86 Sir J. W. Lubbock on Shooting Stars. 



shortness of their periodic times, for shooting stars are seen so 

 frequently that they scarcely excite attention or even a passing 

 remark. The question of their origin seems to have excited 

 more controversy than any other; but for the solution of this 

 difficulty, I apprehend we possess no data which do not apply 

 equally to the moon and to the other bodies of the solar system. 



The motion of the shooting stars is so rapid, that, supposing 

 them to shine by the reflected light of the sun, their relative 

 position to that of the sun may change considerably, even 

 during the short time in which they are visible, and therefore 

 not only they may become larger and more brilliant because 

 their distance from the spectator is diminished, but also be- 

 cause the visible portion of their illuminated disc is increased. 

 Or, on the other hand, their distance from the spectator in- 

 creasing, and the visible portion of their illuminated disc de- 

 creasing, they may cease to be perceptible to the naked eye 

 without Ijeing eclipsed. But in this case it seems probable that 

 their disappearance would not be so sudden ; and it is therefore 

 desirable that the observer should particularly notice and record 

 whether the shooting star disappears suddenly or otherwise. 



If 8x' is the angular velocity in a circle at the same distance 

 r from the centre of the earth, 



8\'= V^r-^lt 



This is greatest in the same orbit when r is least ; r is least 

 when r=a{\ — e) ; therefore 8x is greatest when 



8x= V\-\-e Sx'; 



e cannot exceed unity, therefore Sx cainiot exceed V\ +e8x'. 



If 8a" is the angular motion of the moon in 8/, and if?' is the 



distance of the centre of the moon from the centre of the earth, 



8x'=^8x". 



As the moon revolves in 27"322 days, if 8/=l", and if 



r' = 237000 r = 4000 (in miles), 8x" = -55" (sex.). 



This gives 250" for the angular motion of a satellite in a second 

 of time, as seen from the centre of the earth, and moving at 

 a distance of 4000 miles from the centre. Suppose the satellite 

 to be about 100 miles from the spectator, and to be so situated 

 that the angular velocity, as seen by the spectator, is inversely 

 as this distance compared with the former distance, the appa- 

 rent angular velocity would be 170' per second of time, or 



