The Celestial Sphere

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The Celestial Sphere
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An Ancient Greek Idea

• Imagine the universe to be a hollow sphere with
  the Earth at the center
• The Celestial Sphere
• Idea based upon how things appear to be
• The pattern of stars in the sky does not change
• Stars embedded on the inside
• Pretend the stars are painted on a giant bubble
  around Earth
• Geocentric (Earth centered) model

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Celestial Sphere Extension of the Earths
Coordinate System

• Celestial sphere
• N/S celestial poles
• Celestial equator
• Declination
• Right Ascension

Like a clear bowl over your head!
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The Celestial Sphere
NCP
Earths spin axis
The celestial sphere
Earth
Celestial equator
SCP
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NCP
Celestial Sphere
SS
AE
Path of Sun during the year
Earth
e
Celestial equator
SE
WS
Ecliptic
Ecliptic (2) apparent annual path of Sun on the
CS
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North
East
South
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Key terms

• Celestial sphere
• apparent sphere of the sky (what it looks like)
• Celestial poles joined by line that runs through
  the Earths north and south poles
• (this line is, in fact, the Earths spin axis)
• NCP is directly above Earths north pole
• point about which the stars appear to rotate
• Celestial equator
• projection of Earths equator onto the CS

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Apparent motion Earths rotation

• As Earth rotates, it looks as if the celestial
  sphere is rotating around us
• In the north, the center of the rotation is the
  North Celestial Pole
• In the south, the center of the rotation is the
  South Celestial Pole
• The celestial sphere appears to rotate once every
  24 hours

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Where on Earth

• Definition
• Latitude (l)

The angle between the Observer, the center of the
Earth and the Earths equator
(Measured in degrees)
l 0o (Equator) l 90o (North pole) l
-90o (South pole)
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(l 90o)
North pole
zenith
observer
horizon
l
Equator
(l 0o)
Earth
South pole
(l -90o)
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The latitude of an observer is equal to the
altitude of the visible celestial pole
To NCP

• latitude of observer l
• altitude of NCP A

zenith
B
A
l
B 90o - l
To CE
A B 90o
l
horizon
Earth
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The final steps (hold tight)

• By definition
• angle A angle B 90o

But, angle B 90o - l, so A 90o - l 90o
And hence, A l
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Precession
In addition to its rotation and revolution, the
earths axis also precesses (wobbles) like a top.
The angle between the ecliptic and the equator
remains at 23.5, but the direction changes. The
period of this precession of the equinoxes is
about 26,000 years.
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Because of precession

• Polaris wont always be the North Star
• Orion wont always be a winter constellation
• Horoscopes are all wrong!