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Shadow
Sticks and the Sun
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Background:
Our shadows change over the course of the day and through the seasons.
When is your shadow the shortest? When is your shadow the longest?
Materials:
- Tree or lamp post
that is in the Sun most of the day
- Chalk, stones or
rope
- Pencil or paper
- Clock
- Tape measure or
yard sticks
Activity:
- Select a tree or
lamppost that is in the sun most of the day.
- Mark off the length
of the shadow of the tree several times during the day. Include early
in the morning, noon, and late in the afternoon. You can make these
marks either by drawing a line with the chalk if there is a sidewalk
or parking lot. Or you can lay down small rocks along the length of
the shadow.
- When is the shadow
the shortest? The Sun is highest in the sky at noon and casts the shortest
shadow. Does your shadow stick keep as accurate time as a true sundial?
Is it as accurate as a watch? What happens if you start at the base
of the tree and lay down the rocks? Did the shadow line change too quickly?
Did you end up with a crooked line?
About 2000 years ago,
scientists discovered that if the shadow stick is slanted, it kept more
accurate time than a straight object. The most accurate was if the slanting
post (called a "gnomon") was slanted to the same degree as the
latitude where it is placed. Then the direction was the same at any hour
of the day regardless of the season of the year.
Materials:
- Long stick or post
(gnomon)
- Protractor
- Compass
- Atlas
- Chalk, stones or
rope
- Pencil or paper
- Clock
- Tape measure or
yard sticks
Activity:
- Using the atlas,
look up the latitude of your town. (Minneapolis is about 45 degrees
N. latitude)
- The post or long
stick needs to be inserted into the ground at that angle (45 degrees
for Minneapolis).
- Using the compass,
align the shadow stick so that it points North. The shadow stick or
gnomon is now set up in the same direction as the Earth's axis.
- Just as you did
with the tree or lamppost, mark off the length of the shadow of the
shadow stick several times during the day. Include early in the morning,
noon, and late in the afternoon. You can make these marks either by
drawing a line with the chalk if there is a sidewalk or parking lot.
Or you can lay down small rocks along the length of the shadow. The
students using rocks may want to mark the top of the shadow first and
work their way down to the base of the shadow stick. The shadows can
change quickly and if the students start at the base of the shadow stick
the shadow lines end up being crooked.
Did the shortest
line match when your clock said noon? Probably not! That's because a clock
and a sundial are actually measuring two different things. The clock measures
the passage of time in minutes. The sundial measures the movement of the
earth rotating on its axis. The sundial measures L.A.T. or local apparent
time. The LAT differs from season to season and from place to place. The
actual day length varies from day to day throughout the year. In
addition all sundials do not coincide. That is because the Earth is rotating
to the east on its axis at a rate of one degree every 4 minutes. For that
reason noon in St. Paul is a few minutes before noon in Minneapolis.
A clock measures
L.M.T. or local mean time. LMT measures the average speed at which
the Earth rotates 24 hours. Using clocks has been very important in modern
times. Time differences between railway stations for instance, became
a problem so the standardized method became a necessity.
National
Science Education Standards
Earth
and Space Science
The sun, moon, stars, clouds, birds, and airplanes all have properties,
locations, and movements that can be observed and described. (K-4)
Objects in
the sky have patterns of movement. The sun, for example, appears to move
across the sky in the same way every day, but its path changes slowly
over the seasons.
Most objects
in the solar system are in regular and predictable motion. Those motions
explain phenomena such as the day, the year, phases of the moon, and eclipses.
(5-8)
National
Math Standards
Algebra
Analyze
change in various contexts.
Measurement
Understand
measurable attributes of objects and the units, systems, and processes
of measurement.
Apply
appropriate techniques, tools, and formulas to determine measurements.
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