Earth's Seasons

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Star Gazing
Activity Earths Seasons
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Summary
In this Activity, we will investigate (a) the
Earths orbit around the Sun,(b) the origin of
the seasons on Earth, and(c) the Earths
precession.
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The Ecliptic
The apparent path of the Sun across the sky
during the day is called the ecliptic.
From our heliocentric point of view, this
apparent motion reflects the Earths orbit around
the Sun.
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The plane of the ecliptic is an imaginary planar
surface in space containing the Earths orbit and
the Sun
Earth
Sun
The Earth takes one year to make a complete
orbit around the Sun.
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(a) the Earths orbit around the Sun
The Earths orbit is an ellipse.
Ellipses can have various shapes, from
squashed, or more technically, orbits of
higheccentricity
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to
nearly or completely circular, or more
technically, orbits of low or zero eccentricity
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• Ellipses are characterized by their eccentricity
  e, which varies from

e 1
e 0
e ? 0.8
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The Earths orbit is nearly circular, with e
0.0167.
Its average distance from the Sun is 149 597 900
km.

We usually write awkwardly large numbers like
this in a sort of mathematical shorthand called
scientific notation, as
1.49597900 x 108 km
where the 108 means that you need to multiply
1.49597900 by 10 eight times!
pretty impressive accuracy!
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We have another even more convenient way of
representing the average distance from the
Earthto the Sun
Its defined as one Astronomical Unit, i.e. 1
AU 1.49597900 x 108 km
1 AU
Well find Astronomical Units (AUs) convenient
whenwe compare distances between the Sun other
planets in our Solar System.
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The small eccentricity (0.0167) of the Earths
orbit means that its distance from the Sun
varies by 3.34 x 106 km during the course of a
year.
This is a variation of only about 2 in the
overall orbital radius, but it represents a
distance of approx. 2 600 times the Earths
diameter.
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(b) The origin of the seasons on Earth

• As we saw in the last Activity, one (Earth) year
  is the time it takes for Earth to make a complete
  orbit around the Sun.

Earth
Sun
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However we primarily notice the passing of a year
not by Earths orbital position but by the cycle
of the seasons.
Other planets have seasons too. Investigating
the reasonsfor Earths seasons will help us
understand the conditionson other planets also.
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• Most people are confused about why the Earth has
  seasons.

What is your answer to the question Why is it
warmer in summer than in winter?
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Here are some possible answers
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Its warmer in summer than in winter because ...
(a) the Earth is closer to the Sun then,
(b) the tilt of the Earths axis means that the
hemisphere experiencing summer is closer to
the Sun than the hemisphere experiencing
winter,
(c) the tilt of the Earths axis means that the
hemisphere experiencing summer gets more
concentrated sunlight than does the
hemisphere experiencing winter,
(d) the tilt of the Earths axis means that the
summer hemisphere has longer hours of
sunlight than does the winter hemisphere.
Click on alternatives that seems reasonable to
see if we agree with you! Then continue...
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Did you investigate alternatives (c) and (d) ?
If so, you will have found that there are 2 major
reasonsfor the seasons on Earth
the differing length of daylight hours in each
hemisphere, and
the amount by which sunlight striking the earth
is spread out, depending on whether the sun is
high (summer) or low (winter) in the sky.
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both these effects are due to the tilt of the
Earthsrotational axis
23½o
plane of the ecliptic
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Whether the rotational axis is tilted or
notdetermines whether other planets experience
seasonstoo.
As the Earth orbits the Sun, we can track the
changing of the seasons
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Northern spring
Southern autumn
Northern summer
Northern winter
Southern summer
Southern winter
Northern autumn
Southern spring
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(c) The Earths precession
23½o
The direction of the rotationalaxis of the Earth
is not fixedin space.
Tidal forces due to the Sun Moon cause it to
very slowly rotate
plane of the ecliptic
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This is called precession.
Over a period of 26 000 years, the earths
rotational axis precesses through a complete
cycle.
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It is this precession which has gradually shifted
the positions of the constellations in the sky,
and, in particular, the periods of the year
which correspond to each zodiacal constellation.

Precession also changes the locations at which
seasons occur in the Earths orbit. The Earth
isnow closest to the Sun during southern
Summers,but in about 13 000 years it will occur
duringnorthern Summers. This may cause southern
Summersto become more mild, and northern Winters
to become more severe.
See the previous Activity, Star Patterns
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Image Credits

• NASA View of the Mid-Pacific Ocean
• http//nssdc.gsfc.nasa.gov/image/planetary/earth/g
  al_mid-pacific.jpg

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Hit the Esc key (escape) to return to the Index
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• (a) Its warmer in summer than in winter,
  because the Earth is closer to the Sun then.

This is a very common response.
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It is true that the Earth is closer to the Sun at
one stageof its orbit.
As we saw in the last Section, the Earths
orbital separation from the Sun varies by about
2, equivalent to about 2 600 Earth diameters.
So could summer occur when the Earth is closest
to the Sun?
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But when its summer in the northern hemisphere,
itswinter in the southern hemisphere - and vice
versa.
So summer cant occur at just one particular part
of the Earths orbit.
Back to the alternative answers
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Back to the alternative answers
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(b)
The Earths rotational axis is tiltedby 23½o
with respect to a line drawn perpendicular to
the plane of the ecliptic.
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23½o
Could this tilt make one hemisphere of the Earth
sufficiently closer to the Suncompared to the
other hemisphere for seasons to result?
plane of the ecliptic
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The tilt of the Earths axis only makes a
difference of about 2 000 km - only 1/40 000 th
of the variation in the separation of the Earth
Sun.
So any distance effects due to the Earths tilt
will be overwhelmed by the distance effects due
to the eccentricity of the Earths orbit (which
are considered in (a) ).
Back to the alternative answers
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(c)
The Earths rotational axis is tiltedby 23½o
with respect to the plane of the ecliptic.
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23½o
plane of the ecliptic
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If we take the case when the northern hemisphere
is in summer ...

sunlight
then the tilt of the Earths rotational axis
means that the Sun is higher in the sky in
daytime in the northern hemisphere,
than in the southern hemisphere, which will be
in winter ...
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and in Summer when the Sun is higher in the
sky, each beam of sunlight is spread out less
when it hits the ground ...
Concentrated beamof sunlight in Summer
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than in winter, when the Sun is lower in the
sky.
Diffuse, spread-out beamof sunlight in Winter
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so for the hemisphere experiencing Summer,
sunlight striking the Earth is more concentrated
and this helps to raise the average temperature.
The reverse is true for the hemisphere
experiencing Winter ...
but this isnt the only factor causing the
Earthsseasons. Alternative (d) is a
contributing factor too.
Back to the alternative answers
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(d)
The Earths rotational axis is tiltedby 23½o
with respect to the plane of the ecliptic.
23½o
plane of the ecliptic
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If we take the case when the northern hemisphere
is in midsummer ...
then the north pole has continuous daylight
and locations in the northern hemisphere have
long periods of daylight,
sunlight
equator
whereas locations in the southern hemisphere
have long nights.
the south pole is in continuous darkness
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Long periods of daylight help to warm the
hemisphereexperiencing Summer,
more than the hemisphere experiencing the short
periods of daylight and long cold nights of
winter.
Back to the alternative answers
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Back to the alternative answers