Download Seasons What causes the seasons?

Seasons
Questions:
• What causes the seasons?
• How do we mark the progression of the
seasons?
• What is the seasonal motion of the sun in
the sky ?
• What could cause the seasonal motion of
the sun to change over time ?
Reading: Chapter 2.3
What causes the seasons?
John, who lives in Canada, claims that in June it is
summer in Canada because the sun is closest to the
Earth at this time.
Maya, who lives in Australia claims that in December
it is summer in Australia because the sun is closest to
the Earth at this time.
How to resolve this contradiction ?
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What causes the seasons?
• Seasons are opposite in the N and S
hemispheres, so distance cannot be the
reason.
• The real reason for seasons involves Earth’s
axis tilt.
What causes the seasons?
Direct light causes more heating.
Seasons depend on the orbit (revolution) of the earth
around the sun and how the tilt of the Earth’s axis
affects the directness of sunlight and length of days
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What causes the seasons?
• The tilt of the earths rotation axis relative to the plane
of its orbit remains constant, so its orientation relative to
the sun changes.
What causes the seasons?
• This means that for half the year the Northern
Hemisphere is tilted more towards the sun and for the
other half of the year the Southern Hemisphere is tilted
more towards the sun.
3
What causes the seasons?
• The Hemisphere receiving more direct sunlight will
experience summer, the other Hemisphere, winter.
Marking the progression of seasons
We define four special points:
•
summer solstice: June 21
•
winter solstice: December 21
•
spring (vernal) equinox: March 21
•
fall (autumnal) equinox: September 22
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How do we mark the progression of the seasons?
Summer solstice: June 21
Day when Northern Hemisphere is maximally tilted
towards the sun and gets most direct sun.
How do we mark the progression of the seasons?
Summer solstice: June 21
The Sun appears to be directly overhead at noon for places
situated at latitude 23.44 degrees north, known as the
Tropic of Cancer. (Why?)
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How do we mark the progression of the seasons?
Summer solstice: June 21
Places situated at latitude 66.56 degrees north, known as
the arctic circle will see the Sun just on the horizon during
midnight, and all places north of it will see the Sun above
horizon at any time of the day.
How do we mark the progression of the seasons?
Summer solstice: June 21
Places situated at latitude 66.56 degrees south, known as
the antarctic circle will see the Sun just on the horizon
during midday, and all places south of it will not see the
Sun above horizon at any time of the day.
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How do we mark the progression of the seasons?
Winter solstice: December 21
Day when Southern Hemisphere is maximally tilted
towards the sun and gets most direct sun.
How do we mark the progression of the seasons?
Winter solstice: December 21
The Sun appears to be directly overhead at noon for places
situated at latitude 23.44 degrees south, known as the
Tropic of Capricorn. (Why?)
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How do we mark the progression of the seasons?
Winter solstice: December 21
Places situated at latitude 66.56 degrees south, known as
the antarctic circle, will see the Sun just on the horizon
during midnight, and all places south of it will see the Sun
above horizon at any time of the day.
How do we mark the progression of the seasons?
Winter solstice: December 21
Places situated at latitude 66.56 degrees north, known as
the arctic circle will see the Sun just on the horizon during
midday, and all places north of it will not see the Sun
above horizon at any time of the day
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Marking the progression of seasons
Spring (vernal) equinox: March 21
Fall (autumnal) equinox: September 22
Times when neither Hemisphere is more tilted towards the
sun.
Both hemispheres get same direct sunlight.
Marking the progression of seasons
Spring (vernal) equinox: March 21
Fall (autumnal) equinox: September 22
The Spring equinox marks the transition from the Southern
Hemiphere being tipped towards the sun to the Northern
Hemiphere being tipped towards the sun.
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Marking the progression of seasons
Spring (vernal) equinox: March 21
Fall (autumnal) equinox: September 22
The Fall equinox marks the transition from the Northern
Hemisphere being tipped towards the sun to the Southern
Hemisphere being tipped towards the sun
Marking the progression of seasons
Spring (vernal) equinox: March 21
Fall (autumnal) equinox: September 22
On the fall and spring equinox, where would you expect to
see the sun overhead at noon ?
