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The Celestial Sphere

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Idea based upon how things appear to be. The pattern of ... Orion won't always be a winter constellation. Horoscopes are all wrong! Because of precession: ... – PowerPoint PPT presentation

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Title: The Celestial Sphere


1
The Celestial Sphere
2
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

3
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!
4
The Celestial Sphere
NCP
Earths spin axis
The celestial sphere
Earth
Celestial equator
SCP
5
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
6
North
East
South
7
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

8
(No Transcript)
9
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

10
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)
11
(l 90o)
North pole
zenith
observer
horizon
l
Equator
(l 0o)
Earth
South pole
(l -90o)
12
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
13
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
14
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.
15
Because of precession
  • Polaris wont always be the North Star
  • Orion wont always be a winter constellation
  • Horoscopes are all wrong!
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