Cycles of the Sky
Chapter 3:
Motions of the Planets
Earth
Venus Mercury
All planets in almost circular (elliptical) orbits around the sun, in approx. the
same plane, the ecliptic plane.
(Distances and times reproduced to scale)
The Moon is orbiting Earth in almost the same
plane.
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)
The 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)
Sun Earth in July
Earth in January
The Earth’s distance from the sun has only a very minor influence on seasonal temperature variations.
Earth’s seasons are not because of its eccentricity.
Earth’s orbit (eccentricity greatly exaggerated)
Mercury appears, at most, ~280 from the sun.
It can occasionally be seen shortly after sunset
in the west or before sunrise in the east.
Venus appears, at most, ~460 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
Summary of Planetary Motion • The planets all orbit the sun in the same
direction in the ecliptic plane. • Earth is titled relative to the ecliptic plane.
– This changes the angle the Sun’s light comes at us over the course of the year, resulting in our seasons.
• We always see Mercury and Venus as being relatively near the Sun.
The Orbit of the Moon
As the moon orbits Earth, the same side of the moon is always pointing toward
Earth (tidally locked).
We always see
the same side of the moon!
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.
Phases of the Moon • Half of the Moon
is illuminated by the Sun and half is dark.
• As the moon orbits 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 sidereal period of
27.32 days.
27.32 days
Earth Moon
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 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 sun
light scattered in the 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
The moon’s shadow sweeps across the Earth, over points from where we
can see a solar eclipse.
Eclipses seen from space
Predicting Eclipses
Approximately 1 total solar eclipse per year
Total Solar Eclipse
During a total solar eclipse, the solar chromosphere, corona, and prominences can be seen.
The Diamond Ring Effect
Almost total, annular eclipse of May 30, 1984
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 Eclipses The 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
Annular Eclipses
Conditions for Eclipses
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.
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.
Summary of Moon’s Motion • One side of the moon always faces Earth,
and we see different phases as it orbits. • If the moon, Earth, and sun line up, we see
an eclipse. – Lunar eclipse when the moon moves into
Earth’s shadow. – Solar eclipse when the Earth is in the moon’s
shadow. • The moon’s orbit is elliptical, so solar
eclipses can be full or annular. • The moon’s orbit is at an angle from the
ecliptic, so there isn’t an eclipse every month.