The Earth Rotates

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The Earth Rotates
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Relevant Geography
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Earths Rotation

• 23 hours and 56 minutes relative to background
  stars (sidereal period)
• Spin axis of Earth defines points on the
  celestial sphere (using north and south poles)
• North Celestial Pole
• South Celestial Pole
• Celestial Equator
• Sky appears to rotate east to west about the
  celestial poles because Earth rotates west to
  east.

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Celestial Sphere
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Rotating Celestial Sphere
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The Celestial Sphere
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Definitions

• Celestial Sphere An imaginary sphere where
  celestial objects are projected on the basis of
  their direction from Earth
• Celestial Poles The two points where the spin
  axis of the Earths spin axis intersects the
  celestial sphere
• Celestial Equator The projection of the Earths
  equator onto the celestial sphere
• Great Circle A circle on the sphere's surface
  whose center is the same as the spheres center,
  and divides the sphere into two equal hemispheres
  

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Definitions

• Zenith The point on the sky that is directly
  overhead of the observer.
• Horizon The great circle on the celestial sphere
  that is 90 degrees from the zenith
• Hour circle The great circle through the
  position of a celestial body and the celestial
  poles
• Meridian The hour circle that passes through the
  zenith and both celestial poles

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Directions on the Local Sky

• Altitude The minimum angular distance between
  the position of a celestial body and the horizon
• Azimuth The angular bearing of an object,
  measured from North (0 degrees) through East (90
  degrees), South (180 degrees), West (270
  degrees), and back to North (360 degrees)
• Hour Angle The angle between the meridian and an
  objects hour circle (west is positive)

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Celestial Coordinates

• Vernal Equinox The position of the Sun when it
  crosses the celestial equator from south to north
• Declination The minimum angular distance from
  the position of a celestial body and the
  celestial equator
• Right Ascension The eastward angle from the
  vernal equinox to the intersection of an objects
  hour circle with the equator
• 1 hour of angle 15 degrees

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Celestial Coordinates
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Motion Depends on Declination
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The Sky at the North Pole

• At the North Pole, the North Celestial Pole is at
  the zenith
• Stars never rise or set
• Planets, Moon, and Sun do rise and setWhy?

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Stars Rise and Set at the Equator
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The Sky at Our Latitude
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Circumpolar Constellations

• Circumpolar constellations never set.
• Circumpolar constellations change with latitude
  sky changes with latitude

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The Sky at Southern Latitudes
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Tilt of Spin Axis
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Precession of the Earths Axis
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North Celestial Pole Changes
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Solar vs. Sidereal Day
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An Earth Day

• Sidereal Day 23 hr 56 min 4 secMotion relative
  to background stars
• Mean Solar Day 24 hoursThe average time between
  meridian crossings of the Sun
• Apparent Solar Day variesThe actual time
  between the meridian crossings