Key Idea: The orientation of earth's axis of rotation with respect to the sun changes as the earth moves around the sun.

Students are expected to know that:

  1. The earth moves in a nearly circular path around the sun over the course of one year, approximately 365 days.
  2. The earth’s axis of rotation is tilted about 23.5° relative to a line that is perpendicular to the plane formed by earth’s circular path around the sun.
  3. As the earth moves around the sun and it rotates on its axis, its axis always points to the same place in the sky.
  4. Since the earth’s axis is tilted relative to the plane of its path around the sun and the angle at which it is tilted does not change as the earth moves around the sun over the course of a year, the orientation of the earth with respect to the sun changes gradually and continually over the year.
  5. Changes in the orientation of the earth with respect to the sun can be described by changes in the angle formed by a line between the north pole and the center of the earth and the line from the center of the sun to the center of the earth. This angle continually changes as the earth moves along its path around the sun. The angle is largest on the same day in late December each year, when the north pole is tilted away from the sun (90° + 23.5° = 113.5°) and is smallest on the same day in late June each year, when the north pole is tilted toward the sun (90° - 23.5° = 66.5°).