444 



NATURE 



[September 8, 1892 



brightness depends) by its semi-diameter in seconds of 

 arc. Presuming that the longitude of the perihelion of 

 Mars may be taken as about 334°, the following table 

 will show how the great brilliancy of the planet in 1877 

 and the present year was caused ; other less favourable 

 oppositions are given for purposes of comparison : — 



Date 

 of oppoiition. 



1862, October 5 

 1869, February 13 ... 

 1873, April 27..." ... 

 1877, September 5... 

 1881, December 26 

 1884, January 31 ... 

 1888, April 10 

 1892, August 13 ... 



So much, then, for the distance conditions. At its 

 nearest approach the planet is 35,000,000 miles removed 

 — let us say 150 times more distant than the moon. 



We next come to the conditions of visibility. Mars is 

 nearest to us (the degree of nearness depending upon its 

 position in its orbit) when " in opposition,'' as we have 

 said — that is, when it is in the south at midnight, and 

 opposite the sun, the sun then being, of course, due north 

 below the horizon. It will then appear to us "full," as the 

 moon is said to be full when she occupies an analogous 

 position. At this moment, then, the earth is invisible to 

 the inhabitants of Mars unless she happens to transit the 

 sun's disc. 



The earth appears to Mars precisely as Venus does to 

 us, and if inhabitants there be on Mars, and they study 

 astronomy, a transit of earth to them will be what a 

 transit of Venus is to us. 



Further, as we see Venus as a half-moon, and when 

 neirer to us as a fine large crescent, so the Martians, as 

 the earth approaches them, will see her as a half-moon 

 and then as a crescent, getting finer as the apparent 

 diameter of the completed circle gets greatefr. 



Mars, to see us best, must occupy a point near its 

 perihelion. These things may be gathered from Fig. 2, 

 in which an opposition at Mars' perihelion is shown, the 

 orbits, but not the size of the bodies concerned, being to 

 scale. Before the conjunction of the three bodies (in the 

 line Mars, earth, sun) is approached, Mars will first have 

 the earth as a half-moon at a ; this will gradually melt 

 into a crescent till the moment of conjunction. After- 

 wards the crescent will broaden, and its diameter will be 

 reduced till the point a' is reached, when the earth will 

 appear as a half-moon again. 



It is clear, therefore, that the earth will be a morning 

 and evening star to Mars at the time of their nearest 

 approach. The earth's crescent must not be too fine, or 

 no observation will be possible on a dark background of 

 sky. In other words, although we can observe Mars best 

 when he is nearest, the privilege of seeing the earth 

 when nearest to Mars is denied to his inhabitants. 



We are now, then, in a position to discuss, so far as 

 the mere conditions of visibility are concerned, the two 

 suggestions as to earth-signaUing to which I have already 

 referred. 



Mr Galton's proposal depends upon the observation 

 that a " reflected beam of sunlight sent through a hole in 

 a plate in front of the mirror was just distinctly visible as 

 a faint glint at a distance of ten miles when the hole was 

 a square of one-tenth of an inch in the side." He then 

 adds : " The amount of fog and haze that a beam of light 

 would traverse between us and Mars when the planet was 

 high above our horizon could not exceed that along a 

 terrestrial base of ten miles ; consequently the same pro- 

 portion between the size of mirror and the distance would 

 still hold true. It follows that the flash from many mir- 

 rors simultaneously, whose aggregate width was fifteen 

 yards, and whose aggregate length (to allow for slope) 



NO. I 193, VOL. 46] 



was, say, twenty-five yards, would be visible in Mars if 

 seen through a telescope such as that at the Lick Obser- 

 vatory. With funds and good will, there seems no in- 

 superable difficulty in flashmg from a very much larger 

 surface than the above, and sending signals that the 

 inhabitants of Mars, if they have eyes, wits, and fairly 

 good telescopes, would speculate on and wish to answer. 

 One, two, three, might be slowly flashed over and oyer 

 again from us to them, and possibly in some years, to 

 allow time for speculation in Mars to bear practical fruit, 

 one, two, three, might come back in response. Dr. 

 Whewell, if I recollect right, wrote a paper on the possi- 

 bility of coming to an understanding with lunar inhabi- 

 tants, if there were any. He would begin from the 

 mathematical side. The practical difficulty is by no 

 means insuperable of enabling many independent 

 observers (who need not be near together) to direct their 

 flashes aright. If mirrors could be mounted without 

 much cost as heliostats (and perhaps they can be) it 

 would be easy enough to do this. My own method is not 

 practicable, at least without considerable addition and 



Fig. 2.— The Conditions of Visibility of the Earth from Mars. 



modifications, as it requires the object to be visible to- 

 wards which the flash is directed, but Mars is not visible 

 to the naked eye at day." ^ 



Mr. Galton then uses sunlight and works in the day ; 

 Mr. Haweis, on the other hand, suggests electricity and 

 night-time : — 



" I infer from the astronomers that a signal on our 

 earth about six miles in size of the nature of a brij;;ht 

 light could be seen by the inhabitants of Mars, who by 

 all accounts seem to be making the most systematic and 

 herculean efforts to communicate with us by flashing 

 triangular signals of presumably electric light. Why 

 cannot we answer those signals by something which 

 would resemble the lighthouse intermittent signal ? Here 

 is the method. London every night presents an area of 

 at least twelve miles square brilliantly illuminated. 

 That illuminating power might be enormously increased 

 with only a few additional centres of powerful electric 

 light. But without any additional expense, a little co- 

 operation on the part of the gas companies would 

 suffice to alternate darkness and light at intervals of five 



1 Times, August 6, 1892. 



