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 difficulties. Simultaneous observations are difficult or impossible to obtain. 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 positions 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.
