Does the Sun Really Move Across the Sky? A Shadow Stick Investigation

A stick in the ground is one of the oldest scientific instruments. Ancient sundials work on the same principle. All you need is a stick, some chalk and a sunny day.

Set up a shadow stick

You need: a straight stick or dowel about 30 cm long, set upright in a ball of clay or a bucket of sand, on a flat paved spot or a sheet of paper taped to a board, plus chalk or marker, a ruler and a watch.

Place the stick where the sun will shine on it all day, and mark its base so you can return it to the same place.
Every hour, mark the tip of the shadow with an X and write the time next to it.
Measure each shadow’s length with the ruler.

What you find

The marks form a smooth curve. The shadow is:

Long and pointing west in the morning
Shortest around midday
Long and pointing east in the afternoon

Join the X marks and you’ve drawn the Sun’s path as seen from the ground, traced on the ground.

Graph it

Plot shadow length (cm) against time. The graph dips to a minimum, then rises again. The lowest point marks solar noon, when the Sun is at its highest. It doesn’t always match 12:00 on the clock, because of time zones and daylight saving time.

What it means

The Sun appears to rise in the east, climb and set in the west, because Earth rotates eastward. The Sun’s height changes through the day: low in the morning, high at midday, low again in the evening. A low Sun makes long shadows, a high Sun makes short ones.

Does it change through the year?

Repeat the test in different seasons. In summer, the midday shadow is shorter, because the Sun climbs higher. In winter, it’s longer. Hemispheres: in the Northern Hemisphere away from the tropics, the shadow at noon points north, and in the Southern Hemisphere, south.

Safety

Never look at the Sun. Watch the shadow, not the sky.

The full activity is Does the Sun really move across the sky? from Sun, Earth and Moon. Younger children can try Why is my shadow long in the morning? Sign up for three lessons free.