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The Cycles of the Moon
•The phases of the moon
•The tides
•Lunar eclipses
•Solar eclipses
The Phases of the Moon
From Earth, we see different
portions of the Moon’s surface lit by the sun, causing the
phases of the Moon.
The Phases of the Moon
• The Moon orbits Earth in a sidereal period of
27.32 days.
27.32 days
EarthMoon
Fixed direction in space
Is the moon going to show the same lunar phase after one
sidereal period?
1. Yes.
2. No, it will not have completed a full cycle of phases.
3. No, it will have completed more than a full cycle of phases.
The Phases of the Moon
• The moon’s synodic period (to reach the
same position relative to the sun) is 29.53 days (~ 1 month).
Fixed direction in space
Earth
Moon
Earth orbits around Sun => Direction toward Sun changes!
29.53 days
The moon orbits counterclockwise around Earth (viewed from the North). => It appears to move eastward against
the background of the stars. => The waxing crescent is visible
1. in the morning sky.
2. in the evening sky.
3. the whole night, from sunset to sunrise.
4. only around midnight.
5. never.
The Phases of the Moon
New Moon → First Quarter → Full Moon
Evening Sky
The Phases of the MoonFull Moon → Third Quarter → New Moon
Morning Sky
Waning
The Tides
The tides are caused by the difference of
the Moon’s 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
On the day of full moon, high tides occur …
1. around noon and 6 p.m.2. around noon and midnight.3. around 6 a.m. and 6 p.m. 4. around 6 p.m. and midnight.5. Impossible to tell. The times of tides are not
correlated with the phases of the moon.
Spring and Neap Tides
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
The Tidally Locked Orbit of the Moon
The Earth also exerts tidal forces on the
Moon’s 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.
The Near Side of the Moon
A total lunar eclipse …
1. is a high-performance moon vehicle built by Mitsubishi.
2. occurs when the moon disappears behind the sun.
3. occurs when the moon becomes invisible because it is too close to the sun.
4. occurs when the moon moves through Earth’s shadow.
5. occurs when the moon disappears behind Mars.
Lunar EclipsesThe Earth’s 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.
A Total Lunar Eclipse (I)
A Total Lunar Eclipse (II)
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 Earth’s atmosphere.
Typically, 1 or 2 lunar
eclipses per year.
Solar Eclipses
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.
Total Solar Eclipse
Prominences
Chromosphere and Corona
Diamond Ring Effect
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?
1. Yes.2. No because the sun is much farther away, but also
much larger than the moon.3. No, because the sun is much farther away, but also
much smaller than the moon.4. No because the moon is much farther away, but also
much larger than the sun.
5. No, because the moon is much farther away, but also much smaller than the sun.
Earth’s and Moon’s orbits are slightly elliptical:
Sun
Earth
Moon
(Eccentricities greatly exaggerated!)
Perihelion = position closest to the sun
Aphelion = position furthest away from the sun
Perigee = position closest to Earth
Apogee = position furthest away from Earth
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?
1. A regular total solar eclipse.2. No solar eclipse at all.3. A partial solar eclipse with a crescent appearance.4. A partial solar eclipse with a ring-like appearance of
un-occulted parts of the sun.5. A lunar eclipse.
Annular Solar EclipsesThe angular sizes of the
Moon and the Sun vary,
depending on their distance from Earth.
When the Earth is near perihelion, and the Moon is near apogee, we see an annular solar eclipse.
Perigee Apogee Perihelion Aphelion
Almost total, annular eclipse of May 30, 1984
If the moon was orbiting around the Earth exactly in the plane of the
ecliptic, …
1. lunar and solar eclipses would occur once every day.
2. lunar and solar eclipses would occur once a week.
3. lunar and solar eclipses would occur once a month.
4. lunar and solar eclipses would occur once a year.
5. lunar and solar eclipses would never occur.
Conditions for Eclipses (I)
The Moon’s 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.
Conditions for Eclipses (II)
Eclipses occur in a cyclic pattern.
→ Saros cycle: 18 years, 11 days, 8 hours
Approximately 1 total solar eclipse per year