Earth Motions and the Heavens
What are the basic Earth motions?
What are the apparent celestial motions
associated with Earth’s rotation?
What are
Star Trails?
What is the apparent hourly rate of
motion of the stars?
 Stars appear to move 15°/hr
because the Earth rotates at
Star trails over the 60 inch
dome at Palomar Observatory
3 hour view of night at Palomar
How do star trails change with direction?
 North- circumpolar
 East- rising
 South- arc from east to west
 West- setting
Looking out the window…
Star positions change throughout the night
(except for Polaris) because the Earth
A view of the GEMINI
North dome on Mona Kea
soon after sunset
showing star trails from a
time-lapse exposure. Also
seen are trails from
vehicle headlights as they
drive passed the GEMINI
Lecture Tutorial: Position
You observe a star rising due east. When this star
reaches its highest position above the horizon,
where will it be?
a) high in the northern sky
b) high in the eastern sky
c) high in the southern sky
d) high in the western sky
e) directly overhead
 Imagine you are standing in the northern hemisphere.
Looking directly north, you see a star just above the
horizon. A little later you notice that it has shifted position
slightly. Which way did it move?
a) to the right, (east)
b) to the left, (west)
c) up, (rising)
d) down, (setting)
Lecture Tutorial: Motion
How much of the celestial
sphere can an Earth
observer see at one
a) less than half
b) exactly half
c) more than half
Review Lecture
Celestial Sphere: Geocentric View
How is Polaris different from other stars?
 Polaris doesn’t appear to move (much)
because it aligns with the spin axis (within
about 1 degree).
 Polaris’ position is North, stationary, and its
altitude is equal to the observer’s latitude
The stars positions change throughout the
night (except for Polaris) because the
Earth rotates and Polaris aligns with the
spin axis.
What is special about Circumpolar Stars?
 Circle counterclockwise
around Polaris
 Never rise or set during the
 Visible all year
Why does Orion look different in the Southern Hemisphere?
Orion in Southern Hemisphere
Sirius is blue
Betelgeuse is red
What is the affect of latitude on star
 Near the equator, stars in the eastern sky trail
directly up and over your head.
 At higher latitude stars are tilted with respect to the
rotating "sphere" of the sky.
 At either pole the stars would go around the
View East at Equator
South Pole
This is another picture of the
South Pole, but shorter. Two
nearby galaxies are visible in this
image. The large fuzzy one, just
above the roof of the Commons
Building, is the Large Magellanic
Cloud, and the smaller fuzzy
patch near the top of the image is
the Small Magellanic Cloud.
These galaxies are 200,000 light
years away, yet easily visible to
the naked eye in dark skies!
Lecture tutorial: Seasonal Stars
You go out tonight and see the brightest star in the
constellation Orion just rising above your eastern
horizon at 10 PM. One week later at 10 PM this
same star will be
a) slightly higher in the sky.
b) at the same height as before.
c) below your horizon.
d) setting on your western horizon.
Lecture tutorial: Seasonal Stars
What is the affect of Earth’s
Revolution on the sky?
What is the affect of Earth’s Revolution on
the night sky?
Earth motion is
in this picture
Night stars will appear to have shifted to the westMeaning they have risen earlier
How far do stars shift from night to night?
 Earth revolves around Sun 360° in one year
 So in 12 months Earth moves 360°, so the stars
appear to shift 30° to the west
 And in one day, Earth moves 1°, so the stars
appear to shift 1° to the west
How does Earth’s revolution affect the
times of star rising and setting?
 Stars appear to rise earlier every night
24 hours x 60 minutes = 1440 minutes per day
1440 minutes/day = 4 minutes
 Stars rise 4 minutes earlier each day
What is the ecliptic?
Ecliptic is the
plane of the
solar systemwhere we find
the planets and
the Sun
What can we
see on the
How is the Ecliptic related to the seasons?
When the sun
is crosses the
equator, we
have the first
day of spring
or fall.
What is the affect of Earth’s Revolution on
the constellations blocked by the Sun?
Earth motion is
in this picture
Gives an apparent eastward “motion” of the Sun
The set of
that align on
the ecliptic
What is the Zodiac?
Ophiuchus, 13th sign of Zodiac!
How does Earth’s Precession
affect the night sky?
Slow: 0.5
degrees per
Precession of the Pole Star
Precession of the Equinox
 The “Age” is the constellation that lines up with the
sun on the first day of spring.
Tropic of Cancer
 The sun rose in the constellation Cancer on June 21
(summer solstice) when they named the Tropic of
Cancer. (23.5 degrees N)
 The sun is overhead on June 21 at 23.5 degrees N.
Tropic of Capricorn
 On the Winter Solstice, Dec 21, the sun is directly
overhead at 23.5 degrees S.
 150 BC- The sun rose with the constellation
Capricorn on at that date.

Earth Motions and the Heavens

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