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Summary: how orbital variations affect sunshine
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Obliquity
When obliquity is high, seasonality is enhanced. In
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mid and high latitudes, more sunshine comes in summer, and
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less comes in winter.
Averaged over the whole year, the high
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latitudes receive more sunshine, and the low latitudes receive
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less.
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Precession
When perihelion occurs at summer solstice, the
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contrast between summer and winter is enhanced. When
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perihelion occurs at winter solstice, seasonality is weakened.
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Precession has little effect on annual mean sunshine.
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Eccentricity
Changes in the eccentricity of the earth’s orbit
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have very little effect on the annual mean sunshine anywhere.
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However, they do have a large impact on the amplitude of the
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precessional cycle. If the
earth’s orbit is very eccentric, the
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timing of perihelion in the calendar year becomes more critical.
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