exposure of two minutes is sufficient. In such a case it may be necessary to make the telescope follow on Eros and not on a star. All the stars will then appear as short trails which are easily bisected. If the motion of Eros is large, its position with relation to the comparison stars will differ greatly when east and when west of the meridian. Moreover, it will be necessary to measure the total motion and after subtracting the large motion of Eros determine the small remaining parallax. During the latter part of January Eros culminates at nearly the same time on successive nights and will thus be favorably situated for observations west of the meridian for several weeks. The path of Eros has a loop extending over about 13° in right ascension and 20° in declination, and with a center at about R. A. 2 5m, Dec. +51°. The point of crossing is at R. A. 2h 21m.9, Dec.+34°25′ (1855). It is, therefore, not far from the stars +34°447, magn. 9.3, and +34 ̊448, magn. 8.0. Eros will pass through the point of crossing on September 3, 1900, and again on January 8, 1901. Photometric observations, if made on these dates, will have especial value, since the same comparison stars can be used for both.

A photograph of Eros was obtained on September 6, 1898, with the 11-inch Draper telescope, whose focal length is 153 inches. Stars of the ninth magnitude are readily photographed with this instrument in 5 seconds. The exposure was 10 minutes, the daily motion 18', and the computed magnitude 12.1. Allowing for the difference in motion it would be equally difficult to photograph Eros on this date and on September 17, 1900.

HARVARD COLLEGE OBSERVATORY Circular No. 49.
February 14, 1900.

EDWARD C. PICKERING

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SOLAR CORONA, MAY 28, 1900.

Dolbeer Eclipse Expedition, Siloam, Ga. Photographed by C. Burckhalter,
Chabot Observatory, Berkley, Cal.

POPULAR ASTRONOMY No. 77.

د وتر

Vol. VIII. No. 7.

AUGUST and SEPTEMBER, 1900.

Whole No. 77.

FEASIBILITY OF OBTAINING THE SOLAR PARALLAX FROM
SIMULTANEOUS MICROMETER OBSERVATIONS
OF EROS.

The unusual opportunity offered by the coming opposition of Eros for the determination of the solar parallax has been noticed in several journals, but the apparent facility with which simultaneous observations with filar micrometers at widely separated stations could be utilized has received little or no attention, the photographic method having been given the preference to the exclusion of the former. Although the parallactic factor, by suitable selection of stations may be made considerably larger, there are some inherent difficulties which do not occur in the method here proposed.

The high declination of the planet makes it possible to secure simultaneous observations at all the European observatories west of Pulkowa, and all the American observatories east of Denver, which are provided with sufficiently powerful telescopes. The following scheme in tabular form gives the Greenwich mean time at which the planet will be simultaneously visible, and at sufficient altitude to enable good observations to be made. At the most favorable time with reference to the parallactic factor, however, as well as at the beginning and end of the proposed series, daylight interferes with some of the observatories situated at the ends of the line, and it has been necessary to select two instants, one of which includes all except the extreme eastern, the other all except the extreme western.

The parallactic displacement varies from approximately 17".0 to 30".0 in the case of Pulkowa-Washington, to 14".9 to 24".5 in the case of Greenwich-Washington. The inclusion of Denver somewhat increases these quantities.

The principal difficulty in the proposed scheme is the selection with certainty of the same star or stars by the different observers.

NOTE. This circular was prepared by S. J. Brown, Astronomical Director of the United States Naval Observatory, and it has been sent to the Astronomers of the United States who are willing to coöperate in the observations mentioned, and who are provided with suitable instruments for making them. Director Brown, also prepared a paper on the same subject which was presented before the International Astropholographic Conference held in Paris, July, 1900. Ed.]

The planet will be situated in the Milky Way during the greater part of the time embraced in the series, which will afford many faint stars, but render their proper selection at the time of observation and future identification difficult. Experience shows that in the region of the Milky Way, the field of a telescope pointed at random will contain several stars from the 10th to 12th magnitude within a radius of 6' of arc.

There would seem to be a priori, not much doubt that one or more stars as bright as the 11th magnitude would be found within a radius of 3' of arc, from the true position of the planet as a center. The two or three brightest lying within this distance should be taken, limiting the choice if possible to magnitudes fainter than the 5th.

It will be necessary for the identification of the stars used that a careful sketch map of the field and the region immediately surrounding it, be made at the time of the observation, and that a photograph of the region be taken at about the same time. This would not only serve to identify the stars used, but, in cases where different observers should fail to select the same stars, it would furnish an accurate means of referring the observations to a common origin.

The coöperation of two or more photographic telescopes would obviate entirely the selection of the same reference stars by the different observers, as well as the necessity of adhering closely to the schedule time laid down for the observations, as the motion of the planet could be accurately computed for brief intervals of time thus arising, while the relative places of the stars used could be very accurately measured from the plates. At the same time the positions of the stars could be determined from the plate with sufficient accuracy to furnish the data for computing the coefficients in the equations of condition. The Naval Observatory has no suitable photographic telescope for this purpose, and with the exception of Harvard College Observatory there is none in this country. Except for this reason there would be no gain in resorting to micrometric observations. It will therefore be essensial to the complete success of the proposed scheme that one or more of the European observatories should take this portion of the work.

On account of the rapid motion of Eros, which makes observations of position angle and distance very troublesome, and also the ease and accuracy with which differences in its right ascension and declination can be interpolated, the measures should be