The Celestial Sphere

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The Celestial Sphere

• Paul J. Thomas
• Department of Physics and Astronomy
• UW-Eau Claire

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Celestial Sphere
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Celestial Coordinates
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The Ecliptic and the Celestial Equator
Sun is here June 21
Sun is here December 21
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Ecliptic

• The Suns apparent path through the sky over one
  year, through the constellations of the zodiac.
• At an angle of 23½0 to the celestial equator.
• Summer solstice June 21
• Winter solstice December 21
• Vernal equinox March 21
• Autumnal equinox September 21

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Ecliptic
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Declination and Right Ascension
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Sidereal and Solar Days
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Calendar Issues

• Solar day (between Sun meridian crossings) is 24
  hours.
• Sidereal day (between star meridian crossings) is
  23 h 56 m.
• Tropical year is 365.2422 solar days.
• One leap day in every year divisible by 4, but
  not 100, except if by 400.
• Lunar month is 27.32 days.
• Complicated, isnt it?

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Some astronomical terms

• Celestial sphere
• North and South Celestial Poles
• Celestial Equator
• Ecliptic
• Direct Motion (eastward on ecliptic)
• Retrograde Motion (westward on ecliptic)

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Precession

• Discovered by Hipparchus in 134 B.C.
• Due to the tidal forces of the Sun and Moon, the
  orientation of the Earths axis wobbles.
• A complete wobble takes 26,000 years.

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Constellations of the Zodiac

• Traditional Actual
• Aries 3/21-4/19 4/19-5/13
• Taurus 4/20-5/20 5/14-6/19
• Gemini 5/21-6/20 6/20-7/20
• Cancer 6/21-7/22 7/21-8/9
• Leo 7/23-8/22 8/10-9/15
• Virgo 8/23-9/22 9/16-10/30

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Constellations of the Zodiac

• Traditional Actual
• Libra 9/23-10/22 10/31-11/22
• Scorpius 10/23-11/21 11/23-11/29
• Ophiuchus 11/30-12/17
• Sagittarius 11/22-12/21 12/18-1/18
• Capricorn 12/22-1/19 1/19-2/15
• Aquarius 1/20-2/18 2/16-3/11
• Pisces 2/19-3/20 3/12-4/18