':x 



160 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1913. 



Have we to-day at our disposal sufficiently delicate means of obser- 

 vations to detect these changes? In Newton's time such means were 

 probably lacking. The caprices of our atmosphere furnished a ready 

 explanation of the apparent fluctuations in solar radiation. The 

 spots had been observed on the sun's disk, sometimes few, sometimes 

 many, but no law had been assigned to them. Further, the tradi- 

 tional fixity of the constellations led to the belief that the sun main- 

 tained a complete immobility with reference to the stars. 



But the problem plainly stated aroused new attempts at its un- 

 raveling. Bradley, a fellow countryman and a disciple of Newton, 

 showed that much greater precision could be obtained in the measures 

 of the angular distances of the stars than had before been gained. 

 Less than a century later, W. Herschel could affirm that the con- 

 stellations do alter their form, and the best determination of these 

 changes may be explained by attributing to the solar system a regular 

 rectilinear motion. The ambition of astronomers, increasing with 

 success, tries to-day to show that this movement is not rigorously 

 uniform, and even though shielded from the action of the stars, pays 

 tribute to the universal attraction in periodic oscillations. 



It is pretty safe to predict what will be the most marked of these 

 oscillations. It is not the center of the sun itself which possesses 

 the uniform rectilinear motion, but the center of gravity of the 

 system formed by the sun and all the planets. The oscillation would 

 be small if only the earth need be considered. There is, however, a 

 giant planet, Jupiter, whose mass exceeds that of all of the other 

 planets taken together and is nearly one one-thousandth that of the 

 sun. Describing its orbit at the rate of 12 kilometers per second, 

 Jupiter forces the sun to rotate about an imaginary center with a 

 velocity a thousand times less. This is apparently a very small 

 amount, but not at all negligible with respect to the velocity of 

 translation of the solar system, which is 20 kilometers per second. 

 Consequently the speed of the solar system toward a point in the 

 constellation Hercules is sometimes accelerated, sometimes slowed, 

 by one part in one thousand in an interval of six years. 



Very few of the stars are near enough to us for the parallactic dis- 

 placement relative to the more distant stars and due to this motion 

 of the sun to be appreciable in six years. Consequently, to measure 

 one one-thousandth part of this displacement is beyond the resources 

 of precise astronomy. We may be pretty sure, though, that some 

 day we will thus obtain, at the same time with a measure of the 

 mass of Jupiter, a new confirmation of the principle of the universal 

 attraction of gravitation. 



Meanwhile help comes in another way. What the micrometer for 

 a long time will probably be unable to give, the spectroscope is al- 

 ready furnishing. Although the variation of 30 meters per second, 



