SUN'vS distance from the earth. II 



ferent parts of our Earth. From these differences duly observed 

 the sun's parallax may be determined even to a small part of 

 a second of time and that without any other instruments than 

 telescopes and good common clocks, and without any other 

 qualifications in the observer than fidelity and diligence, with 

 a little skill in astronomy. For we need not be scrupulous in 

 finding the latitude of the place or in accurately determining 

 the hour with respect to the meridian. It is sufficient if the 

 times be reckoned by clocks duly corrected according to the 

 revolutions of the heavens from the total ingress of Venus on 

 the sun's disk to the beginning of her egress from it when her 

 opaque globe begins to touch the bright limb of the sun, 

 which times, as I found by experience, may be observed even 

 to a single second of time. 



" But by the limited laws of motion \^enus is rarely seen 

 within the sun's disk; and for a series of 120 years and 

 upwards is not to be seen there once, namely, from 1639, 

 when Mr. Horrox (Rev. Jeremiah Horrox, 1619-1641 ) was 

 favoured with this agreeable sight, and he the first and only 

 one since the creation of the world down to 1761, at which 

 time according to the theories hitherto observed in the heavens 

 Venus will pass over the sun on May 26 in the morning, so 

 that at London at 6 o'clock in the morning she is to be seen 

 in the middle of the sun's disk and but four minutes more 

 southerly than his centre. The duration of this transit will 

 be about eight hours, namely, from 2 till 10 in the morning. 



Supposing the sun's parallax, as was said, to be twelve 

 seconds and a half, Venus' parallax will be 43 seconds. 



25 



This ratio is 



7.2 



And subtracting the sun's parallax there will remain 

 43" — 12.5" - 30.5" 



