Week 27 · Theme 4 Change and Stability, Unit 4 Patterns in the Sky, Concept 4.2 Patterns of Motion in the Sky (B) · 3 lessons
1
Stars that seem to travel
Discovery Education Science P5: Term 2, Unit 4, pp. 146–149
GoalExplain why stars seem to move, why we see different constellations in different seasons, and why the Pole Star hardly moves.
New words
constellation · a group of stars that people see as a shape in the skyPole Star · a star that lies almost above Earth’s North Pole, so it hardly moves in the skystar · a huge ball of hot gas that makes its own light
Explanation
On a dark night away from city lights, you can see thousands of stars. People long ago imagined shapes in groups of stars and called them constellations. The stars in a constellation are not really near each other: they only look close together from Earth. If you watch the sky for hours, the stars seem to travel from east to west, just like the Sun. But the stars are not moving across the sky; it is Earth that is rotating. Different constellations are visible in different seasons. As Earth goes round the Sun, the side of the sky that we face at night slowly changes, so Orion is seen on winter evenings but not in summer. The other constellations are still there, hidden by daylight. One star is special: the Pole Star lies almost straight above Earth’s North Pole, so it looks nearly still while the stars near it make small circles around it. In the old days travellers used it to find north.
Season
Evening constellation you may see in Egypt
winter
Orion
summer
Scorpius, the scorpion
all year
stars near the Pole Star
Worked examples
Earth turns toward the east→the sky seems to turn the other way→stars rise in the east, set in the west
Mina watches the sky from a desert camp near Siwa for three hours. A bright star starts low in the east and ends higher up, toward the south-east. It looks as if it moved, but Earth turned about 3 × 15 = 45° in that time.
Season
Is Orion in the night sky?
winter
yes, in the evening
summer
no, in the daytime sky
Why can we not see Orion in summer? In summer, at night, the Earth is on the side of the Sun where we face a different part of space. Orion is in the sky only in the daytime, so the Sun’s light hides it. It is still there.
To find north, a traveller can look for the Pole Star. It stays in the same place in the north while the other stars turn slowly around it. Today we can use a compass or a phone, but the old way still works on a clear night.
Class activity
Night sky notes (a grown-up goes with you; stay in a safe place). On a clear evening go outside with a grown-up and look at the sky for five minutes. Draw three bright stars and their position against a house or a tree. Come back after an hour and draw again. Did they move? In which direction? Say why. Star-gazing is just for watching: it is not about star signs or telling your future.
Practice
1
What is a constellation?
A group of stars that people see as a shape in the sky.
2
Why do stars seem to move from east to west during the night?
Because Earth rotates toward the east, so the sky seems to turn the other way.
3
Which star hardly moves, A or B? Say the letter and why.
A, the Pole Star, because it lies almost above Earth’s North Pole.
4
In 4 hours, how many degrees does Earth turn? How far do the stars seem to turn?
4 × 15 = 60°, and the stars seem to turn 60° the other way.
