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The Moon and PlanetsThe Moon and Planets

A Backyard View

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GoalsGoals

• How does the Moon move through the sky?• What causes its phases?• What really is the “dark side” of the moon?• What are eclipses?• What are occultations?• How do planets move through the sky?• What is retrograde motion?• How is the Universe ordered?

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The Moon and Phases• The Moon takes

27.3 days to orbit the Earth.

• The Moon takes 27.3 days to turn on its axis.

• Result: We always see the same face.

• There is no “dark” side of the Moon.

http://antwrp.gsfc.nasa.gov/apod/ap991108.html

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The Moon in the Sky

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Lunar Eclipse• Sometimes the Moon passes through

the Earth’s shadow as cast by the Sun.

22Composite photo of the January 2001 lunar eclipse.

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Solar Eclipse

• Sometimes the Moon passes between the Sun and the Earth (and casts a shadow on the Earth).

August 1999 eclipse. Photo from crew of Mir.

22Composite photo of August 1999 solar eclipse.

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Types of Solar

Eclipses• Partial• Total• Annular

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Occultations

• Eclipse: sometimes the moon passes in front of the Sun.

• Occultation: sometimes the moon passes in front of other things:– Stars– Asteroids– Planets

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The Plane of the Solar System

• The Earth, Sun, Moon and Planets all lie in the same plane.

• This is the plane of the solar system.

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Ecliptic• As viewed from

inside the solar system, the Sun, Moon and planets all move in a band on the sky around the Earth.

• This band is called the ecliptic.

• The constellations that the ecliptic passes through are the zodiac.

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Ecliptic and Occultations

• Result: Moon, Sun and planets aren’t just “anywhere” in the sky.

• Always found somewhere on the ecliptic.

• Distant planets pass behind nearby objects and are occulted.

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The Ecliptic in the Sky

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The Ecliptic

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Retrograde MotionJune, 2000 to May 2001

Credit & Copyright:

Tunc Tezel

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The Wanderers

• The planets (or “wanderers”) move differently from stars– They move with respect to the

stars– They exhibit strange

retrograde motion

• What does all this mean?• How can we explain these

movements?• What does the universe

look like?

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Geocentric System

• The Earth is at the center ofUniverse.

• The Sun, stars, and planets orbit theEarth: explains daily movement.

• To account for unusual planetarymotion epicycles were introduced.

• Fit the Greek model of heavenly perfection – spheres are the perfect shape, circular the perfect motion.

• The accepted model up until ~1630 AD.

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Heliocentric System

• What if?• Sun at center

(heliocentric).• Moon orbits the Earth, the

Earth and planets orbit the Sun.

• Stars don’t move.• The daily motion of the

stars results from the Earth’s spin.

• The annual motion of the stars results from the Earth’s orbit.

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• In the heliocentric model, apparent retrograde motion of the planets is a direct consequence of the Earth’s motion

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Geocentric vs. Heliocentric

• How do we decide between two theories?

• Use the Scientific method:– These are both explanations

based on the observation of retrograde motion

– What predictions do the models make?

– How can these predictions be tested?

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Phases of Venus

• Heliocentric predicts that Venus should show a full phase, geocentric does not

• Unfortunately, the phases of Venus cannot be observed with the naked eye

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Moons of Jupiter

• In a geocentric philosophy,there is only one center ofmotion: the Earth.

• If one thing can be foundnot to orbit the Earth, thenmaybe nothing needs toorbit the Earth.

• Using his new telescope, Galileo:– discovered four moons of Jupiter The Galilean

Moons.– Saw the phases of Venus.

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A Scientific Universe

• The heliocentric model fits the observations best.

• It’s the model of the solar system we have today.

• While our uncritical eyes tell us one thing, the scientific method, and critical thought, reveals another.

• Retrograde motion, phases of Venus, and moons of Jupiter:– We are the center of the Universe.– We as a single part, in no way special.