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Science project idea

Night-sky clock that tells time from the Saptarishi stars

The Saptarishi (Great Bear) pattern turns around the Pole Star once in about 24 hours, like a giant clock hand. A simple two-disc instrument called a nocturnal can use it to tell the time at night.

Make it yourself8 numbered steps, the materials and the science, all free.Go to the stepsGet a ready-made kitReady-made models, kits and spare parts in the store under Science.Browse ScienceAsk on WhatsAppAsk the head office whether a model or kit for this idea is available.Send a message

What it shows

The Saptarishi (Great Bear) pattern turns around the Pole Star once in about 24 hours, like a giant clock hand. A simple two-disc instrument called a nocturnal can use it to tell the time at night.

The science

The sky turns 15 degrees every hour, 360 degrees in a day. The Great Bear’s pointer stars rotate around the Pole Star like the hand of a 24-hour clock. Because the sky also shifts a little each day relative to the clock, because Earth moves around the Sun, the date disc is set to correct the reading by two hours for each month.

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.

  • Two cardboard or thick card discs, 15 cm and 10 cm wide
  • A long thin strip of card for the pointer arm
  • A paper fastener or screw
  • A protractor and a pencil
  • A phone compass and a clock for checking
  • A torch with red cellophane

Safety

For this project: An adult must go with you at night, and keep your hands away from the sharp end of the fastener.

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 rules

Step 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.

  1. Draw a scale of 24 hours on the large disc and a scale of 12 months on the small disc.

  2. Make a hole at the centre of both discs so the Pole Star can be seen through it.

  3. Fix both discs together with the fastener so the small disc turns on the big one.

  4. Attach the long pointer arm to the centre so it also turns.

  5. Turn the date disc to the date, and match it with the large disc’s "midnight" mark.

  6. At night, sight the Pole Star through the centre hole and swing the arm to line it with the Great Bear pointer stars.

  7. Read the time on the large disc where the arm points and compare it with your clock.

  8. Record the error on three nights and note how it changes with the date setting.

What you should see

A card instrument that gives the time at night to within about 30 minutes. The best test is a night when you can see both the Pole Star and the Great Bear clearly.

Viva questions

A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.

1Why can the sky act like a clock?

Earth’s spin makes the stars turn around the Pole Star at a steady rate.

2Why is a date setting needed?

Earth’s orbit shifts the sky by about 2 hours each month.

3How do we find the Pole Star?

Follow the two pointer stars of the Great Bear bowl.

4How accurate is this clock?

Only to about half an hour, so a watch is far better.

Free ebook: Viva Questions for Science Projects

Take it further

Make a second version using the W of Cassiopeia and compare the readings.

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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