lecture03spring2009

lecture03spring2009 - Astronomy100Dr.Rhodes...

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Astronomy 100 - Dr. Rhodes Lecture # 3 -1/16/2009
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Chapter 1: The Quest Ahead The Ecliptic The earth revolves about the sun in a flat surface which is called the ecliptic plane . As the earth revolves about the sun, the sun appears to move eastward around the sky from day to day in a great circle. The apparent path of the sun in the sky is called the ecliptic (which is not to be confused with the ecliptic plane ). The eastward motion of the sun along the ecliptic is called its annual motion .
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Chapter 1: The Quest Ahead Earth’s Orientation in Space A line perpendicular to a plane is called the normal to that plane. The earth’s spin axis is not normal to the ecliptic plane, but is inclined to the normal by an angle of 23 1/2 degrees (i.e., the spin axis is actually tipped by an angle of 66 1/2 degrees out of the ecliptic plane). This 23 1/2 degree tilt angle is called the obliquity of the ecliptic.
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Chapter 1: The Quest Ahead The Equinoxes The obliquity of the ecliptic causes the apparent path of the sun (i.e., the ecliptic) to be tilted by the same 23 1/2 degree angle from the celestial equator in the sky. Any two great circles on a sphere will intersect in two points 180 degrees apart on opposite sides of the sphere. The two intersection points of the ecliptic and the celestial equator on the celestial sphere are called the two equinoxes.
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Chapter 1: The Quest Ahead The Zodiac The zodiac (which means “circle of animals”) is the band in the sky which lies 9 degrees on either side of the ecliptic on the celestial sphere. The zodiac contains the constellations through which the sun appears to move during each year. These 13 constellations are known as the signs of the zodiac .
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Chapter 1: The Quest Ahead The Vernal Equinox The intersection point of the ecliptic and the celestial equator at which the sun is moving from the south to the north across the celestial equator is known as the vernal equinox. The vernal equinox occurs about March 21st, which is said to be the first day of spring. At the vernal equinox the length of the day and the length of the night are each equal to each other (i.e. both are equal to 12 hours ).
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Chapter1: The Quest Ahead The Summer Solstice Three months after the sun was at the vernal equinox it has moved as far to the north of the celestial equator as it can move during the year.
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lecture03spring2009 - Astronomy100Dr.Rhodes...

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