COMMENTATIO A S T R N M I C A. 145 



« The anclent aslronomers, wlio liad no olher way of compiiliag the dislances of the 

 fc Heavenly bodies but by their parallax to the seraidiameter of the earth , and being 

 «neverable to discover any in the fixt stars, did from thence righlly enough infer , 

 « that their distance was very great, and much exceeding that of the planets , but 

 « could go no farther otherwise than by uncertain guess. " 



« Since the Pythagörean syslem of Ibe world has been revived by Copernicus , (and 

 « now by all mathemal icians accepled for the true one) there seemed ground to imagi- 

 « ne that the diamcter of the earth's annual course ( which according to our best as- 

 « tronomers is at least 4001)0 times bigger than the semidiamcter of the earlh ) mi^ht 

 « give a sensible parallax to the fixt stars, tliough the other could not, and thereby 

 « determine their distance more precisely. " 



« But though we have a foundation to build on so vastly exceeding that of the an- 

 « cients , there are some consideralions may make us suspect that even this is not lange 

 « enough for our purpose." 



« Monsieur Rügens (who is very exact in bis astronomical observations) teils us , be 

 « could never discover any visible magnitude in the fixt stars, though he used glasses 

 « Tvhich magnifie the apparent diaraeter above 100 times. "' 



« Now , since in all hkehhood the fixt stars are suns , ( perhaps of a difierent magni- 

 « lüde ) we may as a reasonable medium preäume Ihey are geuerally about the bigness 

 « of our sun. 



« Xel US tTien (for example) suppose the clog-aiar to be so. The distance from us to 

 « the sun being about 100 times the sun's diameter ( as is demonstrable from the eun's 

 « diameter being 32 minutes ) it is evident that the angle under which the dog-star is 

 « Seen in Mr. Rügens 's telescope, must be near the same with the angle of its paral- 

 jdlax lo the sun 's distance, or semidiamcter of the earth 's annual course; so that the 

 « parallax to the whole diameter , can be but double such a qaantity , as even to Mr. 

 « Rügens 's nice Observation is altogether insensible. 



■ «The distance therefore of the fixt stars seems hardly within the reach of any of our 

 « melhods to determine; but from what has been laid dovwi, we may draw some con» 

 « clusions that will mnch illustrate the prodigious vastness of it. 



« I. That the diameter of the earth 's annual orb (which contains at least 160 millions 

 « of roiles) point is but as a point in comparison of it ; at least it must be above 6000 times 

 «I tTie distance of tlie sun. For if a star should appear through the aforesaid telescope half 

 « a minute broad (which is a pretty sensible magnitude) the true apparent Diameter 

 « would not exceed 18 3d minutes, which is less than the 6000 th part of the appa- 

 « -rent diameter of the sun , and consequenlly the sun 's distance not the 6O0O th part 

 ■K of the distance of the star. 



T •«2. That 



