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
appear
• Idea based upon how things _________
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 (________________)
model
Earth centered
Celestial Sphere: Extension of the Earth’s
Coordinate System
Celestial sphere
• N/S celestial poles
N/S Poles
•____________
• Celestial equator
•____________
Equator
•Declination
Latitude
•____________
•Right Ascension
Longitude
•____________
Like a clear bowl over
your head!
The Celestial Sphere
NCP
Earth’s spin axis
The celestial
sphere
Earth
Celestial equator
SCP
NCP
Celestial
Sphere
SS
Path of Sun
during the
year
AE
Earth
WS
e
SE
Celestial
equator
Ecliptic
SCP
Ecliptic (2) == apparent
_____________________________________________
annual path of Sun on the CS
North
Ecliptic
East
Sun’s Path
Celestial Equator
Declination (degrees)
South
Right Ascension (hours)
Key terms
• Celestial sphere:
Apparent 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)
Earth’s North Pole
• NCP is directly above _______________
– Point about which the stars appear to rotate
• Celestial equator:
Projection of Earth’s equator onto the CS
– ___________
Apparent motion: Earth’s 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 - Polaris
• In the south, the center of the rotation is the
_______________________________
South Celestial Pole – No pole star
• The celestial sphere appears to rotate once
24 hours
every _________
Where on Earth
• Definition:
–Latitude (l):
The angle between the Observer, the
center of the Earth and the Earth’s
equator
(Measured in degrees)
l = 0o (Equator)
l = 90o (North pole)
l = -90o (South pole)
(l = 90o) North pole
observer
Equator
l
Earth
South pole (l = -90o)
horizon
(l = 0o)
The latitude of an observer is equal to
the altitude of the visible celestial pole
• latitude of observer = l
• altitude of NCP = A
To NCP
zenith
A
B
l
B=
-l
A + B = 90o
90o
l
Earth
To CE
horizon
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
The Celestial Sphere
Precession
In addition to its rotation and revolution, the earth’s 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
Because of precession:
• Polaris won’t
always be the
North Star
• Orion won’t
always be a
winter
constellation
• Horoscopes
are all wrong!