Transcript The Cycles of the Moon

Chapter 3:
Cycles of the Sky
The Annual Motion of the sun
Due to Earth’s revolution around the sun, the sun appears
to move through the zodiacal constellations.
The sun’s apparent path on the sky is called the ecliptic.
Equivalent: The ecliptic is the projection of Earth’s orbit
onto the celestial sphere.
The Seasons (I)
Earth’s
equator is
inclined
against the
ecliptic by
23.5º.
The different
incidence
angle of the
sun’s rays is
causing the
seasons on
Earth:
The Seasons (II)
The Seasons (III)
Northern summer =
southern winter
Northern winter =
southern summer
The Seasons (IV)
Earth’s orbit (eccentricity
greatly exaggerated)
Earth in
January
Earth in July
sun
Earth’s distance from the sun has only a very minor
influence on seasonal temperature variations.
The Motion of the Planets (I)
The planets are orbiting the sun almost
exactly in the plane of the ecliptic.
Venus
Mercury
The moon is orbiting Earth in
almost the same plane (ecliptic).
Apparent Motion of the Inner Planets
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.
The Tidally Locked Orbit of the moon
 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.
The Phases of the Moon (I)
As the moon orbits
around 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 (II)
New moon  First Quarter  Full moon
Evening Sky
The Phases of the moon (III)
Full moon  Third Quarter  New moon
Morning Sky
The Orbit of the moon (I)
• The moon orbits Earth in a
27.32 days
sidereal period of 27.32 days.
moon
Earth
Fixed direction in space
The Orbit of the moon (II)
Fixed direction in space
29.53 days
Earth
moon
Earth orbits around sun =>
Direction toward sun changes!
• The moon’s synodic
period (to reach the
same position relative
to the sun) is 29.53
days (~ 1 month).
Lunar Eclipses
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 Earth’s full
shadow (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 sunlight
scattered in
Earth’s
atmosphere.
Typically,
1 or 2
lunar
eclipses
per year.
Solar Eclipses (I)
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 moon’s linear
diameter is much smaller than that of the sun.
Solar Eclipses
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.
Total Solar Eclipse
Chromosphere and Corona
Prominences
Diamond Ring Effect
Earth’s and moon’s orbits are
slightly elliptical:
Apogee = position
furthest away from
Earth
Perihelion = position
closest to the sun
Earth
moon
Perigee = position
closest to Earth
sun
(Eccentricities greatly exaggerated!)
Aphelion
= position
furthest
away from
the sun
Annular Solar Eclipses
Perigee
Apogee
Perihelion
Aphelion
The angular
sizes of the
moon and the
sun vary,
depending on
their distance
from Earth.
When Earth is near
perihelion, and the moon is
near apogee, we see an
annular solar eclipse.
Almost total, annular eclipse of May 30, 1984
Approximately 1 total solar eclipse per year
Very Important Warning:
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
Conditions for Eclipses (I)
The moon’s 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.
Conditions for Eclipses (II)
Eclipses occur in a cyclic pattern.
 Saros cycle: 18 years, 11 days, 8 hours
Astronomical Influences on
Earth’s Climate (I)
Factors affecting Earth’s climate:
• Eccentricity of Earth’s orbit around the sun
(varies over period of ~ 100,000 years)
• Precession (Period of ~ 26,000 years)
• Inclination of Earth’s axis versus orbital plane
Milankovitch Hypothesis: Changes in all
three of these aspects are responsible for
long-term global climate changes (ice ages)
Astronomical Influences on
Earth’s Climate (II)
Last
glaciation
Polar
regions
receiving
less than
average
energy
from the
sun
End of
last
glaciation
Polar
regions
receiving
more than
average
energy
from the
sun