562 NORMAN LOCKYER LECTURE 



agreement with the temperatures calculated on the assumption that for 

 each planet there is a balance between the radiation received from the 

 Sun and the radiation re-emitted into space. The temperature differences 

 from one part of the surface to another depend very much, however, upon 

 the extent and nature of the atmosphere. A dense atmosphere greatly 

 reduces the variations of temperature across the surface and the range 

 of temperature between day and night. The Moon provides an extreme 

 example of rapid variations. At the lunar eclipse of January 14, 1927, 

 Pettit and Nicholson found that the temperature of the surface dropped 

 from + 70° c. to — 80° c. in a little more than an hour, as the result 

 of the radiation from the Sun being cut off by the interposition of the 

 Earth. During 2^ hours of totality, the temperature dropped a further 

 40° c. But after totality had ended, the temperature rose to almost its 

 initial value in about an hour. Venus, on the other hand, despite its 

 long day, shows only a moderate range of temperature. 



To determine the composition of the atmosphere of Venus, or of any 

 other planet, recourse must be had to the spectroscope. Absorption 

 in the atmosphere of the Earth is a complicating and troublesome factor. 

 Ozone, though present in the Earth's atmosphere in very small amount, 

 with an equivalent thickness of but a few millimetres, completely cuts 

 off the whole spectrum below A 2900, so that the extreme ultra-violet 

 region is completely inaccessible to observation. Oxygen reveals itself 

 by some strong absorptions in the near infra-red and red regions, in- 

 cluding the A and B bands of Fraunhofer and some weaker absorptions 

 in the visible spectrum. Water-vapour has some extremely strong 

 ' absorptions in the infra-red. The terrestrial origin of these various 

 absorptions can be established in two ways. First, by observing the 

 spectrum of the Sun at different altitudes, the terrestrial absorptions 

 become stronger the lower the altitude, because the air-path is cor- 

 respondingly increased. Secondly, if the spectra of light from the 

 east and west limbs of the Sun are compared, the absorptions of solar 

 origin show a slight relative displacement caused by the solar rotation, 

 whilst the absorptions of terrestrial origin are undisplaced. 



The absorptions of terrestrial origin in the spectrum of the Sun 

 having been identified, the absorptions produced in the atmosphere 

 of a planet can be investigated by photographing the spectra of the planet 

 and the Moon on the same night and at the same altitude. An absorp- 

 tion present in the spectrum of the planet and not in that of the Moon, 

 or much stronger in the spectrum of the planet than in that of the Moon, 

 must originate in the atmosphere of the planet. Another, and more 

 delicate, method of investigation is to photograph the spectrum of the 

 planet at a time when it is approaching or receding from us most rapidly. 

 The motion will displace the absorptions due to the planet's atmosphere 

 with respect to those due to our own atmosphere, and in this way the 

 planetary absorptions may be revealed. 



Complete information about the constitution of any planetary atmo- 

 sphere is not obtainable, however, because many possible constituents 

 of the atmosphere show no absorptions in the region accessible to study. . 

 Amongst such undetectable constituents are hydrogen, nitrogen, helium, 

 neon and argon. 



