NEW MEASUREMENTS OF DISTANCE OF SUN. Ill 



fairly typical case by transplanting ourselves in inuii>'ination to the 

 jilanet Eros on the evening of the 'id of December, 1!)()0, armed with 

 an imaginary telescojie ridiculously out of proportion to the real size 

 of the planet, which is probably not more than 20 miles in diameter 

 (fig. 4), The earth is past inferior conjunction with the sun, and 

 appears as a crescent. The North Pole of the earth is tilted towai-d 

 us, and by the aid of this projection of the meridians and j^arallels of 

 latitude we can with great ease trace the path of each observatory 

 as it is carried round by the rotation of the earth and can measure 

 from the scales the angular distance at any moment of an observatory 

 from the center, or the distance 1)etween two observatories, which 

 angular distances as seen from the planet are the precise equivalents 

 of the parallactic diplacements of the i)lanet as seen from the earth. 



In the programme of observations there was one novel and prom- 

 ising feature — the application of pliot()grai)hy. With the exception 

 of the transit of Venus olxservations. in which its success was not 

 striking, photograjjhy had not l)een previously applied in a deter- 

 mination of the solar parallax, for the very good reason that in 

 1889, at the time of (he opposition of Victoria, there was practically 

 only one telescope in existence which was capable of taking photo- 

 graphs for exact measurement, tliat j)ioneer photographic e(iuatorial 

 made by the Brothers Henry at Paris. The fact that there Avere in 

 11)00 eighteen photographic telescopes engaged in observing Eros 

 shows how rapidly the equipment of astronomy has gi-owu in the last 

 few years. We were so fortunate at Cambridge as to have our new 



