What it shows
A planisphere is a star map on a dial with a window shaped for one latitude. Turning the dial to the date and time shows exactly which stars are above the horizon, which demonstrates how the sky shifts through the night and through the year.
The science
The Earth turns once a day and travels once round the Sun in a year, so a star rises about 4 minutes earlier each night. The star chart is drawn with the celestial pole at its centre, and a horizon mask with an oval window shows the part of the sky above the horizon for one latitude. Rotating the chart under the mask by date and hour copies both motions at once, and the edge of the window is the horizon.
Materials
Cost: under Rs 50. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- A printed all-sky star chart with dates round the edge
- Stiff card
- A sheet of transparent plastic for the horizon window
- A paper fastener or split pin
- A compass and a protractor
- Glue and scissors
Safety
This project uses safe, everyday materials. Still, read the rules below with a parent before you start.
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
9 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.
Print or draw a star chart centred on the north celestial pole with months marked round the rim.
Glue the chart to card and cut it out as a circle.
On a second card draw a circle of the same size with the hours of the night marked round the rim.
Draw the horizon window: an oval whose centre lies a distance from the pole equal to 90 degrees minus your latitude, scaled to the chart.
Cut out the window and glue the plastic sheet behind it so the chart is protected.
Fix the two cards together through their centres with the paper fastener.
Turn the chart until today's date lines up with the present hour on the mask.
Go outside, face south with the compass, hold the planisphere overhead and match the stars in the window to the sky.
Check it a month later at the same hour and note how the sky has moved.
What you should see
A dial that correctly shows Orion high in the south on January evenings and Scorpius in the south in June, matching the real sky.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1Why must the window match your latitude?
The pole star's height above the horizon equals the latitude, so the visible part of the sky depends on it.
2Why does a star rise 4 minutes earlier each night?
The Earth moves about 1 degree along its orbit each day.
3Which stars never set from Delhi?
Those within about 28 degrees of the north celestial pole, such as the Pole Star.
4Could the same planisphere be used in Chennai?
Not accurately; Chennai is at 13 degrees north, so a different window is needed.
Take it further
Make a second mask for a southern Indian city and compare which stars each can see.
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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