Home laboratory

I invite you to measure the sun. More precisely, we shall attempt to determine the amount of energy emitted and the surface temperature of the sun. The methods we intend to employ being rather primitive, simplifications are unavoidable. Therefore our results should be taken with due caution.

The measurement boils down to the determination of the solar constant, i.e. the amount of energy received in unit time by unit area (perpendicular to the direction of solar rays) placed beyond the atmosphere at earth's mean distance from the sun. We shall compare sun radiation with that of a, say, 200 watt transparent light bulb.

Closed eyes will play the role of a photometer. On a sunny day turn the light on. When the bulb gets hot, close your eyes and bring your face nearer to it (watch your nose!). You will see red and you will feel the heat (infrared radiation). Don't open your eyes yet! Turn to the sun. You will experience similar impression. Now, move your face (with respect to the bulb) until you find a spot such that the shade of red which you see through your closed pupils is the same as the one caused by the sun. The similarity in shade would correspond to the equality of radiation intensity of the sun and the bulb. Obviously, this equality concerns the visible light and partly infrared radiation too, under the additional assumption that the spectrum of electric light is similar to the solar spectrum and radiation distribution is homogeneous in all directions, as is the case with the sun.

The measurement of the distance d between your eyes and the bulb at the spot of equal shades of red completes the experimental part of our project.

(The intensity of a light source is measured comparatively. The simplest implementation of this method is provided by Bunsen's photometer in the form of a sheet of paper with a greasy stain. Under equal illumination the stain disappears.)

The computations

a) The solar constant S

b) The energy

c) The temperature