Question

1) define retrograde motion and explain if it is observed, nightly, weekly, monthly, or on a yearly basis and

2) explain how Ptolomy and Copernicus each explained retrograde motion.

Answer 1

1. Retrograde motion:

Retrograde motion is defined as the apparent backward motion of planet caused by its being lapped by another planet, or vice-versa. Both the planets move in direct (eastward) motion around the Sun, but the planet which has the inside (smaller) orbit moves faster than the planet on the outside (larger) orbit, and when it passes the slower-moving planet, each sees the other one as apparently moving backwards relative to its usual motion around the sky. In this Retrograde motion, neither the planet is actually moving backwards, it only appears that way during the time that one laps the other.

Retrograde motion of Jupiter or Mars or Saturn in our sky is an illusion, caused by Earth’s passing these slower-moving outer worlds.

An outer planet's apparent motion is always retrograde for the month or more before and after opposition. The duration of retrograde motion depends on the planet; it's shortest for Mars, and generally longest for Pluto. The moment when a planet's apparent motion changes direction is called a stationary point, because at that instant the planet appears to be more or less stationary with respect to the stars. An outer planet always has one stationary point before opposition, and another stationary point after opposition.

2.

Ptolemy used epicycles to explain the retrograde motion of the planets.

in the Geocentric (Ptolemaic) model ,the idea was:

• the Earth is the center of the solar system (universe)
• all orbits are perfect circles. However, the retrograde (reverse) motion clearly falsifies these first two elements of the model! So, to fix this, we propose that planets don't actually go round the circular orbits! They orbit a point which tracks the circular orbit (think the moon going round an invisible Earth). This is an epicycle, and it explains the observations!

In the heliocentric model given by copernicus, he explained retrograde motion of planets. According to him, the retrograde motion occur naturally if planets further from the sun move more slowly.