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