The reduction of the data was accomplished in 1883, but as was expected, a necessity for reobservation appeared in certain cases. This work was done between Oct. 9, 1883 and Aug. 9, 1884. The observer from the beginning had been Prof. William A. Rogers. He resigned his position as assistant professor in the observatory in 1886, but continued to serve as editor of the published results. The whole series makes half a dozen or more volumes of the annals. The European supervisors of the general undertaking, well pleased, apparently, with the early instalments of manuscript returns, assigned to Harvard the work of reobservation of another zone, that between 9° 50′ south and 14° 10′ south. This work is still in progress. Each zone comprises about 8000 stars or nearly 17,000 in all.
The publications of the observatory during the present term in the form of annals, and as contributions by members of the corps to various journals of science, have been numerous and extensive. At the beginning of the term but four volumes of annals had been issued, though about an equal number were in some stage of progress in the printers’ hands, publication having gone on slowly from lack of means. At the present time the continuous series of 22 volumes has been issued, excepting the second or supplementary parts in two or three instances. These parts are nearly ready, and the manuscript for about half a dozen more volumes in regular succession has, in part or whole, been given to the printer.
A review of what has been done during the present term in the departments of photometry, spectroscopy and photography will be comprised in the next and closing number of this series.
VII.
Agreeably to the announcement of the annual report of Harvard College Observatory for 1877, as to photometry, a beginning was made by constructing a photometer suitable to be attached to the great telescope. Other photometers have been devised at different times for use independently. One of the earliest was applied during the year beginning Oct. 12, 1877, in measuring the light of all known satellites excepting the two inner ones of Uranus, which are too faint to be discerned, even by the great telescope. The first prolonged observation entered upon was of the eclipses of Jupiter’s satellites.
As there are four satellites and as the plane of their orbits is nearly the same as that of the planet itself, eclipses are frequent. The plan proposed the observation of all these eclipses visible during a revolution of Jupiter around the sun, a period of about 12 years. The work was begun June 23, 1878, and has been regularly pursued. The final result will be of the highest value in that, among its utilities it will permit a new and independent computation to be made of the earth’s distance from the sun, which distance is a prime factor in theoretical astronomy.
Computations hitherto made, based upon data derived from these eclipses, are not authoritative, because of disagreements among different observers using different telescopes, and because of defects in the method of observation.
The director’s report for 1878 says: “Errors of this kind are much lessened by photometric observations of the satellites as they gradually enter or emerge from the shadow of Jupiter, using the planet itself or another satellite as a standard. Each comparison thus obtained gives an independent determination of the time of the eclipse, free from the errors due to the condition of the air or the power of the telescope employed and less likely to be affected by personal equation than the observation of a disappearance or a reappearance. By the ordinary method an observation during twilight can have little value, while good photometric observations may be made as well then as at any other time. It is even possible to make them before sunset.”
In 1879 a work of magnitude was begun—the photometric observation of all stars down to those of the sixth magnitude visible in this latitude. For greater facility, and particularly to avoid loss of time in identifying stars of small magnitude, it was decided to make a new departure in method and in construction of an instrument. The new instrument was called the meridian photometer, and stars were observed by it only when near the meridian. The position of any star being well known, the time of its appearance in the field of the telescope could be foreseen.