WEIGHING THE WORLDS 



of only eight years. Thus transits occurred in the 

 years 1761 and 1769; but the next transit did not 

 occur until 1874, followed by the transit of 1882. 



It was thought by Halley, and following him by 

 astronomers in general, that by noting the precise 

 instant when the planet touched the limb of the sun, 

 as viewed from different observation stations, it 

 would be possible, owing to the difference of time 

 at which this so-called occultation would occur as 

 viewed at different longitudes, to compute accurately 

 the distance of Venus from the earth, and, secondari- 

 ly, to obtain the desired parallax of the sun. But 

 although important expeditions were planned, where- 

 by astronomers took up their location with proper 

 instruments at various points of the earth from which 

 the transit of Venus would be visible, the final results 

 of all series of observations were very disappointing. 

 To say nothing of uncertainties of observation 'due 

 to clouds, which are likely to obscure the sun at 

 precisely the wrong moment, there are inherent diffi- 

 culties due to the refractive effects of the atmosphere 

 of Venus ; so even after the transit of 1882 there still 

 remained elements of doubt as to the sun's precise 

 parallax, and therefore, of course as to his exact 

 distance. 



AID FROM MARS AND EROS 



Another methoid of attempting to solve the prob- 

 lem would be to take the parallax of our neighbor 

 Mars at one of those periods when he chances to be 

 relatively near us. This attempt has been made again 

 and again, and with a certain measure of success; 



75 



