BOLOMETRIC INVESTIGATION The Sun's Total Radiation.—One of the most important charac teristics of the sun is its practice of radiating energy. Of the 3.79 x io" ergs which it discharges every second in the form of light, heat, and other ethereal vibrations, its attendant planets and their satellites receive about one part in 120 million, so that the significance of the process is cosmic rather than human. This enormous quantity of energy is sent forth regularly and ceaselessly, and goes no one knows whither or to what end. In seeking further knowledge of this fundamental process the first step to be taken is the measurement of the precise rate of radiation, for which purpose use has been made of various instruments bearing the generic name, bolometer.
Owing to natural limitations we can measure only the radiation received by a small area of the earth's surface, but, knowing the dimensions of the solar system, we can readily deduce the total amount. The general method is to convert the sun's radiation into heat, which can be accurately measured—making an allow ance, which must be estimated from separate investigation, for absorption by the earth's atmosphere. The result is usually ex pressed as the solar constant, which is defined as the amount of energy which would fall perpendicularly per square centimetre per minute on a surface placed just outside the earth's atmos phere. The value of this constant, according to Charles Greeley Abbot, is about 1.94 calories (81 million ergs), but it is subject to slight variations are related to the sunspot periodicity and possibly to other factors also. The rate of radiation appears to increase with the prevalence of sunspots, but further observa tions are required to determine the relation precisely.
The question of the source and maintenance of this ceaseless efflux of energy will be dealt with in a later section of this article. For the present we will limit ourselves to the consideration of the immediate use which can be made of the measurements in the light of ascertained physical principles. It is known that surfaces in a certain condition—known as that of a perfect radiator or black body—when they radiate energy, do so at a rate strictly proportional to the fourth power of their absolute temperature.
There are strong theoretical reasons for believing that the sun radiates as a black body, and its absolute temperature is therefore calculable. Now the sun cannot be uniform in temperature throughout, so that it is necessary to consider what region has the temperature so determined. Since the greatest part of the radiation is in the form of visible light, this region must be the photosphere, and the photosphere is a relatively thin superficial layer at the base of the atmosphere. Thus the temperature measured from the total radiation is that of the outside of the sun. It is called the effective temperature, and has a value of 5,75o absolute centigrade degrees.
The Sun's Analysed Radiation.—The bolometer also provides the data for another method of estimating the sun's effective tem perature. The radiation from a black body is distributed among the various wave-lengths in its continuous spectrum in a manner definitely related to the temperature of the body. Thus, if the bolometer be passed along the solar spectrum, and the amount of energy at each point in the continuous background thereby measured, the distribution of energy throughout the spectrum may be determined and the absolute temperature deduced. (Al lowance must again be made for absorption by the earth's atmos phere, which is not uniform for all wave-lengths.) There are two partially independent methods of analysing the observations to obtain the temperature, which yield the respective values, 6,15o° and If the sun behaved exactly as a perfect radiator, these determinations would agree with one another and with the result obtained from the undissected radiation. The slight departures from agreement show that the sun is not quite in this condition, but is sufficiently near to it to enable us to say with confidence that the temperature near its surface is about 6,000°.