OF THE KERGUELEN REGION OF THE GREAT SOUTHERN OCEAN. 497 



would never be in darkness even at the winter solstice, so that poleward of the parallels 

 of 66° there would be no night whatever at any time of the year. 



When the sun was reduced to such a size that its rays just grazed the pole at the 

 winter solstice, the thermal history of the earth would have reached an important stage, 

 for this would mark the close of perpetual sunshine at the poles, provided we do not 

 consider the refraction of the earth's atmosphere. When a ray from the sun's limb is 

 tangential to the pole at the winter solstice, that is when the pole is turned away 

 23° 28' from the sun, it follows that the angular distance of the limb from the centre 

 of the sun would also be 23° 28', and the sun would subtend an angle of 46° 56' in the 

 heavens, its diameter being therefore 72,818,000 miles. This is nearly equal to the 

 diameter of the orbit of Mercury. The precipitation of water, the formation of the first 

 stratified rocks, and the advent of life on the surface of the earth, may possibly have 

 taken place before or have coincided with this stage of the sun's development, and because 

 of the great size of the sun there would be a universal tropical climate on the earth. 



At the present time a very little over 50 per cent, of the earth's surface is under 

 illumination ; Ijut with a sun in the heavens having an angular diameter of 46° 56', 69 "9 

 per cent, of the earth's surface would be illuminated. If we assume that the total amount 

 of radiant energy from such a large sun was the same as from the present sun, the amount 

 of light and heat radiated from a unit surface of such a nebulous sun would be much less 

 than from a unit surface of the present sun. Assuming that the insolation due to any 

 element of area of the sun's disc is proportional to the cosine of its zenith distance, then it 

 may be readily shown that, if the total radiation from the large sun be the same as from 

 the existing one, the insolation at any place due to the whole sun is independent of the 

 sun's apparent magnitude, provided the whole of the sun's disc is visible. When, 

 however, only part of the sun's disc is visible, as at sunrise and sunset, the size of the sun 

 has a marked eftect on the distribution of heat, the ultimate result being that, at sunrise 

 and sunset, the earth receives an additional amount of energy entirely on account of 

 the sun's greater magnitude. 



At the equator the duration of sunrise and sunset, that is the length of time during 

 which only part of the sun is above the horizon, is short compared with other latitudes ; 

 the additional heat received at the equator during one whole day, therefore, on account 

 of the increased magnitude of the sun, would be comparatively small, being indeed an 

 absolute minimum at the equinox when the sun's apparent path in the heavens cuts 

 the horizon at right angles. The nearer to the poles we go, the longer does the sun 

 take to emerge completely above the horizon, and the longer is the period during which 

 the sun is only partly seen ; the greater therefore would be the quantity of additional 

 energy received on account of the sun's great size. There might thus have been 

 a large luminary, the rate of insolation of which at any place on the earth's 

 surface was equal to that of the sun at present; but from that sun, on account of 

 its great size, the earth as a whole would receive more energy, distributed in the most 

 advantageous manner possible, viz., a very slight increase at the tropical regions, and 



