Radiation Pressure. 405 



We may illustrate the possible effects of radiation pressure 

 without proceeding to such fineness as this. Let us imagine 

 a particle of the density of the earth, and a thousandth of an 

 inch in diameter, going round the sun at the earth's distance. 

 There are two effects due to the sun's radiation. In the first 

 place, the radiation-push is T ^ of the gravitation-pull ; and 

 the result is the same as if the sun's mass were only 99/100 

 ot" the value which it has for larger bodies like the earth. 

 Hence the year for such a particle would be longer by ^h, or 

 about 367 instead of 3(J5| days. In the second place, the 

 radiation absorbed from the sun and given out again on all 

 sides is crushed up in front as the particle moves forward and 

 is opened out behind. There is thus a slightly greater pressure 

 due to its own radiation on the advancing hemisphere than on 

 the receding one, and this appears as a small resisting force in 

 the direction of motion. Through this the particle tends to 

 move iu a decreasing orbit spiralling in towards the sun, and 

 at first at the rate of about 800 miles per annum. 



Further, if there be any variation in the sun's rate of 

 emitting energy, there will be a corresponding variation in 

 the increase of the year and the decrease of the solar distance, 

 and the particle, if we could only observe it, would form a 

 perfect actinometer. 



Though, unfortunately, we cannot observe the motion of 

 independent small particles circling round the sun at the 

 distance of the earth, there is good reason to suppose that 

 some comets at least are mere clouds of dust. If we are right 

 in this supposition, they should show some of these effects. 

 Encke's comet at once suggests itself as of this class ; for, as 

 everyone knows, it shortens its journey of 3| years round the 

 sun on every successive return, and on the average by about 

 2 1 hours each revolution. Mr. H. C. Plummer * has lately 

 been investigating this comet's motion ; and he finds that if it 

 were composed of dust particles, each of the earth's density 

 and about j~ mm. or rather less than a thousandth of an inch 

 in diameter, the resisting force due to radiation pressure would 

 account for its accelerating return. But the sun's effective 

 mass would be reduced by about 1/80 ; and on certain sup- 

 positions he finds that the assumed mean distance as calculated 

 from Kepler's law, without reference to radiation, is greater 

 than the true moan distance by something of the order of 1 in 

 400, and he thinks such a large error is hardly possible. So 



* Monthly Notices R.A.S., Jan. 1905, " On the Possible Effects of 

 Radiation on the Motion of Comets, with special reference to Encke's 

 Comet.'" 



