Week 24 · Theme 4 Change and Stability, Unit 4 Patterns in the Sky: Get Started, Unit Project Preview, Concept 4.1 Effects of Gravity (A) · 3 lessons
1
The shadow that went missing
Discovery Education Science P5: Term 2, Unit 4, pp. 101–103
GoalExplain how a shadow forms, why shadows change through the day, and what a human sundial will use.
New words
shadow · a dark shape made when an object blocks lightpattern · something that repeats in a way you can predictsundial · a clock that tells the time with a shadow
Explanation
Look up: the Sun seems to cross the sky each day, the Moon changes shape, and stars appear at night. Patterns like these repeat, so we can predict them. A shadow is one pattern we can see every day. Light travels in straight lines. When an object blocks it, a dark shape appears on the side away from the light. If the light comes from the left, the shadow falls on the right. Imagine a photo where a house throws a long shadow to the right but a tall pole beside it has none. Something is wrong: in sunlight every object that blocks light has a shadow, so the picture must have been changed. Because the Sun seems to move, a shadow changes length and direction through the day. This unit will teach you why, and at the end you will use your own shadow in a human sundial. Never look at the Sun to study it.
Worked examples
Time
Shadow of the stick
morning
150 cm
midday
40 cm
afternoon
100 cm
A 1-metre stick stands in the yard. Its shadow is 150 cm long in the morning, 40 cm at midday and 100 cm in the afternoon. The shadow is shortest when the Sun is highest. From morning to midday it got shorter by 150 − 40 = 110 cm.
The three Suns in the picture are the morning, midday and afternoon positions, in order from left to right. The Sun looks lowest at the start and the end of the day and highest in the middle.
upright object→light from the Sun→moving shadow→time of day
Ancient Egyptians used the shadows of tall stone obelisks and of small shadow clocks to tell the time. The idea is the same as a human sundial: an upright thing casts a shadow that moves.
Class activity
Shadow watch (a grown-up helps; never look at the Sun). Stand a pencil upright in a lump of clay on a sheet of card in a sunny spot. Mark the tip of its shadow with a dot, then do the same one hour later. Measure each shadow in centimetres, write the numbers and say which way the shadow moved. Wear a hat. You are looking at the shadow, not at the sky.
Practice
1
How is a shadow made?
An object blocks light, so a dark shape appears on the side away from the light.
2
The light comes from the left. On which side of the teddy will its shadow fall?
On the right, away from the light.
3
A pole’s shadow is 90 cm at 9:00 and 30 cm at midday. How much shorter did it get?
90 − 30 = 60 cm.
4
When is a shadow shortest in the day, and where is the Sun then?
