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Measuring Sunlight

Measuring Sunlight
Image courtesy of Forrest M. Mims III
Web Id: P17
Purpose: Sunlight is the key to life on Earth. This project will show you how sunlight changes during the day and during the seasons. It will also show you how sunlight is absorbed and reflected by clouds, vegetation and trees.

This project describes a simple sunlight monitor that you can easily make. With this monitor, which is known as a solar radiometer, you will be able to measure sunlight in open fields, under trees and in the shade of buildings at any time during the day and during the seasons. You will learn first hand how sunlight is altered by clouds, haze, smog, dust and smoke in the sky. You will also learn surprising facts about how clouds can both reduce and increase sunlight.

Age Range: 11-Adult
Time Required: After the necessary supplies are obtained, you should be able to begin measuring sunlight in less than an hour. You will quickly learn how sunlight is altered by clouds and trees. For best results, try some or all of the suggested activities below. You can learn much by using the solar radiometer for a few minutes during both clear and cloudy days. You can do some real science by doing this project for several minutes a day over a full year.

The energy in sunlight warms the Earth and prevents the oceans, lakes and rivers from freezing solid. Sunlight powers the weather systems that produce clouds, rain and snow. The blue and red colors of sunlight provide the energy that drives the photosynthesis that allows plants to produce starch from water and carbon dioxide.

Many years ago scientists used mercury thermometers to measure sunlight. They coated a thermometer bulb with lampblack (soot) or black paint and then exposed the thermometer to sunlight for a set period of time. The increase in temperature was directly related to the intensity of the sunlight striking the thermometer.

Today you can measure sunlight much more accurately than the old-fashioned method with a silicon solar cell and a multimeter. The silicon solar cell converts sunlight into electricity. This electrical current is measured by a multimeter, a device that displays current on a digital readout or by sweeping a moving needle across a scale of numbers. Together, the solar cell and the multimeter form a sunlight monitor or solar radiometer.


Sunlight varies throughout the day and reaches a peak at or near noon when the sun is at its highest position in the sky. Sunlight also varies across the seasons. In the Northern Hemisphere, sunlight reaches a peak near the end of June (the summer solstice) and a low near the end of December (winter solstice). Sunlight is also altered by clouds, haze, air pollution and forest fire smoke. This project will allow students to measure the changes caused by the angle of the sun over the horizon and by the effect of clouds, haze and air pollution.

While the sunlight monitor described here is very simple, it is also very stable and is an ideal tool for demonstrations, brief studies of sunlight and long term studies suitable for doing real science and for science fair projects.

Project Links:

  • You can learn much about sunlight at these sites:
  • Learn about SOHO, a satellite that provides amazing images of the sun from space.
  • Learn all about the sun at the Stanford Solar Center.
  • Find out how sunlight affects tree rings and how the ozone layer, haze and water vapor are measured using sunlight at Sun and Sky.
  • Learn all about photovoltaics (solar cells).
  • See daily sunlight measurements made by the UV-B Monitoring and Research Program, a program of the US Department of Agriculture’s Cooperative State Research, Education and Extension Service (CSREES).

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