Astronomy in-class activity: planetary motion models, first principles versus scientific models

Astronomy 210 In-class activity 7 v.15.08.16, fall semester 2015
Cuesta College, San Luis Obispo, CA

Students find their assigned groups of three to four students, and work cooperatively on an in-class activity worksheet to analyze how the Ptolemaic and Copernican models reproduce prograde and retrograde motion of planets, and to distinguish between first principles and scientific models.

Astronomy quiz question: heliocentric model of retrograde motion

Astronomy 210 Quiz 2, Fall Semester 2008
Cuesta College, San Luis Obispo, CA

[4.0 points.] How does Copernicus' model explain the retrograde motion of a planet?
(A) The only perfect shape is a circle, and the only perfect motion is uniform motion.
(B) A planet slows down in its orbit when it is further from the sun, and speeds up in its orbit when it is closer to the sun.
(C) A planet moves around a circle that itself moves along an orbit around the Earth.
(D) A planet has a different orbital distance and speed around the sun from the Earth's orbital distance and speed around the sun.

Correct answer: (D)

Copernicus' model of planetary motion explains retrograde motion as a "lapping illusion" when planets in different orbits traveling at different speeds move past each other. Response (A) is Ptolemy's model of retrograde motion; response (B) is Kepler's second law; response (C) is based on Aristotle's first principles, but does not explain retrograde motion.

Student responses
Section 70158
(A) : 9 students
(B) : 11 students
(C) : 14 students
(D) : 18 students

"Difficulty level": 39%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.35

Astronomy quiz question: geocentric model of retrograde motion

Astronomy 210 Quiz 2, Fall Semester 2008
Cuesta College, San Luis Obispo, CA

[4.0 points.] How does Ptolemy's model explain the retrograde motion of a planet?
(A) The only perfect shape is a circle, and the only perfect motion is uniform motion.
(B) A planet has a different orbital distance and speed around the sun from the Earth's orbital distance and speed around the sun.
(C) A planet moves around a circle that itself moves along an orbit around the Earth.
(D) A planet slows down in its orbit when it is further from the sun, and speeds up in its orbit when it is closer to the sun.

Correct answer: (C)

Ptolemy's model of planetary motion used epicycles moving along deferents to explain the retrograde loops of planets. Response (A) is based on his (and Aristotle's) first principles, but in and of itself is not a model of retrograde motion; response (B) is Copernicus' heliocentric explanation of retrograde motion; and response (D) is Kepler's second law.

Student responses
Section 70160
(A) : 1 student
(B) : 2 students
(C) : 22 students
(D) : 8 students

"Difficulty level": 69%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.89

Astronomy clicker question: retrograde/prograde rise/set

Astronomy 10, Summer Session 2007
Cuesta College, San Luis Obispo, CA

Astronomy 10 learning goal Q3.2

Students were asked the following clicker question (Classroom Performance System, einstruction.com) near the end of their learning cycle:

[0.3 points.] As seen from San Luis Obispo, CA, when a planet is in retrograde motion, how will it move in a single night?
(A) It will rise from the east horizon, and set in the west horizon.
(B) It will rise from the east horizon, and shortly afterwards set in the east horizon.
(C) It will rise from the west horizon, and set in the east horizon.
(D) It will rise from the west horizon, and shortly afterwards set in the west horizon.

Correct answer: (A).

Most students are probably thinking that retrograde motion (planet moves a very small distance east-to-west relative to the background stars) is the reason why it will rise in the east horizon, and set in the west horizon. The next clicker question will test for this misconception.

Student responses
Section 8027
(A) : 11 students
(B) : 1 student
(C) : 2 students
(D) : 0 students

[0.3 points.] As seen from San Luis Obispo, CA, when a planet is in prograde motion, how will it move in a single night?
(A) It will rise from the east horizon, and set in the west horizon.
(B) It will rise from the east horizon, and shortly afterwards set in the east horizon.
(C) It will rise from the west horizon, and set in the east horizon.
(D) It will rise from the west horizon, and shortly afterwards set in the west horizon.

Correct answer: (A).

This follow-up question causes a lot of consternation for students, who must realize that in prograde motion, a planet moves a very small distance west-to-east relative to the background stars, which themselves over a single night rise from the east horizon, and set in the west horizon. Thus a planet will be seen to rise from the east horizon, and set in the west horizon in a single night, regardless if it is undergoing retrograde or prograde motion!

Student responses
Section 8027
(A) : 3 students
(B) : 1 student
(C) : 10 students
(D) : 0 students