This demonstration will show how the orbit of a planet around the Sun
and the tilt of a planet's rotation axis lead to the seasons.
Time requirement: 30 minutes as a demonstration.
- Large globe with tilted rotation axis
- Turn off the lights in the classroom.
- Have one person hold a globe with the rotation axis tilted like the
rotation axis of the Earth. The amount of this tilt is 23.5 degrees.
- Have a second person illuminate the globe with a flashlight.
- Making sure not to change the orientation of the globe with respect
to the rest of the classroom, have the first person carry the globe slowly
in a circle around the flashlight. As the globe is carried around the circle,
the person with the flashlight should continue to illuminate the globe
directly. If viewed from above, the globe should be carried
counter-clockwise around the circle because that is the direction which
all the planets orbit the Sun.
- Observe how the tilt of the axis changes with respect to the
flashlight over the course of an "orbit." At one point during the orbit,
the North Pole should be tilted toward the "Sun" and at one point during
the orbit the South Pole should be tilted toward the "Sun." Observe that
the Sun never shines directly on the Poles, however.
- Have the first person make one more orbit around the Sun, but this
time have that person spin the globe around its rotation axis while orbiting
the Sun. Make sure not to change the orientation of the rotation axis
while the globe is spinning. Also, remember that Earth makes many (365)
rotations for every revolution around the Sun. Observe that the seasons
do not change because the globe is spinning, but that the length of the
day (amount of daylight) in each hemisphere does change over the course
of an orbit around the Sun.
- Why is it important not to change the orientation of the globe
while it makes its orbit around the "Sun?" How long does it take the
Earth to make its orbit around the Sun?
- Which point in the orbit corresponds to the (Northern) summer
solstice? On which day would this event take place on our calendar?
Why is this the longest day (of daylight) of the year in the Northern
hemisphere? Why does the Sun shine 24 hours a day at the high Northern
latitudes during the Northern summer?
- Which point in the orbit corresponds to the (Northern) winter
solstice? On which day would this occur on our calendar? Why is this
the shortest day (of daylight) of the year in the Northern hemisphere?
- Why is it winter in one hemisphere while it is summer in the other?
- Why is the weather hottest is summer time and coldest in winter time?
- Which points in the orbit correspond to the vernal and autumnal
equinoxes (think about this carefully so that you can tell one from
the other)? On which days do these events occur on our calendar?
How much daylight is there on these days at all latitudes?
Last updated: September 04, 1997
Joe Twicken /