552 PEOFESSOR J. H. P05"NTING ON RADIATION IN THE SOLAR SYSTEM. 
Let a stream of energy density E l)e incident on a fully absorbing surface moving 
towards the source with velocity u. Let the surface be at 0°A, so as to obtain the 
eftect of the incident radiation only. AVben the surface is at rest, we may regard the 
stream as bringing up momentum E per second, or as containing momentum ol 
density E/U brought up vdtb velocity U to it. If the surface is moving towards 
the source, it takes up in one second the momentum in length U + n,, or receives 
^ (U + n), and the pressure on it is y/ = E ( I + = p( I + 
It is easy to show tliat when a jierfect reflector is moving, the ]iressure upon it is 
altered from p to 71 ( 1 + ) • 
In tlie paper, tlie case of a full radiator in an enclosure at zero has alone been 
considered, so that the correcting factor is I + ^ or 1 + when the motion is 
at X to the line of radiation. Hence the forces obtained in the paper when the factor 
was I fl- are all double those olitained with the factor now given. The process of 
drawing in small particles to the sun is correspondingly lengthened out. 
It is, perhaps, worth noting that the motion of a body round the sun produces a 
small aberration effect. If the body is a sphere, the sunlight does not fall on the 
hemisphere directly under the sun, hut on one turned round through an angle ujX]' 
The pressure of the radiation, though still straight from the sun, does not act through 
0 
tlie centre but through a point X radius of sphere in front of the centre. thus, 
in the case of the earth, it will tend to stop the rotation. But tlie eftect is so minute 
tliat if present conditions as to distance and radiation were maintained, it would take 
something of the order of years to stop the whole of the rotation.] 
