parallax diagram

The Seasons

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

Directed light, cardboard with square hole.

Background and Theory
One of the simplest 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. Myself, I wonder how much of 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 your average Harvard grad, we are going to do some work on this problem right here today!

Print out the worksheet.

Part A: The Seasons Here on Earth:

  1. Write down on your worksheet your prediction for the cause of the seasons. Do not worry if you are not sure---that's the point of this exercise! Just write something reasonable down. This is your working model of the 'reason for the seasons'.
  2. Now, answer the following questions about YOUR model:
    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?)
  3. Now for some information about the Earth. You may know any number of the following facts:
    1. The seasons are reversed in the Northern and Southern Hemispheres, so that the Southern Hemisphere experiences summer in February.
    2. The number of daylight hours is greater in the summer than in the winter.
    3. In the summer months, at the poles of the Earth, the daytime lasts for 24 hours. (At the North Pole, the sun rises on March 21, and does not set again until Sept 21---the sun is above the horizon for six straight months!)
    4. The Earth's orbit is only very slightly elliptical, and the Earth is actually closest to the Sun in the northern wintertime.
    Are each of these observations of the Earth consistent with your model? Why or why not?
  4. How would you like to change your model to be more consistent with the actual observations of the Earth?
  5. In one sentence, give the reason for the seasons.
  6. Write down one fact you learned in this Part. What is the difference between a fact and a model (aka theory)?
Part B: The Intensity of Sunlight:
  1. Get a card, with a square cut out of it. 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, parallel to the card. Now move your hand so that the light strikes it at an angle, as shown in the two figures at right.
  2. When you hold your card parallel to your hand, does the square of light cover more or less of your skin than when you hold it at a slant?
  3. In which case does your hand feel warmer?
  4. In January, which hemisphere (North or South) is most perpendicular to the Sun?
  5. Explain how the tilt of the Earth's axis causes the seasons to vary over the course of the year.
Part C: The Seasons on Uranus:
  1. Uranus is a unique planet, for many reasons. But probably the most interesting thing about it is that it has an extreme axial tilt. In fact, the rotation axis of Uranus is tilted by 98o to the orbital plane. That is, Uranus' poles sometimes point nearly directly at the Sun! See the figure below if this is not clear (in this figure, Uranus rotates in and out of the page, around the rotation axis---moving your fingers around might help you figure out what's happening!). Uranus rotates once around its axis in 0.7 Earth days, and revolves once around the Sun in 84 Earth years.

  2. In position A, above, how many daylight hours would a typical Uranian have each day? To what Earth season(s) might this position correspond (Winter, Spring, Summer, Fall)?
  3. In position D, how many daylight hours would a Uranian on the 'left' half of the planet have each day?
  4. In position B, how many daylight hours would a Uranian on the 'left' half of the planet have each day?
  5. Where on Earth would you have to live to experience seasons like those of a typical Uranian?
Part D: The Seasons on Planet _______:
  1. You discover a new extra-solar planet! This planet orbits its star at about 1AU (the distance from the Earth to the Sun), and is about the same mass and size as the Earth. It is a great candidate for looking for life! What do you name your new planet?
  2. Through lots of very careful observations and clever deductions, you discover that ________ is tidally locked to its Sun, much like the Moon is tidally locked to the Earth. Describe the seasons on ___________. Use diagrams if necessary to aid your explanation.
  3. How are ________'s seasons different from the Earth's seasons?
  4. Where would you expect life to flourish on ________? Where would you expect it to fail miserably?
  5. There is a way to make ______ somewhat more Earth-like in its seasons, without altering the fact that it is tidally locked. What property of ______ would make its seasons more Earth-like?