The solar system, the spinning Earth and the seasons
الأسبوع 13 · الوحدة الرابعة «نظام (الأرض - الشمس - القمر)» (افتتاحية) والدرس الأول «الأرض والنظام الشمسي»: الكواكب ودوران الأرض وميلها والظلال والفصول · 3 دروس
2
Earth’s spin, tilt and shadows
Science Prep 1 Term 1, Unit 4: pp. 79–87
GoalExplain day and night, the apparent motion of the Sun and the changing length of shadows.
New words
axis · the imaginary line through the Earth’s poles that it spins aroundapparent motion · motion that only seems to happensundial · a clock that tells time from the shadow of a stick
Explanation
The Earth turns once in about 24 hours (a little less, 23 hours 56 minutes) around its axis, the line through the poles. The axis is tilted by 23.5°. The turning gives us day and night: the side facing the Sun has day, and the other side has night. The Sun seems to move across the sky from east to west, because the Earth turns from west to east. This is an apparent motion. The Sun is highest in the sky at noon. So a shadow is shortest at noon and longest at sunrise and sunset. A sundial tells the time from the shadow of a stick. Safety: never look at the Sun. We study shadows, not the Sun itself.
Worked examples
the Earth turns west to east→the Sun seems to move east to west
A train passenger sees trees running backwards although the trees stand still. In the same way the Sun seems to move across the sky although the Earth is the one turning.
Time
Sun in the sky
Shadow
sunrise
low
long
noon
highest
shortest
sunset
low
long
A stick is 1 m tall. In the morning its shadow is long, at noon it is short, and in the afternoon it is long again on the other side. The Sun’s height decides the shadow length.
Why do we say “about 24 hours” and not “exactly 24 hours”? The Earth turns once in 23 hours 56 minutes relative to the stars, and a solar day is a little longer. The rough value 24 is enough for daily life.
Class activity
Shadow clock, with an adult. On a sunny day, stand a pencil or a stick upright in a lump of clay on a sheet of paper outside. At three times, for example morning, noon and afternoon, mark the end of the shadow and measure its length with a ruler. Make a table. Which time was shortest? Never look at the Sun.
Practice
1
What causes day and night?
The Earth spinning on its axis, so the side facing the Sun has day.
2
In which direction does the Earth turn, and in which direction does the Sun seem to move?
The Earth turns from west to east; the Sun seems to move from east to west.
