ECLIPSES OF JUPITER'S SATELLITES, P. S. T.

(Off left-hand limb as seen in an inverting telescope to May 27th, then off right.)

NOTICES FROM THE LICK OBSERVATORY.*

USE OF

PREPARED BY MEMBERS OF THE STAFF.

THE CROSSLEY REFLECTOR FOR PHOTOGRAPHIC
MEASUREMENTS OF POSITION.

Although the observation of asteroids is not intended to be a regular part of the work of the Lick Observatory, a good many observations of these bodies have been made here during the past year. All the Watson asteroids have been observed for the National Academy of Sciences, and in the course of this work a number of new asteroids have been discovered and regularly observed.

It sometimes happens that an asteroid cannot be found readily with visual or with small photographic telescopes. It may be situated in the Milky Way, where the number of stars is confusing, or it may be faint or moving very slowly. If found, there may be no catalogue-stars near enough to it for purposes of comparison.

The

In such cases the Crossley reflector has furnished a convenient means for both finding the asteroid and determining its position. The method of observation is as follows: The field having been identified by means of a DM. chart, an exposure of half an hour is given to the photographic plate, in the usual manner. slide is then closed, the guiding eye-piece of the double-slide plate-holder is slipped along two or three hundredths of an inch. and again clamped, and the guiding-star is brought to the new position of the cross-wires. Another exposure of (say) two minutes is then given, the times of beginning and end being accurately noted. Finally, the slide is partly drawn, the drivingclock stopped, and stars are allowed to trail over the lower third of the plate for several minutes.

On the resulting negative, all the star-images are double. The stronger images serve for purposes of identification, and the

* Lick Astronomical Department of the University of California.

weaker images for exact measurement. The diameter of the smallest star-discs should not exceed 1′′.5 or 2".0. The corresponding images of the asteroid are respectively a trail, and an image which is not distinguishable from a disc, and which, in the subsequent measurement of the plate, is referred to the shortexposure images of the stars. The trails serve for the accurate

orientation of the plate.

On a photograph of the asteroid (439) Ohio, made on the night of December 27, 1899, and measured by Mr. CODDINGTON, there are eleven catalogue-stars. The mean position of the asteroid, at the mean time of exposure, 13h 10m 35′ P. S. T., is = = R. A. 7 27 42.221; Decl. 6° 9′ 26′′.79.

Comparison of the mean with the individual measures gives the following residuals (Mean – Obs.);

The probable error of the mean is, therefore, in R. A. ± 0.010; in Decl. o". 12. It will be seen that this result is comparable with the best measurement by visual methods,

J. E. K.

THE CROCKER-LICK OBSERVATORY ECLIPSE EXPEDITION. By the generosity of Mr. WILLIAM H. CROCKER, of San Francisco, the Lick Observatory will be able to send a party to Georgia, to observe the total solar eclipse of May 28th. Only two observers, Messrs. W. W. CAMPBELL and C. D. PERRINE, will be sent out from the Observatory; but several European astronomers have expressed a desire to join the party, and similar requests have also been received from astronomers connected with American colleges which do not intend to send out expeditions of their own.

I take this opportunity to acknowledge the indebtedness of the Lick Observatory to Mr. CROCKER for his generous assist

ance.

It is especially gratifying to the members of the Observatory staff that this practical token of appreciation and confidence comes from a life member of the Astronomical Society of the Pacific, and a prominent citizen of our own State.

The instrumental equipment of the expedition will be quite complete. The principal instrument for photographing the corona will be the five-inch telescope of forty feet focal length, used by the Lick Observatory parties in South America and India. For photographing the corona on a smaller scale there will be several cameras of from five to six inches aperture and others of smaller size. One slit spectrograph, and two objective spectrographs, arranged to give a continuous record of the changing spectrum at the beginning and end of totality, are also included in the equipment. Observations of contacts will be made.

Any observers having experience in astronomical or physical work, who wish to join the party at their own expense, like the gentlemen referred to farther above, are invited to communicate with the Director of the Lick Observatory before April 20th, and after that date with Professor W. W. CAMPBELL, Lick Observatory Eclipse Expedition, Atlanta, Georgia.

JAMES E. KEELER.

ELEMENTS OF COMET a 1900 (GIACOBINI).

This comet was discovered by GIACOBINI at Nice, on January 31st, the position of discovery being R. A. 2h 57m 44′, Decl. -7° 55'. The first accurate position received at the Lick Observatory was that of February 3d, obtained by JAVELLE at Nice. After the receipt of the announcement of discovery the weather at Mount Hamilton was stormy, and only cleared when the Moon was too near the comet's place to warrant a search. The first observation obtained here was on February 16th, and the next on the 21st. From these three observations I have deduced the following parabolic elements: