Retrograde motion of a planet when viewed from Earth is caused by
A) the relative motions of Sun and planet.
B) its elliptical orbital path.
C) the relative motions of Earth and planet.
D) the inclination of its orbit to the ecliptic plane.

Question

Quizplus · Accepted Answer

Retrograde motion of a planet when viewed from Earth is caused by the relative motions of Earth and planet. As Earth takes about 365 days to orbit the Sun while some planets take longer or shorter periods, the relative motion of Earth and planets causes the appearance of retrograde motion, which is an optical illusion. When Earth overtakes the slower-moving outer planets, they appear to move backward (retrograde) in the sky, relative to the fixed stars.

Retrograde Motion of a Planet When Viewed from Earth Is