Planetary Motion. Wanderers The Sun, Moon and planets appear to move relative to the stars....
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Transcript of Planetary Motion. Wanderers The Sun, Moon and planets appear to move relative to the stars....
Planetary Motion
Wanderers
• The Sun, Moon and planets appear to move relative to the stars.
• Planetary motion is confined to the ecliptic.
– Generally west to east
– Not exactly on the ecliptic
Conjunction
• A conjunction occurs when two objects line up as seen from Earth.
• Inferior planets have two solar conjunctions each cycle. – Mercury, Venus
• An opposition occurs when a planet is directly opposite the Sun.
• Superior planets have one conjunction and one opposition.– Mars, Jupiter, Saturn
Retrograde
• Planets sometimes move east to west.– Backwards against the
stars
• This retrograde motion was difficult to explain.
wikipedia
• Geocentric means the Earth is at the center and motionless.
• Described as the Ptolemaic view (after Ptolemy).
• Stars orbit the Earth on a very distant celestial sphere.
• The Sun and other planets orbit the Earth on circles within circles.
Mars
epicycle
deferent
Earth
Geocentric View
Ptolemaic Simulator
Heliocentric View
• Heliocentric means the Sun is at the center and motionless.
• Described as the Copernican view.
• The Earth revolves on its axis once per day.
• All planets move about the Sun on circles and epicycles.
• The Catholic church adopted the Ptolemaic view as the “revealed truth” in the 13th Century. The Copernican model was published in 1543 with detailed comparisons to observations.
• Heliocentric motion has the Earth move faster than the superior planets.– Retrograde motion at opposition.
• This is a simpler solution – Occam’s razor.
Mars
EarthSun
Opposition
Copernican Simulator