THE TEMPERATURE OF MARS 9 



I do not think that this statement is quite just to Prof. Poynting, 

 in whose paper, as we shall see, the idea certainly occurs. 

 It is in the relative importance which they attach to it that 

 the two investigators differ so completely. 



Prof. Poynting's paper not only deals with the temperatures 

 of different planets compared with that of the earth ; it also 

 gives a method by which the mean temperature of the earth 

 may be obtained absolutely. Strictly speaking, his results 

 apply only to an ideal planet, for which certain assumptions 

 are rigorously true. Most of these assumptions present no 

 difficulty as a basis for an approximate result. But with 

 regard to the absorbing power of the planet, Prof. Poynting 

 assumes that the reflection at each point is one-tenth of the 

 radiation received, justifying the assumption by the following 

 remarks : " This is probably of the order of the actual reflection 

 from the earth. According to Langley the moon reflects about 

 one-eighth of the radiation received. The earth certainly reflects 

 less. The temperatures determined hereafter are proportional 

 to the fourth root of the coefficient of absorption. Even if this 

 coefficient is as low as 0*9, its fourth root is 0*974. Hence if the 

 actual value is anywhere between 0*9 and 1, the assumed value of 

 o'9 will not make an error of more than 2\ per cent, in the value 

 of the temperature." 



But is it between 09 and 1 ? The albedo (or fraction of 

 incident light reflected) varies enormously for different planets. 

 For Venus it is 0*92 and for Mars 0*27. In these cases the 

 proportion of incident light absorbed is therefore only 8 per 

 cent, for Venus, and actually 73 per cent, for Mars. It is of 

 course quite true that this applies only to the visible part of 

 the spectrum, and that there is a much greater proportion 

 of absorption in the infra-red; but on the face of it it seems 

 very unlikely that in the case of Venus the absorption is 

 anywhere near the value 90 per cent. 



Unfortunately the value of the earth's albedo is highly 

 uncertain. The latest value, that of Prof. Very, 1 obtained 

 from the " earth-shine " on the moon, is 0-89 — i.e. nearly that 

 of Venus; but there are enormous difficulties in the determina- 

 tion. Prof. Lowell's value, estimated from the reflecting powers 

 of air, cloud, and the earth's surface, is 075. This has to be 



1 Astronomische Nachrichten, 4696. 



