Lecture 2 Motions in the Sky

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Lecture 2Motions in the Sky
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Yearly Motions

• The Earth travels around the Sun once every year.
• The Earth is tilted by 23.5 degrees with respect
  to the orbital plane.

E
E
E
E
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• The Sun lines up with different constellations
  during the year.

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Ecliptic

• Path of the sun through the celestial sphere.
• Not aligned with Celestial Equator due to tilt of
  the Earth

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Zodiac

• The 13 constellations through which the sun
  passes during the course of a year.

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Planetary Motion
The planets all move in the same plane about the
Sun so they all appear to move along the zodiac.
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The Real Signs of the Zodiac
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Solstices and Equinoxes

• Solstice
• position in Earths orbit at which the Sun is
  furthest from the Celestial Equator.
• Longest and shortest days of the year.
• Equinox
• position in Earths orbit at which the Sun
  crosses the Celestial Equator.
• Day and Night are the same length.

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The Ecliptic (Path of the Sun)
North Celestial Pole
Summer Solstice Sun reaches highest point in sky
for N. Hemisphere. (June 21)
Autumnal Equinox Sun crosses Celestial Equator.
(September 21)
Winter Solstice Sun at lowest point in sky for
N. Hemisphere. (December 21)
Vernal Equinox Sun crosses Celestial Equator.
(March 21)
South Celestial Pole
Celestial Equator
Ecliptic
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Coordinate Systems

• Declination Degree north () or south (-) of
  the Celestial Equator.

North Celestial Pole
90
60
30
Celestial Equator
0
-30
South Celestial Pole
-60
-90
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• Right Ascension Position east or west of the
  position of the Sun at the Vernal Equinox (like
  longitude).
• Sky split into 24 hours such that 1 hour15

North Celestial Pole
The Sun moves through 1 hour of Right
Ascension during 1 hour.
Ecliptic
Celestial Equator
Position of Sun at Vernal Equinox
2hr
1hr
0hr
22hr
23hr
South Celestial Pole
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The Seasons

• The Seasons are due to the tilt of the Earth with
  respect to the orbital plane.
• Not distance from sun

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Why are the seasons warm and cold?

• During the Summer the sun is higher in the sky so
  the light is more direct.

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Why are the seasons warm and cold?

• Sun is above the horizon longer in the summer.

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Earth in June (Summer Solstice)
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Earth in December (Winter Solstice)