Astro 201 Exercise 3 - questions and answers

Questions

  1. Draw a diagram that shows how earth's tilt causes the seasons. If several diagrams are thought of, which one is the best (and why)? Or are they equivalent?
  2. At what precise compass direction (azimuth) does the vernal equinox point rise?
  3. Does the sun ever pass directly overhead here in Iowa?
  4. At what latitude on earth does the sun pass directly overhead, but only on (exactly) one day a year?
  5. For an observer at the equator, describe where the sun transits at the equinoxes and solstices.
  6. Jupiter has an axial tilt of 3 degrees (compared to Earth's 23.5). Would you expect Jupiter's seasons to be more or less extreme than earth's? What about Uranus, whose axial tilt is nearly 90 degrees?

Answers

  1. Either of two drawings get the trick done. From an earth-based perspective, use the celestial sphere, with the sun climbing north in the summer:

    Or you can adopt an "outside" viewpoint in which the sun is at the center of the solar system:

    The essential point is the 23.5 tilt between the earth's equatorial plane and the plane in which the earth moves around the sun. With the tilt in place, seasons are inevitable.

  2. Always due east. (Azimuth 90 degrees.)
  3. No. The sun passes directly overhead only if you live between the two "tropics," plus and minus 23.5 degrees from the equator.
  4. Right on a tropic, the sun passes overhead on the day of summer solstice.
  5. At the equator the sun passes overhead on the equinoxes. On summer solstice the sun passes 23.5 degrees north of directly overhead (zenith), on winter solstice passes 23.5 degrees south of zenith.
  6. Based on axial tilt alone, Jupiter should have very mild seasons (since tilt causes the seasons) and Uranus should have extreme seasons because of its extremem tilt. [In reality, internal heat sources dominate for these gas giants, so what the sun is or is not heating has relatively little effect on the planet's cloud-top temperature, at least as far as we can tell.]
Last modified: Wed Sep 2 16:25:50 CDT 1998