The Cycles of the Moon

1
The Cycles of the Moon

• The phases of the moon
• The tides
• Lunar eclipses
• Solar eclipses

2
The Phases of the Moon
27.32 days

• The Moon orbits Earth in a sidereal period of
  27.32 days.

Earth
Moon
Fixed direction in space
3
The Phases of the Moon (II)
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).

4
The Phases of the Moon (III)
From Earth, we see different portions of the
Moons surface lit by the sun, causing the phases
of the Moon.
5
The moon orbits counterclockwise around Earth
(viewed from the North). gt It appears to move
eastward against the background of the stars. gt
The waxing crescent is visible

• in the morning sky.
• in the evening sky.
• the whole night, from sunset to sunrise.
• only around midnight.
• never.

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The Phases of the Moon (IV)
New Moon ? First Quarter ? Full Moon Evening Sky
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The Phases of the Moon (V)
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Full Moon ? Third Quarter ? New Moon Morning Sky
Waning
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The Tides

• The tides are caused by the difference of the
  Moons gravitational attraction on the water on
  Earth
• Between the near side and the center of the
  Earth
• Between the center and the far side of the Earth
• ? 2 tidal maxima
• ? 12-hour cycle

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On the day of full moon, high tides occur

• around noon and 6 p.m.
• around noon and midnight.
• around 6 a.m. and 6 p.m.
• around 6 p.m. and midnight.
• Impossible to tell. The times of tides are not
  correlated with the phases of the moon.

10
Spring and Neap Tides
0

• The Sun is also producing tidal effects, about
  half as strong as the Moon.
• Near Full and New Moon, those two effects add up
  to cause spring tides
• Near first and third quarter, the two effects
  work at a right angle w.r.t. each other, causing
  neap tides.

Spring tides
Neap tides
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The Tidally Locked Orbit of the Moon
0

• The Earth also exerts tidal forces on the Moons
  rocky interior.
• ? It 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 Near Side of the Moon
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13
A total lunar eclipse

• is a high-performance moon vehicle built by
  Mitsubishi.
• occurs when the moon disappears behind the sun.
• occurs when the moon becomes invisible because it
  is too close to the sun.
• occurs when the moon moves through Earths
  shadow.
• occurs when the moon disappears behind Mars.

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14
Lunar Eclipses
0
The Earths shadow consists of a zone of full
shadow, the Umbra, and a zone of partial shadow,
the Penumbra. If the Moon passes through the
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)
0
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A Total Lunar Eclipse (II)
0
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 sun light scattered
in the Earths atmosphere.
17
0
Typically, 1 or 2 lunar eclipses per year.
18
Solar Eclipses
0
The Sun has approx. the same angular diameter of
0.50 as the Moon. Thus, when the Moon passes in
front of the Sun, it can cover it completely,
causing a total solar eclipse.
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0
Total Solar Eclipse
Chromosphere and Corona
Prominences
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0
21
0
Diamond Ring Effect
22
If the sun and the moon have the same angular
diameter on the sky, does that mean that the sun
and the moon actually have about the same size?

0

• Yes.
• No because the sun is much farther away than the
  moon.
• No because the moon is much farther away than the
  sun.

23
Earths and Moons orbits are slightly elliptical
0
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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What do you expect to see if at the time of a
solar eclipse the moon is near apogee, and the
Earth is near perihelion?

• A regular total solar eclipse.
• No solar eclipse at all.
• A partial solar eclipse with a crescent
  appearance.
• A partial solar eclipse with a ring-like
  appearance of un-occulted parts of the sun.
• A lunar eclipse.

0
25
Annular Solar Eclipses
0
The angular sizes of the Moon and the Sun vary,
depending on their distance from Earth.
Perigee
Apogee
Aphelion
Perihelion
When the Earth is near perihelion, and the Moon
is near apogee, we see an annular solar eclipse.
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0
Almost total, annular eclipse of May 30, 1984
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If the moon was orbiting around the Earth exactly
in the plane of the ecliptic,

• lunar and solar eclipses would occur once every
  day.
• lunar and solar eclipses would occur once a week.
• lunar and solar eclipses would occur once a
  month.
• lunar and solar eclipses would occur once a year.
• lunar and solar eclipses would never occur.

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Conditions for Eclipses (I)
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The Moons orbit is inclined against the ecliptic
by 50. 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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0
Approximately 1 total solar eclipse per year