THE TRUE GRANDEUR OF THE SUN 231 



Coppernican theory as heretical and impious, and blasphemy 

 against the Most High God. 



Instead of endeavouring to smother the truth, a Catholic 

 monarch in a Catholic land could now contribute from the 

 revenues of his Catholic subjects to advance it. A century later, 

 when a yet more favourable opportunity presented itself to 

 determine the sun's parallax, not one government but half-a- 

 dozen would contribute. This was during the transit of Venus 

 in 1761 and 1769. Halley, the friend of Newton, had pointed 

 out the advantage of observations on this planet ; like the 

 moon, it crosses the sun's face, but its apparent magnitude is 

 too small to cause any serious diminution in the sun's light J 

 the moment it touches, the sun's disk is more sharply defined. 

 But the plane of the earth's orbit is somewhat inclined to that 

 of Venus, so that we are able to witness the transit of Venus 

 only twice in a little more than a century. 



When at last it came again, the interest manifested in the 

 event was extraordinary observing parties were scattered from 

 the Cape of Gobd Hope to Siberia and India ; at the second 

 transit, from Hudson's Bay to Madras, from Siberia to Cali- 

 fornia, from far northern Norway to the South Sea Isles. The 

 results, however, were far from satisfactory. They did not 

 greatly improve upon the accuracy attained a hundred years 

 before. It is only within the last half-century that, by the 

 concurrence of a variety of methods, it has been possible to 

 attain a result which no further investigations can materially 

 change. 



These methods were grounded upon bases very diverse. 

 Even while Cassini was observing the transit of Mars, Roemer 

 was deducing the velocity of light. Later on, ingenious con- 

 trivances in the laboratory have made it possible to fix this 

 velocity with great precision, so that, knowing the time which 

 light takes to cross the diameter of the earth's orbit, it is pos- 

 sible to deduce the distance of the sun by this means. Two 

 others were worked out by Leverrier from the observed varia- 

 tions in the earth's orbit one due to the gravitational influence 

 of the moon, the other to that of the near planets. These and 

 several others of less value unite with more recent and more 

 accurate observations on Venus to fix the solar parallax at 

 &" 8, with a probable error of less than one five-hundredth of 

 the distance. 



