Transcript The Sky and the Motions of the Earth

Motions of the
Earth and Sky I
Outline for Today
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History: flat vs. spherical earth
Map of the sky
Constellations
Diurnal and Yearly Motion
The seasons
Precession
Phases of the Moon
Flat or Spherical Earth?
• Proposed spherical earth: Aristotle (384-322 B.C.)
• Evidence: curved shadow earth casts on the Moon
• Additional evidence: new stars seen when sailing south
• Spherical earth widely believed since time of Aristotle
• Columbus set out to prove the world is round?
Myth created by fictional biography of Columbus by
Washington Irving
Constellations
• Constellations are patterns
of stars that have a historical
reference to a mythological
figure, animal, or object.
• The stars are usually not
near each other in space;
they just lie in the same
direction.
101 LY
~80 LY
124 LY
Ursa Major, The Big Bear (Big Dipper)
The Big Dipper is an
asterism within the
constellation of Ursa
Major. An asterism is
a prominent pattern
of stars that is smaller
than a constellation.
The Celestial Sphere = map of the sky
Daily Motion
• The earth’s axis of rotation
also defines some places on
the celestial sphere.
• The celestial North pole is
over the earth’s north pole.
There happens to be a
bright star near it, which is
called Polaris, or the North
Star.
• The celestial equator is the
extension of the earth’s
equator.
Daily Motion
• Each day, the earth rotates
once (west-to-east) on its
axis. This causes us to face
different directions and see
different stars. The stars’
daily (diurnal) motion
reflects the earth’s spin.
stars appear to rotate about the north & south
celestial poles
Yearly Motion
In addition to rotating, the Earth also revolves about the Sun.
As the earth revolves the
Sun is projected in front
of different constellations
at different times of year.
The path the Sun takes
across heavens is called
the ecliptic. The
constellations which the
Sun passes through are
zodiac constellations.
Because the Sun is bright, we can only see some constellations at
certain times of year.
The Seasons
• The earth’s spin axis is tilted 23.5° with respect to the
plane of the earth’s orbit (the ecliptic).
• Because of tilt, the “directness” of sunlight changes
during the year.
• northern hemisphere tilted toward the Sun
summer solstice
• southern hemisphere tilted toward the Sun
winter solstice
• Neither is tilted toward the Sun
vernal and autumnal equinox
• Seasons not caused by earth changing its distance from
the Sun
The Seasons
During summer, the Sun moves higher in the sky, so sunlight is more
direct, and it is visible for a longer period of time.
QuickTime™ and a
Video Format cvid
decompressor
are needed to see this picture.
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.
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.
QuickTime™ and a
Sorenson Video 3 decompressor
are needed to see this picture.
Because of precession:
• Polaris won’t always
be the North Star
• The season during
which a constellation
appears at night
changes very slowly
over time
3rd
quarter
sunrise
noon
midnight
sunset
new
Moon’s phase + position in sky
 Time
full
1st quarter
http://www.csulb.edu/~gpickett/ps112_ems.html
http://www.astro.psu.edu/users/kluhman/a5/Lunar_Nav.swf
Phases of the
Moon
The Moon reflects light
from the Sun, so its
phase tells you the
relative position of the
Sun.
You can therefore tell
time from the Moon!
Phases of the
Moon
Full Moon occurs when
the Moon is opposite in
the sky from the Sun.
This is called opposition.
New Moon occurs when
the Moon is in the same
direction as the Sun, i.e.,
in conjunction.