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.

3) explain how the orbit of a planet with an eccentricity value close to 1 compares to a planet with an eccnentricity close to the value of 0.

4) explain how distance influences the orbital speed of a planet in orbit and

5) explain the role of distance and mass and the influence it has on the orbital period of a planet.

Answer 1

a) Most of the rotational and orbital motions in the solar system are in the same "eastward" direction. Motions in this direction are referred to as direct motions, while motions in the opposite direction are referred to as retrograde.

b) let's assume two planets move in a direct (eastward) motion around the Sun, but the planet with 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 planet is actually moving backwards; it only appears that way during the time that one laps the other.

c) If the eccentricity of an ellipse is close to one (like 0.8 or 0.9), the ellipse is long and stretched. If the eccentricity is close to zero, the ellipse is more like a circle. It means a planet in an orbit with eccentrcity as zero will be at the same distance from the central body at all times and sue to this, the speed will be same at all points.

However, if eccentricity is close to 1, the orbit will have two focus and major, minor axis. The speed of the planet will not be constant. The speed will increase as it approaches the central body and will decrease as it goes away.

d) As explained above in part (c) , larger the distance between central body and planet, lesser will be the speed. Shorter the distance, higher is the speed. A planet's orbital speed changes, depending on how far it is from the Sun. The closer a planet is to the Sun, the stronger the Sun's gravitational pull on it, and the faster the planet moves. The farther it is from the Sun, the weaker the Sun's gravitational pull, and the slower it moves in its orbit.

e) As per kepler's law,

T² = r³

aldo,

T = sqrt (2*pi²*r³ / GM)

Therefore,  increase in the mass of the orbited (central) body causes a decrease in the orbital period. Increase in the orbital radius will increase the orbital period