What it shows
A vertical stick's shadow is longest at sunrise, shrinks until the Sun is highest, then grows again. The shortest shadow lies along the north-south line, pointing north for most of India for most of the year, and the time it happens is local solar noon, which is usually not 12 o'clock by the clock.
The science
As the Earth turns, the Sun appears to climb from the east, reach its highest point on the north-south line (in the south for most of India for most of the year) and sink in the west. A vertical stick casts its shortest shadow when the Sun is highest, and that shadow points along the north-south line. In the far south of India, for a few weeks around the summer solstice, the noon Sun passes slightly north of the point overhead and the shortest shadow points south. Indian Standard Time is set for 82.5 degrees east longitude, so local noon arrives earlier in the east of the country and later in the west, and the stick shows that difference.
Materials
Cost: free. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- A straight stick about 30 centimetres long
- A flat open patch of ground or a large sheet of card
- Pebbles or chalk
- A clock
- A string and a ruler
- A compass to compare
Safety
For this project: Never look straight at the Sun. Drink water and wear a cap if staying outside long.
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.
Push the stick straight into the ground, checking with the string that it is vertical.
From 10 in the morning, mark the shadow tip with a pebble every 20 minutes and write the time beside it.
Continue until about 3 in the afternoon.
Measure the length of each shadow with the string and ruler and write it in a table.
Find the shortest shadow; draw a line from the stick along it and mark the far end north (in the far south of India in summer, check with the compass, because the noon shadow may point south).
Write down the clock time of the shortest shadow; this is your local solar noon.
Compare the north line with a compass; the compass may differ by a degree or two because it points to magnetic north.
Join the pebbles with chalk to see the curved path the shadow tip traced.
What you should see
A table of shadow lengths that falls to a minimum and rises again, a north line that matches the compass closely, and a local noon time that depends on town and date: roughly 12:20 to 12:55 in Mumbai, or 11:05 to 11:40 in Guwahati, rather than 12:00.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
01When is the shadow shortest?
At local solar noon, when the Sun is highest in the sky.
02Which way does the shortest shadow point in India?
North for most places and most of the year; in the far south it can point south for a few weeks in summer.
03Why is solar noon not at 12 o'clock?
Clock time is the same across India, but the Sun reaches its highest point at different times in the east and the west.
04Why does the compass differ slightly?
A compass points to magnetic north, which is not exactly at the geographic pole.
Take it further
Repeat the same day a month later and compare the length of the noon shadow to see the Sun's changing height.
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.
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