1 Motion of Earth and Moon around the Sun Physics 113 Goderya Chapter(s): 2 and 3 Learning Outcome: The Sun and Its Motions 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. Due to Earth’s revolution around the sun, the sun appears to move through the zodiacal constellations. The Rotation of Earth Earth’s rotation is causing the day/night cycle.
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Motion of Earth and Moon
around the Sun
Physics 113 Goderya
Chapter(s): 2 and 3
Learning Outcome:
The Sun and Its Motions
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.
Due to Earth’s revolution around the sun, the sun
appears to move through the zodiacal
constellations.
The Rotation of Earth
Earth’s rotation is causing the day/night cycle.
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Precession (1)
The Sun’s gravity is doing the same to Earth.
The resulting “wobbling” of Earth’s axis of rotation around the
vertical w.r.t. the Ecliptic takes about 26,000 years and is
called precession.
At left, gravity is pulling on a slanted top. => Wobbling around the vertical.
Precession (2)
As a result of precession, the celestial north pole follows a circular pattern on the sky,
once every 26,000 years.
It will be closest to Polaris ~ A.D. 2100.
There is nothing
peculiar about Polaris
at all (neither
particularly bright nor
nearby etc.)
~ 12,000 years from
now, it will be close to
Vega in the
constellation Lyra.
The Cause for Seasons
Earth’s axis of rotation is inclined vs. the normal to its
orbital plane by 23.5°, which causes the seasons.
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Seasons
(SLIDESHOW MODE ONLY)
Temperature & Sun Rays
Sun Rays & Incidence Angle
They are not related to Earth’s distance from the sun. In fact, Earth is slightly closer to the sun in (northern-
hemisphere) winter than in summer.
Light from the sun
Steep incidence
→ Summer
Shallow incidence
→ Winter
The Seasons are only caused by a varying angle of
incidence of the sun’s rays.
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Daylight Hours
Northern summer = southern winter
Northern winter = southern summer
The Phases of the Moon (1)• The Moon orbits Earth in a sidereal period of
27.32 days.
27.32 days
EarthMoon
Fixed direction in space
The Phases of the Moon (2)
• 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
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The Phases of the Moon (3)
From Earth, we
see different
portions of the Moon’s surface
lit by the sun,
causing the phases of the
Moon.
Lunar EclipsesEarth’s shadow
consists of a
zone of partial shadow, the
Penumbra, and a zone of
full shadow, the Umbra.
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
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Why the Red Glow? 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 Earth’s
atmosphere.
Lunar Eclipses: 2002-2012
Typically, 1 or 2 lunar
eclipses per year.
Solar Eclipses
The sun appears approx. as large in the sky (same angular diameter ~ 0.50) as the moon.
→ When the moon passes in front of the sun, the moon can cover the sun completely, causing a total solar eclipse.
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Solar Eclipses: 2002-2012
Approximately 1 total solar eclipse per year
Total Solar Eclipse
Prominences
Chromosphere and Corona
Diamond Ring Effect
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Conditions for Eclipses
A solar eclipse can only occur if the moon passes a
node near new moon.
The moon’s orbit is inclined against the ecliptic by ~ 50.
A lunar eclipse can only occur if the moon passes a