244 W. Harkness — Magnitude of the Solar System. 



used to find the solar parallax, either from the time required 

 by light to traverse the semi-diameter of the earth's orbit, or 

 from the ratio of the velocity of light to the orbital velocity of 

 the earth. 



Any periodic correction which occurs in computing the place 

 of a heavenly body, or the time of a celestial phenomenon, is 

 called by astronomers, an equation, and as the time required 

 by light to traverse the semi-diameter of the earth's orbit first 

 presented itself in the guise of a correction to the computed 

 times of the eclipses of Jupiter's satellites, it has received the 

 name of the light equation. The earth's orbit being interior to 

 that of Jupiter, and both having the sun for their center, it is 

 evident that the distance between the two planets must vary 

 from the sum to the difference of the radii of their respective 

 orbits, and the time required by light to travel from one planet 

 to the other must vary proportionately. Consequently, if the 

 observed times of the eclipses of Jupiter's satellites are com- 

 pared .with the times computed upon the assumption that the 

 two planets are always separated by their mean distance it will 

 be found that the eclipses occur too early when the earth is at 

 less than its mean distance from Jupiter, and too late when it 

 is further off, and from large numbers of such observations 

 the value of the light equation has been deduced. 



The combination of the motion of light through our atmos- 

 phere with the orbital motion of the earth gives rise to the 

 annual aberration, all the phases of which are computed from 

 its maximum value, commonly called the constant of aberra- 

 tion. There is also a diurnal aberration due to the rotation of 

 the earth on its axis, but that is quite small and does not con- 

 cern us this evening. When aberration was discovered the 

 corpuscular theory of light was in vogue, and it offered a 

 charmingly simple explanation of the whole phenomenon. 

 The hypothetical light corpuscles impinging upon the earth 

 were thought to behave precisely like the drops in a shower of 

 rain, and you all know that their apparent direction is affected 

 by any motion on the part of the observer. In a calm day 

 when the drops are falling perpendicularly, a man standing 

 still holds his umbrella directly over his head, but as soon as 

 he begins to move forward he inclines his umbrella in the same 

 direction, and the more rapidly he moves the greater must be 

 its inclination in order to meet the descending shower. Simi- 

 larly, the apparent direction of an oncoming light corpuscle 

 would be affected by the orbital motion of the earth, so that 

 in effect it would always be the resultant arising from combin- 

 ing the motion of the light with a motion equal and opposite to 

 that of the earth. But since the falsity of the corpuscular 

 theory has been proved that explanation is no longer tenable, 



