Seasons
parallax diagram

The Seasons

Purpose
The student will explore various explanations for the causes of the seasons.

Materials
Directed light, cardboard with square hole, planetarium.

Background and Theory
One of the most fundamental and yet most confusing astronomical issues is the 'reason for the seasons'. Years ago, a group of film-makers went to a graduation ceremony at Harvard University, and asked students AND FACULTY! what caused the seasons. Clearly, this would not be interesting if they all got it right. In fact, nearly everyone got it wrong. This has led to consternation and dismay in the scientific community. Perhaps that was just due to the fact that people hadn't thought about it for a while, and, most likely, had been partying a little too hard to have a good thought about it on the spur of the moment. In any event, just so that you can all be better educated than the average Harvard grad, we are going to answer this question (correctly!) today.

Procedure
Part A: Observing the Seasons:
Let's start by gathering some information about the seasons using the planetarium.

  1. We begin by looking at the sky at the same time of day (6 pm), at different times of year. We will set the planetarium for 6 pm on March 21. Where is the Sun in your sky?
  2. Now we will move the date to June 21. Where is the Sun in your sky? Compare your answer to the answer for spring. Are there more hours of daylight in the summer, or in the spring?
  3. Now your professor will move the date to December 21. Where is the Sun at 6 pm in your sky?
  4. Are there more daylight hours in a given day in the summer or the winter?
  5. Now, let's change our location. Remaining on December 21st, we will travel to Chile, in the southern hemisphere. Where is the Sun at 6 pm on December 21st in Chile? Is December a summer or a winter month in Chile?
  6. Do the seasons occur at the same time of year in the Northern and Southern Hemispheres?
  7. Now we will go you someplace REALLY exotic---the South Pole. We run the time forward, until one entire day passes. What do you notice about the Sun?
  8. Let's check what's happening at the North Pole. Again, we'll run the time forward for an entire day. What do you notice about the Sun?
  9. There's one last thing to notice about the seasons, and we can go back home to take a look. We set up the planetarium so the Sun is on the meridian, on June 21st, and turn on the meridian line and scale. What is the altitude of the sun on this day?
  10. Now we put the sun on the meridian on December 21st. What is the altitude of the Sun?
  11. The diagram on the worksheet shows the Earth, with our approximate latitude labeled. Draw and label the position of the Sun on June 21, and on December 21. In which case is the sun more nearly 'above' our latitude?
  12. Draw a line from the Sun to our location on June 21 and on December 21. In which case is the line most nearly perpendicular to the surface of the Earth at our home location?
Part B: One Possible Explanation:
Often, when you ask people at random, they'll say that the reason for the seasons is that the Earth is farther from the Sun in the winter, and closer to the Sun in the summer. For this part, use this idea as your working theory of the 'reason for the seasons'.
  1. Answer the following questions about this working theory:
    1. Are the seasons experienced at the same time in the Northern and Southern Hemispheres?
    2. Does the length of the day change with season?
    3. Are there any special places that will have a unique experience at any time of year (i.e. the sun does not rise in a given 24 hour period?)
  2. Is your working theory consistent with each of the observed facts from Part A? For each one, explain why or why not.
  3. Is your working theory correct or incorrect?
  4. Propose a theory to explain the seasons which is consistent with all the known facts.
  5. The idea that the Earth revolves around the Sun is a theory. The idea that gravity holds the Moon in orbit around the Earth is a theory. The idea that the tilt of the Earth's axis causes the seasons is a theory. 'Theory' means that they are among the most well-tested of scientific ideas. Over and over again, we've tried to prove them wrong; we've failed. Facts and theories are not the same. You've 'learned' several facts in this lab, and tested a theory against them. In your own words, explain the difference between a fact and a theory.
Part C: The Intensity of Sunlight:
Get a card with a square cut out of it from your professor. Stand by the overhead projector. Hold the card up so that the light from the projector passes straight through it. Hold up your hand in the square of light, perpendicular to the light coming through the card. Now, switch hands and move your illuminated hand so that the light strikes it at an angle, as shown in the two figures at right.
  1. Which hand had a smaller area illuminated---the perpendicular one or the angled one?
  2. Which hand felt warmer where it was illuminated?
  3. In January, is the Northern or Southern Hemisphere most perpendicular to the light coming from the Sun?
  4. Explain how the tilt of the Earth's axis causes the seasons to vary over the course of the year.
Part D: The Seasons on Uranus:
Possibly the most interesting thing about Uranus is that it has an extreme axial tilt. In fact, the rotation axis of Uranus is tilted by 97.9o to the orbital plane. That is, Uranus' poles sometimes point nearly directly at the Sun! See the figure at right if this is not clear. In this figure, Uranus rotates in and out of the page, around the black line, which represents the rotation axis. Uranus rotates once around its axis in 0.7 Earth days, and revolves once around the Sun in 84 Earth years.
  1. In position A, at right, how many daylight hours would a typical Uranian have each day?
  2. In position D, how many daylight hours would a Uranian on the 'left' half of the planet have each day?
  3. In position B, how many daylight hours would a Uranian on the 'left' half of the planet have each day?
  4. Where on Earth would you have to live to experience seasons like those of a typical Uranian?