Week 26 · Theme 4 Change and Stability, Unit 4 Patterns in the Sky, Concept 4.2 Patterns of Motion in the Sky (A) · 3 lessons
3
Shadows through the day
Discovery Education Science P5: Term 2, Unit 4, pp. 142–145
GoalMeasure a shadow at different times, graph the results and explain the pattern.
New words
data · measurements and facts collected in an investigationpattern · a regular change that repeatsangle · the amount of turn between two lines, measured in degrees
Explanation
In this investigation we stand a straw upright in clay on a sheet of card in a sunny spot, and mark the end of its shadow every hour. The card must stay in the same place and face the same way all day, or the results would not be fair. Looking at the marks, we see a pattern. In the morning the shadow is long and points west. It gets shorter as the Sun climbs, and it is shortest around midday. In Egypt the midday shadow points north, because the Sun is to the south. In the afternoon the shadow grows longer again and points east. The shadow always falls on the side opposite the Sun. Its length depends on how high the Sun is, and its direction depends on which side of the sky the Sun is on. Plotting the lengths on a graph makes the pattern easy to see. Never look at the Sun to check: look at the shadow.
Worked examples
Time
Shadow (cm)
9:00
27
10:00
19
11:00
15
12:00
14
Farida’s results with a 10 cm straw on one winter day: 27 cm at 9:00, 19 cm at 10:00, 15 cm at 11:00 and 14 cm at 12:00. From 9:00 to 12:00 the shadow got shorter by 27 − 14 = 13 cm. The graph is like a valley with its lowest point near midday.
Time of day
Shadow points …
morning
west
midday
north
afternoon
east
The shadow direction also changes. In the morning it points west, at midday north and in the afternoon east. If you mark the shadow tip every hour through one day, the dots make a curve across the card.
Why must the card stay in the same place and turned the same way? If it moves, the marks no longer show the Sun’s motion. Only the time should change, so we keep the straw, the card, the place and the direction the same.
Class activity
Shadow card (a grown-up helps; hats on; never look at the Sun). Use clay to stand a straw upright in the middle of a sheet of card on flat ground. A grown-up shows you which way is north with a compass or a map app. Every hour mark the tip of the shadow with a dot and write the time. Measure the distance from the straw to each dot in centimetres. Draw a line graph of the lengths. Leave the card in the same place and direction all day.
Practice
1
When is the shadow shortest, and why?
Around midday, because the Sun is highest in the sky then.
2
Look at the table. By how many cm did the shadow shorten from 10:00 to 12:00?
Time
Shadow (cm)
10:00
22
11:00
18
12:00
16
22 − 16 = 6 cm.
3
Why must the card stay in the same place all day?
So that only the time changes and the marks show the Sun’s motion.
4
In the afternoon in Egypt, which way does a shadow point, east or west?
