One day you went hiking with your family and
came across the rock in the picture. What
scientific process probably caused the rock to
break down the middle?
a.
b.
c.
d.
Weathering
Erosion
Condensation
Deposition
What scientific process is responsible for
moving rocks and sediment from one place to
another?
a.
b.
c.
d.
Weathering
Deposition
Erosion
Soil
One of the best uses of a soil that contains
nutrients from decaying plants and animals, or
humus, is:
a.
b.
c.
d.
Filling in holes on a concrete road
Preserving the remains of dinosaurs
Growing plants
Making pots out of clay

Light travels in
straight lines from
the Sun.

When an object
gets in the way, a
shadow is formed.

As the earth turns on its axis, the sun appears to move across the sky. The
shadows the sun casts move in a clockwise direction for objects in the
Northern Hemisphere.

If the sun rose and set at the same time and spot on the horizon each day,
shadow sticks would have been accurate clocks. However, the earth is always
spinning like a top. It spins around an imaginary line called its axis. The axis
runs through the center of the earth from the North Pole to the South Pole.
The earth's axis is always tilted at the same angle.

Every 24 hours the earth makes one
complete turn, or rotation. The earth rotates
on its axis from west to east. The earth's
rotation causes day and night. As the earth
rotates, the night side will move into the
sunlight, and the day side will move into the
dark.

On the earth's yearly trip around the sun the North Pole is tilted toward the
sun for six months and away from the sun for six months. This means the
shadows cast by the sun change from day to day.

Because the earth is round, or curved, the ground at the base of a shadow
stick will not be at the same angle to the sun's rays as at the equator. Because
of this the shadow of the shadow stick will not move at a uniform rate during
the day.

Eventually man discovered that angling a gnomon and aiming it north made a
more accurate sundial. Because its angle makes up for the tilt of the Earth, the
hour marks remained the same all year long. This type of gnomon is called a
style. After this discovery, people were able to construct sundials that were
much better at keeping accurate time.

A sundial is a device that determines the time of day by the
position of the Sun.

As the sun moves across the sky, the shadow-edge aligns
with different hour-lines.

All sundials must be aligned with the axis of the Earth's
rotation to tell the correct time. In most designs, the style
must point towards true celestial north
http://www.youtube.com/watch?v=tI0GqYJha1Q
http://www.youtube.com/watch?v=jGwArN-cQxk

Is there a pattern to following the track of the sun?

How is a shadow used to tell time?

Today, we are going to track the sun’s shadow and how it
changes during the course of the day.

http://www.schoolsobservatory.org.uk/astro/esm/shadows

Remember, it is very dangerous to look at the Sun. So
never look directly at the Sun!






Plate
Straw
Pencil
Ruler
Scissors
Tape

Distribute materials

Students need to find the center of the paper plate and
mark it with a dot.

With a pencil, have students
pace 4 marks along the edge
of the paper plate. (These
marks will help students
reposition their sundial for
taking measurements
throughout the day.)

Make 4 half inch cuts on one end of the straw section.

Flare out the cut portion of the straw and tape it onto the
center of the paper plate. The straw should be
perpendicular to the surface of the plate.

Measure and cut the straw to a
2 inch length.

Mark the playground with chalk at the 4 registration points on the edge of
their sundials. You may want to mark them with North, South, East, and
West.

Have students carefully trace the straw's shadow with a pencil. Ask them
to darken in the shadow with the pencil and write the time at the tip of the
shadow.

Ask students to note where the Sun is in the sky. Warn them NOT to look
directly at it.

After tracing ask students measure the shadow length and fill in their
data sheets.

Students will take more measurements later in the morning, at midday,
and in the early afternoon.

At the end of the day, ask students to predict where they think shadows
will fall in the late afternoon. Ask them to look at their sundials and guess
where the shadow would be at 2 PM (or any other afternoon time). Using
a red pencil, have them outline the predicted shadow on their sundials.

What did you observe?

What did the shadows do?

When was the shadow the longest?

Where was the Sun?

When was the shadow the shortest?

Where was the Sun?

Why do you think the shadows change length?

How can you explain what you are observing?

How could you use a shadow to tell the time of day?

How accurate were your predictions?

What could you do to make them more accurate?