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Summer Days….
Although the Northern Hemisphere gets its most direct
sunlight on the summer solstice in June, the hottest days of
summer are in July/August.
Summer Days….
Although the Northern Hemisphere gets its most direct
sunlight on the summer solstice in June, the hottest days of
summer are in July/August.
This is because it takes time to heat up the earth and oceans.
Can you explain the coldest months of winter similarly ?
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Summer Days….
The tilt of the Earth’s axis also
explains the length of days and
nights…
On the equinoxes, day and
night are roughly of equal
length.
Between the Spring equinox
and the Summer solstice, days
in the Northern Hemisphere
grow longer as it is tilted
towards the sun, and hence this
hemisphere gets more energy
The opposite is true in the
Southern Hemisphere.
Summer Days….
On the summer solstice,
places in the Northern
Hemisphere have their
longest day (vice versa in
the Southern Hemisphere).
Between the Summer
solstice and the Fall
Equinox, days in the
Northern Hemisphere grow
shorter as its tilt towards
the sun decreases, and
hence this hemisphere gets
less energy. The opposite is
true in the Southern
Hemisphere.
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Winter Wonderland….
Based on the previous slide
can you explain the length
of days and nights from the
Fall equinox to the Winter
solstice to the Spring
equinox ?
High Noon
Because of the tilt of the earth’s axis, the apparent path of the sun
(ecliptic) is tilted at an angle of 23.5 degrees with respect to the
celestial equator.
This tilt causes the sun to appear to follow different paths in the
sky in summer and winter.
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High Noon
We can recognize solstices and equinoxes by Sun’s path across
sky:
Summer solstice:
Highest path, rise and set
at most extreme north of
due east.
Winter solstice: Lowest
path, rise and set at most
extreme south of due
east.
Equinoxes: Sun rises
precisely due east and
sets precisely due west.
The Ecliptic and the Equinox
The apparent path of the sun (ecliptic) is tilted at an angle of 23.5
degrees with respect to the celestial equator.
Hence the ecliptic crosses the celestial equator twice a year
The crossing points mark the Spring Equinox and Fall Equinox.
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The Zodiac
• Because of its relationship to changing seasons, ancient
cultures attached great significance to the ecliptic.
– It marks the center line of the zodiac (‘circle of animals’),
a band of the sky divided into 12 parts of width 30
degrees.
– The signs of the zodiac are no longer of special
importance in astronomy.
The Planets
• The planets can be seen along the ecliptic
appearing like bright stars.
– Why are they seen along the ecliptic?
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The Planets
• As Venus and Mercury orbit inside Earth’s orbit, they
never get far from the sun and are visible in the west
after sunset or in the east before sunrise.
• Venus can be very bright, but Mercury is difficult to
see near the horizon.
The Planets
• By tradition, any planet in the sunset sky is
called an evening star.
• Any planet in the dawn sky is called a morning
star.
• Venus seasonally switches from morning to
evening star
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Precession of Earth’s axis
• Precession may affect the severity of the different
seasons and may cause extreme climate changes
such as ice ages
Earth’s axis
precesses like
the axis of a
spinning top
Summary
• What causes the seasons?
– The orbit of the earth and the tilt of the Earth’s
axis causes sunlight to hit different parts of the Earth
more directly during the summer and less directly
during the winter
• How do we mark the progression of the seasons?
– The summer and winter solstices are when the
Northern Hemisphere gets its most and least direct
sunlight, respectively. The spring and fall
equinoxes are when both hemispheres get equally
direct sunlight.
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Summary
• What is the seasonal motion of the sun in the sky?
– The tilt of the earth causes the days in the Northern
hemisphere to be longer in summer and shorter in winter
and vice versa in the Southern Hemisphere.
– The tilt of the ecliptic causes the sun to appear to follow
different paths in the sky in summer and winter
• What could cause the seasonal motion of the sun to change over
time ?
– The tilt remains about 23.5 degrees (so the season pattern is
not affected), but Earth has a 26,000 year precession cycle.
– This precession may cause extreme climate changes
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