What it shows
The number of stars you can see falls when street lights and glare brighten the sky. Counting stars in a fixed patch of sky at different places gives a simple measure of light pollution.
The science
The eye can see stars of magnitude 6 at best in very dark skies, about 3,000 stars across the whole sky. Light scattered by dust and haze and street lights brightens the sky background so only the brightest stars stand out from it. A tube blocks side light and shows a small fixed patch, so counts at different places can be compared. In a very polluted town centre only a few of the brightest stars can be seen, while in a dark village many hundreds are visible.
Materials
Cost: free. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- A cardboard tube such as a kitchen roll, painted black inside
- A star chart or an app showing the Pleiades and Orion’s belt
- A notebook and a red torch
- A watch
- A map of your area
- A friend and an adult
- A table for place, time, count and sky condition
Safety
For this project: An adult must go with you at night. Choose safe open places, keep away from roads and ditches, and use a red torch to protect your night vision.
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.
Decide a fixed target patch of sky, such as the square of Pegasus or the region around the three stars of Orion’s belt. Learn its position in the sky from a chart.
Wait 15 minutes outdoors so that your eyes become dark-adapted and avoid any bright phone screen.
Hold the tube to one eye and point it at the patch. Count the stars visible inside the circle for one minute. Do not count stars outside the tube.
Record the number, place, time, moon phase, haze and clouds.
Repeat at three or more places: a street corner, a park, the outskirts or a village if possible. Use the same patch and the same tube each time.
Make a second count for the Pleiades: write how many of the cluster stars you can see by eye and compare with the chart (6 to 7 in good skies).
Plot the star count against distance from the town centre on a graph.
Mark each place on the map with a coloured dot for the count and draw a simple light pollution map.
What you should see
A bar chart and a map showing more stars at darker places, for example 5 in a lit street and 30 in a dark field, with the Moon and haze noted as causes of difference.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1What is light pollution?
Excess artificial light that brightens the night sky and hides faint stars.
2Why use a tube?
It cuts stray light and gives a fixed small patch so counts can be compared.
3Why wait 15 minutes before counting?
Eyes take time to become dark-adapted and see faint objects.
4How can light pollution be reduced?
By shielded lamps that point downward, lower brightness and switching off lights not needed.
Take it further
Repeat at the same place on a moonless and a full-moon night and find how much the Moon reduces the count.
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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