Transcript Planetary Motion

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.
• An opposition occurs
when a planet is directly
opposite the Sun.
• Inferior planets have two
solar conjunctions each
cycle.
• Superior planets have one
conjunction and one
opposition.
– Mercury, Venus
– Mars, Jupiter, Saturn
Retrograde
• Planets sometimes move
east to west.
– Backwards against the
stars
• This retrograde motion
was difficult to explain.
wikipedia
Geocentric View
•
•
•
•
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
Earth
deferent
epicycle
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.
Opposition
• Heliocentric motion has the Earth move faster than the
superior planets.
– Retrograde motion at opposition.
• This is a simpler solution – Occam’s razor.
Mars
Sun
Earth
Copernican
Simulator