your shadow longer in winter than in summer? It's easy to see the
answer if you have a "sun" and an orbiting "earth"
to demonstrate. Like many other ancient people, the ancient Chacoans
used the annual changes in shadows to measure the passage of time
and the change of seasons.
Time: 50 minutes
See our Teacher Tips
for standards, discussion ideas, and more.
this activity you'll see how the earth's tilt on its axis changes
the length of shadows. The earth's position relative to the sun also brings
us the seasons. Modeling the Seasons
is another easy activity you can do to see why those changes happen.
• a dark room
• a basketball
• 6–8 small paper clips (size = #1)
• a large empty jar or coffee can (no lid)
• masking tape
• register tape
• 100-watt bulb (clear, unfrosted) with a stand
• a pen or pencil
• a protractor
Linda Shore demonstrates how the earth's tilted axis affects
where sunlight strikes the earth as the planet orbits the sun. (Watch
Place the basketball on top of the open end of the jar (or the coffee
can). The jar will act as a stand for your basketball that is now
a model of the earth. Place the lightbulb (the sun) about 6 feet
away from the “earth.”
6 feet -->
cut a strip of register tape long enough to go from one pole of
the basketball to the other. Using the masking tape, tape the ends
of the register tape at “the poles” and note the position
of the “earth’s” equator.
shows proper clip-bending technique. (Watch
each paper clip so that the loops are at a 90-degree angle.
each of the paper clips under the edge of the register tape so that
one of the loops is pointing straight up. Use a small piece of masking
tape to hold the paper clip in place. Each paper clip represents a
tall object that will cast a shadow at noon. Make sure
that you place paper clips above, below, and at the equator of your
basketball. You might also want to place a paper clip at the approximate
latitude of your hometown and at the north and south poles. In our
example, we've labeled the basketball with the equator and the location
of Chaco Canyon.
locations of Chaco Canyon and the equator are labeled
on the ball. (Watch
the basketball (earth) in its stand and tip the northern hemisphere
toward the lightbulb (sun) so that the North Pole is tipped 23.5°
from the vertical base (see June 21 below). Use a protractor to
measure the angle. This represents the earth’s position on
or about June 21 (the summer solstice). NOTE: For December 21 (winter
solstice), the northern hemisphere should be pointed away from the
sun. On March 21 and September 21 (the equinoxes), the earth’s
northern hemisphere isn't tipped either toward or away from the sun.
|Image courtesy NASA|
each of the latitudes where you placed a paper clip marker, make a
note of the shadow of the marker at noon. Is the paper clip
casting a shadow? If so, is the shadow pointing north or south?
Where on the earth are the shadows longer and where are the shadows
noon at Chaco Canyon, the shadow points due north. (Watch
why the shadows change length over the course of the year. (Watch
fact that length and orientation of noontime shadows are not the
same over the course of the year is due to the tilt of the earth's
axis. For example, at the equator, you may have noticed that on
the spring and autumnal equinoxes, there are no shadows cast at
noon. However, noontime shadows point north for northern latitudes
(the shadow we're pointing to at Chaco) and south for southern latitudes.
The closer you are to the poles, the longer the noontime shadows
Else Can You Do?
Here are some things you might encourage your students to discover
as they investigate sundial shadows all around the world:
• Where does your shadow point during the course of the day?
Except for a few times during the year for latitudes between the
Tropics of Cancer and Capricorn, shadows have a southerly projection
during the day in the southern hemisphere. In the northern hemisphere,
shadows have a northerly projection. Between the Tropics (23.5°N
and 23.5°S latitudes), there are dates where the shadow points
east-west, with no northerly or southerly projection. These are
also the dates when the sun does not cast a shadow at noon (see
• Where should you go on the earth if you want to cast no
shadow at noon? That depends on the time of year. On June 21 (summer
solstice), you want to be at 23.5°N (Tropic of Cancer); on December
21 (winter solstice), you want to be at 23.5°S (Tropic of Capricorn).
On March 21 and September 21 (the vernal and autumnal equinoxes,
respectively), you want to be at the equator. Any other date in
the year, you’ll need to travel somewhere between these latitudes.
The sun is never directly overhead for latitudes greater than 23.5°N
or 23.5°S latitude.
• What kind of shadow do you cast at the north and south poles
on the solstices? On the winter solstice (December 21), the sun
does not rise at the North Pole—so there is no shadow. At
the South Pole, the sun will be up all day on December 21—so
you will cast a shadow all day. This shadow will trace out a 360°
circle. Similarly, at the South Pole during the summer solstice
(June 21), there is no sun so there is no shadow. And on that date
at the North Pole, because the sun is up all day you will cast a
shadow all day and it will also trace out a circle.