What it shows
India uses one standard time, but the Sun rises up to about two hours earlier in far-eastern Arunachal Pradesh than in western Gujarat. A globe and torch show why places east and west of each other have different local times.
The science
Earth turns from west to east, so sunrise reaches eastern places first. Each 15 degrees of longitude is one hour of Sun time. India stretches about 30 degrees from west to east, so the sunrise gap between its far ends is close to two hours even though every clock shows the same time.
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 globe or a ball with India marked on it
- A bright battery torch or battery LED lantern
- A table in a dark room
- Sticky dots in two colours
- A printed table of sunrise times for a few cities
- A notebook
Safety
For this project: Never shine the torch or lantern into anyone’s eyes, and keep the room dark only with a teacher or parent nearby.
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.
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.
Mark Dwarka or Bhuj in Gujarat and Itanagar in Arunachal Pradesh on the globe with two different coloured dots.
Place the lamp one metre from the globe in a dark room and tilt the globe to match its axis.
Turn the globe slowly from west to east and watch which dot enters the light first.
Stop when the eastern dot just meets the light edge and note where the western dot lies.
Turn the globe further until the western dot reaches the light and count how much you turned.
Use 15 degrees per hour to convert the turn into hours and compare with the sunrise table.
Write the longitude of each city and find the difference between them.
Explain in a paragraph why one time zone for the whole country has advantages and problems.
What you should see
A model in which the eastern dot enters sunlight before the western one, with a measured gap of roughly one and a half to two hours (depending on the two places chosen) matching the table of sunrise times.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
01Which direction does Earth spin?
West to east.
02How many degrees of longitude make one hour?
15 degrees.
03Which meridian is used for Indian Standard Time?
82.5 degrees east.
04Why does Arunachal see sunrise earlier than Gujarat?
It lies about 25 to 30 degrees of longitude further east.
Take it further
Work out the local solar time at five cities and compare it with the clock time.
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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