the “nature of things” in the solar system why does the sky look the way it does?
TRANSCRIPT
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The “Nature of Things” in the Solar System
Why does the sky look the way it does?
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The existence of the ecliptic
The Earth and the other planets orbit the Sun, in nearly the same plane for all planets
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The inner solar system 3 months from now
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February 3
May 3
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Why we see the Sun “projected against” different constellations at different times
of the year
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Then, why is the ecliptic tilted with respect to the celestial equator (the
reason for seasons)
Answer: the obliquity of the ecliptic (or more simply
“obliquity”, or even more simply, “tilt of the Earth’s axis”)
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Northern hemisphere in summer
Northern hemisphere in winter
The obliquity of the ecliptic and the seasons
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The obliquity of the ecliptic and habitability of the Earth
• The obliquity is 23.5 degrees, and changes over time by about +/- 1.5 degrees
• This is a mild variation, and allows moderate, but not extreme seasonal variations.
• It is conjectured that this modest variation is a consequence of the Earth having a relatively large moon, which may be a low-probability event.
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Eclipses: observational properties
A lunar eclipse (eclipse of the Moon)
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Progress of a lunar eclipse
What’s going on? Big mystery for the ancients
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A solar eclipse: one of the great
spectacles of nature
What is going on? More pictures: Fig 9.5- 9.8
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Eclipses caused by shadows of one solar system object on another
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Solar eclipses and lunar eclipses
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Solar eclipses are rare, lunar eclipses relatively common
• The Earth is a large object and casts a big shadow that covers a lot of space
• The Moon is a small object and casts a small shadow. It is much less likely to hit a given place on Earth.
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Lunar eclipses: dark ones and bright ones
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Why don’t eclipses occur every month? 5 degrees makes a difference
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Eclipses for 2010
• January 15 --- annular eclipse of Sun--Indian Ocean
• June 26 --- partial lunar eclipse --around 5-6 AM around moonset
• July 11--- eclipse of the Sun ---China and western Pacific
• December 21--- total lunar eclipse--maximum eclipse around 3 AM Iowa time