Astronomy clicker question: reason for the seasons

Astronomy 10, Spring Semester 2008
Cuesta College, San Luis Obispo, CA

Astronomy 10 learning goal Q1.5

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

[0.3 points.] What causes different seasons in San Luis Obispo, CA?
(A) The Sun making very low, or very high paths across the sky.
(B) There being less than, or more than 12 hours of daylight.
(C) The changing distance from the Earth to the Sun.
(D) The axis of the Earth tilting away from, or towards the Sun.

Correct answer: (D)

The different paths of the Sun across the sky, and the amount of daylight hours, are both ultimately caused by the axis of the Earth tilting away from, or towards the Sun.

Student responses
Section 4160
(A) : 0 students
(B) : 0 students
(C) : 1 student
(D) : 14 students

After results have been compiled and the correct answer given, students are shown a simulation using Starry Night Pro 3.1 (on a CD-ROM packaged with their textbook, Astronomy: Journey to the Cosmic Frontier, 4/e by John D. Fix).

Earth, as seen from the Sun, June 21

Earth, as seen from the Sun, December 21

Note that the Earth is noticeably smaller in June 21 than in December 21, as the Earth is at its nearest distance (perihelion) to the Sun on January 4--thus the changing distance from the Sun to the Earth is not the reason for the seasons! The northern hemisphere of the Earth is tilted towards the Sun during June, and away from the Sun during December, producing both the higher/lower paths of the the Sun across the sky, and the greater/lesser amount of daylight hours. Ergo, the seasons.

Students may ask why the hottest month is August, despite the summer solstice being in late June; while the coldest month is February, despite the winter solstice being in late December. This is the same reason why it takes a while for a warm beer to cool off in the refrigerator, and why it takes time for frozen meat taken out of the freezer to defrost--there is a time response lag between thermal loading and temperature.