What it shows
Stars appear to turn around the Pole Star because Earth spins on its axis once a day. Sketches made hours apart reveal arcs, and the amount of turning matches about 15 degrees an hour.
The science
Earth turns 360 degrees in 24 hours, so every star seems to move 15 degrees each hour. Stars near the Pole Star trace small circles, and those far from it trace bigger arcs. Because the Pole Star lies almost exactly above Earth’s axis, it hardly moves.
Materials
Cost: free. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- A fixed window frame or a camera stand made from a bamboo stick
- Black chart paper and a white gel pen
- A protractor and a ruler
- A torch with red cellophane over it
- A watch
- A phone compass to find north
Safety
For this project: An adult must stay with you for night observing; do not climb on parapets.
Never use mains electricity: power models with cells, batteries or a low-voltage adapter. Sharp and hot tools, flames and lab chemicals need an adult or your teacher beside you. Keep magnets, button cells and small parts away from young children, and wash your hands after making.
All the safety rulesStep by step
8 steps. Do them in order, and test each part before you go on to the next. Write what you notice in a notebook as you go.
Pick a window or spot facing north and fix your head position with a stick or a mark on the floor.
Find the Pole Star using the two end stars of the Saptarishi bowl as pointers.
On black paper mark a dot for the Pole Star and sketch five bright stars near it with their names or numbers.
Write the time beside the sketch, then leave without moving the stand or the window.
Return after one hour and draw the same stars again in a different colour.
Repeat after a second hour, using the same viewing spot.
Join each star’s dots with a curved arc with the Pole Star as the centre.
Measure the angle between the first and last positions with a protractor and divide by the hours.
What you should see
Arcs of stars around a nearly still Pole Star, with an angle of roughly 15 degrees per hour. This shows that the sky turns because Earth turns.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1Why does the Pole Star hardly move?
It lies almost in line with Earth’s axis of rotation.
2How many degrees does the sky turn in an hour?
About 15 degrees.
3Why use a red torch?
Red light keeps your eyes adapted to the dark.
4Do the stars really move in circles?
No, it is Earth’s spin that makes them appear to.
Take it further
Fix a phone on a stand and take a ten-minute time-lapse of the northern sky to compare with your sketch.
Change one thing, measure what happens, and add a table or chart of your readings: that is what turns a model into your own investigation.
Free ebooks for this project
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