650 POPULAR SCIENCE MONTHLY. 



east, and another twelve hours later, when it is about to set in the 

 west. In the meantime, the observer will have been carried to a 

 position 8,000 miles removed from that which he occupied in the 

 morning. Each of the three methods has certain objections and dif- 

 ficulties. Simultaneous observations are difficult or impossible to ob- 

 tain. Between the different observations both earth and Eros are 

 sweeping along in their orbits, and this introduces complications which 

 must be allowed for with great care. Also the size of the earth is not 

 perfectly known, nor the distance apart of any two stations upon its 

 surface, though the error introduced from this cause is very small. 



For the determination of the position of Eros on each day during 

 opposition, as recommended by the Paris committee, the precise posi- 

 tions of very many stars must be known. A few of these have already 

 been determined, but most of them must be measured at the present 

 time. For this purpose the positions of several hundred stars will 

 be determined and the highest precision at different observatories with 

 the meridian circle, and, from these as standards, many hundreds more, 

 by photographs. For the positions of Eros itself with relation to these 

 stars, no doubt the micrometer, the heliometer and the photograph will 

 be used, and a comparison of the results by these three instruments will 

 be of the greatest interest. 



Observations of Eros, made during the recent opposition, or in the 

 future, will doubtless give the most exact determination of the solar 

 parallax possible by the geometrical method, applied to any known 

 member of the solar system. Indeed, Eros, at the most favorable times, 

 is perhaps as good an object as can be desired. If it came still nearer 

 to the earth, its motion would doubtless be more rapid, so that little 

 would be gained. According to Professor Newcomb, Eros comes 

 'about as near to us as observations can advantageously be made.' 

 Nevertheless, it is doubtful whether any geometrical determination of 

 the solar parallax will ever be accepted as final. When the astronomical 

 world was preparing to observe the transit of Venus in 1874, Leverrier 

 refused to take any part in it, declaring that the determination by 

 gravitational means would make all geometrical methods of no further 

 value. This may be true for the future, but it will not lessen, for the 

 present, at least, the high value of the determinations now going on. 



The solar parallax is about 8". 80, correct within approximately 

 0".01. That is, the distance of the sun is about 92,897,000 miles, 

 correct within 100,000 or 150,000 miles. It is difficult to appreciate 

 an angle of 0".01, within which limit the determination must come 

 to be of value. A foot rule forms an angle of 0".01, when placed 

 at a distance of 20,626,481 feet, or over 3,900 miles. If the present 

 work shall reduce the margin of doubt, astronomers will be well paid 

 for their efforts. 



