Earth Motions and the Heavens
Rotation
Revolution
Precession

What are the basic Earth motions?
Rotation
Revolution
Precession
http://www.snyder7hills.org/units/ss/ch1/rotrev.html

What are the apparent celestial motions
associated with Earth’s rotation?
http://en.wikipedia.org/wiki/File:AxialTiltObliquity.png
http://3.bp.blogspot.com/-hlq7nqAtZlk/Tt5FHTu6WI/AAAAAAAADEE/ukVSOFugs2w/s1600/startrails.jpg
What are
Star Trails?
http://zuserver2.star.ucl.ac.uk/~idh/apod/image/0609/gemstartrails_schulz_f.jpg
What is the apparent hourly rate of
motion of the stars?
 Stars appear to move 15°/hr
because the Earth rotates at
15°/hr.
Star trails over the 60 inch
dome at Palomar Observatory
3 hour view of night at Palomar
http://www.astro.caltech.edu/palomar/images/star.trails.jpg
How do star trails change with direction?
 North- circumpolar
 East- rising
 South- arc from east to west
 West- setting
http://www.astro.caltech.edu/palomar/images/haletrails.jpg
Looking out the window…
North
South
East
West
Star positions change throughout the night
(except for Polaris) because the Earth
rotates
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
dome.
http://gemini.physics.ox.ac.uk/photos/gemini-trails-mr.gif
Lecture Tutorial: Position

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

Motion
Motion
How much of the celestial
sphere can an Earth
observer see at one
time?
a) less than half
b) exactly half
c) more than half
Review Lecture
Tutorial
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
night
 Visible all year
Orion
Why does Orion look different in the Southern Hemisphere?

http://www.dimijianimages.com/Africa-p10/star-trails-south-TX-gallery.jpg
Orion in Southern Hemisphere
Sirius is blue
Betelgeuse is red
http://www.starrynightphotos.com/star_trails/images/star_trails_orion.jpg
What is the affect of latitude on star
trails?
 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
horizon.
http://ganymede.nmsu.edu/tharriso/ast110/class05.html
http://images.google.com/imgres?imgurl=http://www.dimijianimages.com/Africa-p10/star-trails-southTX-gallery.jpg&imgrefurl=http://www.dimijianimages.com/Africa-p10/star-trails-southTX.htm&usg=__Ko6DW_HfnHz6RuO4benYIjBf914=&h=592&w=850&sz=167&hl=en&start=54&um=1&itbs
=1&tbnid=x7KgdoC9kpKAM:&tbnh=101&tbnw=145&prev=/images%3Fq%3DSTAR%2BTRAILS%26start%3D40%26um%3D
1%26hl%3Den%26safe%3Dactive%26sa%3DN%26rls%3Dcom.microsoft:enus%26ndsp%3D20%26tbs%3Disch:1
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

Intro
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?
http://bigbendnow.com/wp-content/uploads/2012/06/Night-Sky-Changes-with-the-Seasons.jpg

http://ganymede.nmsu.edu/tharriso/ast110/ch02f14.0_0.jpg
What is the affect of Earth’s Revolution on
the night sky?
Earth motion is
counterclockwise
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
day
hours
1440 minutes/day = 4 minutes
360°/day
 Stars rise 4 minutes earlier each day
What is the ecliptic?

http://www.eso.org/public/outreach/eduoff/vt-2004/Background/Infol2/vt2004-if11-fig2.jpg
http://www.calendrier-lunaire.fr/en_US/eclipses-de-lune-et-de-soleil-comprendre-le-calendrier-lunaire.html
http://www.calendrierlunaire.fr/uploads/images/Anglais/UNDERSTA
NDING/eclipse.jpg
http://star.wind.mystarband.net/bib/images/m
oon_orbital_phases.gif
http://lcogt.net/files/styles/fourcolimage/public/spacebook/Ecliptic%20repair.png
Ecliptic is the
plane of the
solar systemwhere we find
the planets and
the Sun
http://cse.ssl.berkeley.edu/img/eclip.gif
http://physics.uoregon.edu/~jimbrau/BrauImN
ew/Chap06/FG06_05.jpg
What can we
see on the
Ecliptic?
http://ganymede.nmsu.edu/tharriso/ast110/class05.h
tml
http://upload.wikimedia.org/wikip
edia/commons/b/bd/Earths_orbit
_and_ecliptic.PNG
How is the Ecliptic related to the seasons?
When the sun
is crosses the
celestial
equator, we
have the first
day of spring
or fall.
http://i131.photobucket.com/albums/p301/gl
actus/space%20album%20three/ecliptic310x2
72.jpg
What is the affect of Earth’s Revolution on
the constellations blocked by the Sun?
Earth motion is
counterclockwise
in this picture
Gives an apparent eastward “motion” of the Sun
The set of
constellations
that align on
the ecliptic
What is the Zodiac?
http://www.starchamber.com/paracelsus/images/seasons.gif

Ophiuchus, 13th sign of Zodiac!
http://ganymede.nmsu.edu/tharriso/ast110/ch02f14.0_0.jpg
How does Earth’s Precession
affect the night sky?
http://www.astro.virginia.edu/class/oconnell/astr1230/4.1-precession.html

Slow: 0.5
degrees per
century
Precession
http://www.astro.virginia.edu/class/oconnell/astr1230/im/precess-NS.gif

Precession
http://www.epm.ethz.ch/research/experimental_studies/exp_prec/precession_earth.jpg?hires
http://cache.eb.com/eb/thumb?id=94898
Precession of the Pole Star
http://religion.lilithezine.co
m/images/EarthPrecession.gif
Precession of the Equinox
Age
 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.
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Earth Motions and the Heavens