الأسبوع 26 · المحور الرابع «التغير والثبات»، الوحدة الرابعة «الأنماط في السماء»، المفهوم 4.2 «أنماط الحركة في السماء» (أ): الليل والنهار والظلال · 3 دروس
1
A spinning, travelling Earth
Discovery Education Science P5: Term 2, Unit 4, pp. 130–137
GoalTell rotation from revolution and explain how Earth’s spin makes day and night.
New words
rotation · the spinning of an object around its own axisrevolution · the travel of one object in a path around anotheraxis · an imaginary line through the centre of an object, around which it spins
Explanation
We cannot feel it, but Earth is always moving in two ways. First, it spins like a top around its axis, an imaginary line through the North and South Poles. One full spin, called a rotation, takes about 24 hours, which is one day. Second, Earth travels around the Sun in a path called an orbit. One full trip, called a revolution, takes about a year. The Sun lights only half of the Earth at a time. The half facing the Sun has day, and the half facing away has night. As Earth rotates, a place on the surface turns into the sunlight and then out of it. That is why we have day and night in a regular cycle. The Sun, Moon and stars seem to rise in the east and set in the west, because Earth spins toward the east.
Worked examples
Hours
Earth turns
1
15°
6
90°
12
180°
24
360°
Earth turns a full circle, 360°, in 24 hours. In one hour it turns 360 ÷ 24 = 15°. In 6 hours it turns 6 × 15 = 90°, a quarter turn. After 12 hours a place that faced the Sun faces away from it.
Example
Type of motion
child spinning on the spot
rotation
Moon going round Earth
revolution
top spinning in place
rotation
Rotation or revolution? A child spins on the spot: that is rotation. The Moon goes round Earth: that is revolution. A marble rolls around the rim of a cup: revolution. A top spins in place: rotation.
Earth spins toward the east→the Sun seems to rise in the east→and set in the west
Why does it feel as if we are not moving? Everything on Earth moves with it, like sitting in a smooth car or a plane: you can hardly feel it moving. The Sun looks as if it travels across the sky, but really we are turning.
Class activity
Spin and travel (a grown-up helps). Stand in a room with a lamp or torch on a table (do not look into it). Turn slowly on the spot, as Earth rotates. Notice when you face the light (day) and when your back is to it (night). Then walk once around the lamp while still facing the same wall: that is revolution. Say which is rotation and which is revolution.
Practice
1
How long does one rotation of Earth take, and what does it cause?
About 24 hours. It causes day and night.
2
How many degrees does Earth turn in 3 hours?
3 × 15 = 45°.
3
Is each motion a rotation or a revolution? A: a wheel turning on its axle; B: a planet going round the Sun.
A is rotation; B is revolution.
4
Why do we have night in Egypt when the Earth rotates away from the Sun?
Egypt is then on the half of Earth that faces away from the Sun, so no sunlight reaches it.