The Cycles of the Moon

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Chapter 3
Cycles of the Sky
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The Annual Motion of the sun
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Due to Earths revolution around the sun, the sun
appears to move through the zodiacal
constellations. The suns apparent path on the
sky is called the ecliptic. Equivalent The
ecliptic is the projection of Earths orbit onto
the celestial sphere.
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The Seasons (I)
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Earths equator is inclined against the ecliptic
by 23.5º.
The different incidence angle of the suns rays
is causing the seasons on Earth
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The Seasons (II)
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The Seasons (III)
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Northern summer southern winter
Northern winter southern summer
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The Seasons (IV)
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Earths orbit (eccentricity greatly exaggerated)
Earth in January
Earth in July
sun
Earths distance from the sun has only a very
minor influence on seasonal temperature
variations.
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The Motion of the Planets (I)
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• The planets are orbiting the sun almost exactly
  in the plane of the ecliptic.

Jupiter
Venus
Mars
Earth
Mercury
The moon is orbiting Earth in almost the same
plane (ecliptic).
Saturn
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Apparent Motion of the Inner Planets
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Mercury appears at most 28º from the sun. It can
occasionally be seen shortly after sunset in the
west or before sunrise in the east. Venus appears
at most 46º from the sun. It can occasionally
be seen for at most a few hours after sunset in
the west or before sunrise in the east.
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The Tidally Locked Orbit of the moon
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• ? The moon is rotating with the same period
  around its axis as it is orbiting Earth (tidally
  locked).
• ? We always see the same side of the moon facing
  Earth.

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The Phases of the Moon (I)
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As the moon orbits around Earth, we see different
portions of the moons surface lit by the sun,
causing the phases of the moon.
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The Phases of the moon (II)
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New moon ? First Quarter ? Full moon Evening Sky
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The Phases of the moon (III)
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Full moon ? Third Quarter ? New moon Morning Sky
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The Orbit of the moon (I)
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• The moon orbits Earth in a sidereal period of
  27.32 days.

27.32 days
Earth
moon
Fixed direction in space
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The Orbit of the moon (II)
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Fixed direction in space
29.53 days
Earth
moon
Earth orbits around sun gt Direction toward sun
changes!

• The moons synodic period (to reach the same
  position relative to the sun) is 29.53 days ( 1
  month).

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Lunar Eclipses
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Earths shadow consists of a zone of full shadow,
the umbra, and a zone of partial shadow, the
penumbra. If the moon passes through Earths full
shadow (umbra), we see a lunar eclipse. If the
entire surface of the moon enters the umbra, the
lunar eclipse is total.
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A Total Lunar Eclipse (I)
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A Total Lunar Eclipse (II)
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A total lunar eclipse can last up to 1 hour and
40 min. During a total eclipse, the moon has a
faint, red glow, reflecting sunlight scattered in
Earths atmosphere.
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Typically, 1 or 2 lunar eclipses per year.
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Solar Eclipses (I)
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The angular diameter of the moon ( 0.5o) is
almost exactly the same as that of the sun.
This is a pure chance coincidence. The moons
linear diameter is much smaller than that of the
sun.
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Solar Eclipses
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Due to the equal angular diameters, the moon can
cover the sun completely when it passes in front
of the sun, causing a total solar eclipse.
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Total Solar Eclipse
Chromosphere and Corona
Prominences
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Diamond Ring Effect
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Earths and moons orbits are slightly elliptical
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Apogee position furthest away from Earth
Earth
Perihelion position closest to the sun
moon
Perigee position closest to Earth
sun
Aphelion position furthest away from the sun
(Eccentricities greatly exaggerated!)
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Annular Solar Eclipses
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The angular sizes of the moon and the sun vary,
depending on their distance from Earth.
Perigee
Apogee
Aphelion
Perihelion
When Earth is near perihelion, and the moon is
near apogee, we see an annular solar eclipse.
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Almost total, annular eclipse of May 30, 1984
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Approximately 1 total solar eclipse per year
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Very Important Warning
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Never observe the sun directly with your bare
eyes, not even during a partial solar eclipse!
Use specially designed solar viewing shades,
solar filters, or a projection technique
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Conditions for Eclipses (I)
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The moons orbit is inclined against the ecliptic
by 5º. A solar eclipse can only occur if the
moon passes a node near new moon. A lunar eclipse
can only occur if the moon passes a node near
full moon.
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Conditions for Eclipses (II)
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Eclipses occur in a cyclic pattern. ? Saros
cycle 18 years, 11 days, 8 hours
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Astronomical Influences on Earths Climate (I)
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Factors affecting Earths climate

• Eccentricity of Earths orbit around the sun
  (varies over period of 100,000 years)

• Precession (Period of 26,000 years)

• Inclination of Earths axis versus orbital plane

Milankovitch Hypothesis Changes in all three of
these aspects are responsible for long-term
global climate changes (ice ages)
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Astronomical Influences on Earths Climate (II)
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Last glaciation
End of last glaciation
Polar regions receiving less than average energy
from the sun
Polar regions receiving more than average energy
from the sun