Cycles of the Sky Chapter 3:
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 (IV)
sun
Earth in JulyEarth in January
Earth’s distance from the sun has only a very minor influence on seasonal temperature variations.
Earth’s orbit (eccentricity greatly exaggerated)
The Motion of the Planets (I)
The planets are orbiting the sun almost exactly in the plane of the ecliptic.
The moon is orbiting Earth in almost the same plane (ecliptic).
Jupiter
MarsEarth
Venus
Mercury
Saturn
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.
Apparent Motion of the Inner Planets
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 Orbit of the moon (I)• The moon orbits Earth in a
sidereal period of 27.32 days.27.32 days
Earthmoon
Fixed direction in space
The Orbit of the moon (II)
• 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
Lunar EclipsesEarth’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 (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.
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.
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
Annular Solar EclipsesThe 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.
Perigee Apogee Perihelion Aphelion
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)