248 HISTORY OF SCIENCE. 



reasonable to suppose that a line 180,000,000 miles long (for such 

 is the diameter of the earth's orbit) would subtend some measurable 

 angle to the distance of at least the nearest fixed stars. Triough 

 the absence of any annual parallax of the fixed stars was not allowed 

 as a proof against the earth's annual motion, yet it was felt that 

 if even the smallest amount of parallax could be detected, it would 

 furnish an unanswerable argument in favour of the Copernican theory. 

 Galileo had suggested a mode of observing whether some selected 

 star always crossed the meridian at the same altitude, and a modifica- 

 tion of his method consisted in permanently fixing a telescope on some 

 immovable support in such a position that the passage of the star 

 across a fixed triangular field may be noted. Should the altitude of 

 the star vary, the interval of time required for it to pass across the tri- 

 angle will of course vary also. It might be supposed that the effects 

 producible by the annual parallax being perfectly understood, it would 

 be an easy matter to distinguish them. But though the greatest at- 

 tention has been given to this subject by a multitude of observers, the 

 amount of the parallax is so small, that no satisfactory indications what- 

 ever were obtained by the earlier observers. There are, in fact, several 

 other causes which give rise to apparent changes in the position of the 

 fixed stars, and when several of these causes were as yet unknown, it 

 was of course impossible to eliminate their effects from the observa- 

 tions. Hooke, Picard, Romer, Horrebow, and Cassini observed and 

 announced the observation of what they took for the effects of parallax. 

 It was in conducting observations to ascertain the parallax that a dis- 

 covery very important for astronomy was made. Here it will perhaps be 

 better to anticipate the chronological course of our history by stating 

 what is known of stellar parallax at the present day. Only about a 

 dozen stars are known to possess a sensible parallax. The parallax of 

 the star a Centauri is about o'g", and this corresponds with a distance 

 from the earth about 224,000 times the distance of the latter from the 

 sun. Sirius, the brightest star in the heavens, has a parallax of fifteen 

 hundredths of a second of arc, and that corresponds with a distance 

 from us of 1,375,000 times our distance from the sun, while the star 

 Capella is at four times the distance of Sirius. It is of course impos- 

 sible to form any adequate conception of distances so vast, but the 

 velocity of light furnishes a standard of comparison which it may be 

 interesting to apply to this case. Light travels at the rate of 180,000 

 miles per second, and it takes about eight minutes to traverse the space 

 which separates the earth from the sun. Thus the light from a Centauri 

 travels for 3! years before it reaches the earth, and that from Sirius 

 reaches us 2i| years after it has been emitted, while the time occupied 

 in the journey of the luminiferous impulses from Capella is actually the 

 space of a human life, for the interval required is no less than seventy 

 years. It is curious to reflect that we see the stars not by the light they 

 are now giving off, but by the light they emitted scores of years before, 



