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The Celestial Sphere (The sphere should really be rotating, not Earth)
The Celestial Sphere
(The sphere should really be rotating, not Earth)
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 _________ to be
–The pattern of stars in the sky ________________
• Stars ‘embedded’ on the _________–Pretend the stars are painted on a
giant ‘bubble’ around Earth• Geocentric (________________) model
appear
does not change
inside
Earth centered
Celestial Sphere: Extension of the Earth’s Coordinate System
Celestial sphere
• N/S celestial poles
•____________• Celestial equator
•____________•Declination
•____________•Right Ascension
•____________
Like a clear bowl over your head!
Latitude
Longitude
Equator
N/S Poles
The Celestial Sphere
Earth
NCP
SCP
Celestial equator
Earth’s spin axisThe celestial
sphere
SE
AESS
WS
Earth
CelestialSphere
Ecliptic
Celestialequator
Ecliptic = apparent annual path of Sun on the CS
NCP
Path of Sunduring the
year
SCP
Ecliptic (2) = _____________________________________________
North
South
Right Ascension (hours)
Celestial Equator
Ecliptic Sun’s Path
Eas
t
Declination (degrees)
Key terms
• Celestial sphere: – _________ sphere of the sky (what it looks like)
• Celestial poles joined by line that runs through the Earth’s north and south poles
(this line is, in fact, the Earth’s spin axis)
• NCP is directly above _______________– Point about which the stars appear to rotate
• Celestial equator: – ___________ of Earth’s equator onto the CS
Apparent
Earth’s North Pole
Projection
Apparent motion: Earth’s rotation
• As Earth ________, it looks as if the celestial sphere is _________ around us
• In the north, the center of the rotation is the _______________________________
• In the south, the center of the rotation is the _______________________________
• The celestial sphere appears to rotate once every _________
rotatesrotating
North Celestial Pole - Polaris
South Celestial Pole – No pole star
24 hours
Where on Earth• Definition:
–Latitude ():The angle between the Observer, thecenter of the Earth and the Earth’sequator
(Measured in degrees)
= 0o (Equator) = 90o (North pole) = -90o (South pole)
Earth
Equator
North pole
South pole
observer
zenith
= 0o)
= -90o)
= 90o)
horizon
The latitude of an observer is equal to the altitude of the visible celestial pole
A B
To NCP
zenith
horizon
Earth
B = 90o - To CE
A + B = 90o
• latitude of observer = • altitude of NCP = A
The final steps: (hold tight)
• By definition:angle A + angle B = 90o
But, angle B = 90o - , so
A + 90o - = 90
And hence,A =
The Celestial Sphere
PrecessionIn addition to its rotation and revolution, the earth’s axis also __________ (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 ____________.
precesses
26,000 years
• Polaris won’t always be the North Star
• Orion won’t always be a winter constellation
• Horoscopes are all wrong!
Because of precession:
