of 66.5ºN you can see 23.5ºN above
the horizon by looking north! Below 66.5ºS
you can see 23.5ºS by looking south!
At these latitudes
the Sun sometimes
never sets!
North of 66.5ºN
 The Arctic
South of 66.5ºS
Midnight Sun in Barrow, Alaska
 The Antarctic
Tropics and Arctics
Arctic Circle
Tropic of Cancer
Equator
Tropic of Capricorn
Antarctic Circle
Next Time
Eclipses and the
Lunar Cycle