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 Diurnal Motion
 Annual Motion
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Daily East / West motion of the sky
Due to the Earth’s rotation (15°/hour)
[360°/24 hours = 15°/hour]
[1° in 4 minutes]
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Stars fall into two groups
Circumpolar - never rise or set
Equatorial
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Circle going through north and south
points on the horizon and the zenith
Z
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The sky appear like a dome overhead.
Zenith-directly
overhead
Meridian – imaginary
line running north &
south through the
zenith
Horizon – where the
sky appears to intersect
the ground.
Altitude – height of a
star above horizon
(degrees).
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Objects on the Meridian are at their highest
point in the sky (transit)
Astronomical noon is when Sun is on the
Meridian.
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Does not mean the Sun is directly overhead.
Midnight is when the Sun is on the Meridian below
the horizon.

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Sky looks like a
dome with the star
painted on the
inside.
88 constellations
such as Ursa Major,
Orion, etc.
Asterisms are not
constellations for
example the Big
Dipper, the Teapot,
etc.
Stars are not a same distances.
So, constellations are not real places.
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North Celestial Pole (NCP) –
extension of Earth’s rotation
axis onto the sky.
South Celestial Pole - ditto
Celestial Equator – extension
of Earth’s equator onto the
sky.
Ecliptic – apparent yearly
path of Sun.
NCP
Celestial Sphere
Autumnal
Equinox
23.5°
Ecliptic
Vernal
Equinox
SCP
Celestial Equator
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The stars, sun, moon
and planets all
appear to rise in the
east and set in the
west.
Reflection of Earth’s
rotation from west
to east.

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We live on the side
of the Earth.
NCP’s altitude
equals your latitude.
Circumpolar stars
never rise or set but
circle the NCP
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We live on the side
of the Earth.
Sky appears tilted at
an angle equal to
our latitude.
Stars appear to
move in arcs across
the sky that are not
perpendicular to
horizon.
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The sky appears to rotate around NCP.
Altitude of NCP is always equal to your
latitude on Earth's surface.
A space ship lands on an unknown planet.
The occupants notice that the stars never rise
or set but appear to move in circles parallel
to the horizon.
Where on the planet did the space ship land?
a) At the equator.
b) At 45 degrees latitude.
c) At one of the celestial poles.
d) Cannot be determine.
Atlanta has a latitude of about 33 north degrees.
The star Polaris is very close to the Celestial North Pole.
What is the altitude of Polaris when viewed from Atlanta?
a) 33 degrees
b) 45 degrees
c) 57 degrees
d) 90 degrees
This picture is a time exposure of the night sky showing star trails as the
stars appear to rotate around the NCP.
What is the name of the bright star near the center of the rock arch?
Where are the circumpolar stars?