REDUCTION OF THE LONGITUDE OF VAN RENSSELAER HARBOR OBSERVATORY.

From observations of the moon and the moon culminating stars.

RECAPITULATION OF RESULTS, FROM MOON CULMINATIONS, OF THE LONGITUDE OF VAN RENSSELAER

The value of December 8 must be rejected on the ground of imperfect transits; it is also thrown out by Peirce's Criterion; if included it would make the final result 13s greater.

The weighted mean from nine observations is 4h 43m 34 +13. If we combine the results according to the moon's limb, we find:—

Record and Reduction of the Occultations and Eclipse, observed at Van
Rensselaer Harbor.

The observations of occultations and of an eclipse were made at the Winter Quarters in latitude 78° 37′ 04′′, and in approximate west longitude 4h 43m 323.

In the following record, the times are given by chronometer, uncorrected for error and rate, in the two accounts of it in Appendix No. IX., second volume of the Narrative, pages 398 and 399, and in No. 1017 of the Astronomische Nachrichten, pages 135 and 136, the time is mean local time, as made out by Mr. Sonntag.

Note.-Moon's limb much undulating. The telescopes were one of English make and one by Fraunhofer, each of thirty-inch focal length. At immersion the time was noted when the last point of Saturn's ring disappeared behind the moon's limb; at the emersion, the time is given when the last point of the ring parted from the moon's limb. Chronometer comparisons :

Occultation of Saturn. January 8, 1854.

Immersion at 10h 10m 30s

Emersion at 11 07 06.5

Pocket chronometer. A. Sonntag, observer.

Doubtful, perhaps obscured by a cloud.
Pocket chronometer. A. Sonntag, observer.

Moment when the last point of Saturn's ring parted from the moon's limb.

For points of contact see note to the occultation of the 12th of December. Chronometer comparison Pocket chronometer 10h 18m 308.0

No. 2143 10h 45m

Last contact of ring. The moon's limb was much undulating.

The sky was very clear. Thermometer at immersion -53° (corrected temp. =— -58°.2), at the emersion -52° (corrected temp. =—54°.6).

Occultation of Mars. February 13, 1854.

Immersion, first contact at 1h 04m 59s.0 Pocket chronometer. A. Sonntag, observer.

66

Pocket chronometer. A. Sonntag, observer.

Note.-Moon near the horizon, limb much undulating. Immersion reliable, emersion doubtful; perhaps too late. The planet reappeared at a different place than expected.

The observations for time, for the above occultations, were made with the eighteen inch transit instrument.

Observation of the solar eclipse. May 15, 1855.

The original entry of the observations of this eclipse I could not find: the following has been copied from the published account in the Narrative:—

Altitude of the sun at beginning 10° 17', at ending 8°. The time was obtained from observations of equal altitudes of the sun.

Reduction of the preceding Observations for Longitude.

Converting the chronometer error on sidereal time to its equivalent on mean time, we have, from the preceding transit reductions, the following results:

The differences in the values of E are due to the imperfect transits arising from the difficulty of obtaining a reliable azimuthal determination, and since the rate of the chronometer has been found by frequent comparisons with the other mean time chronometers to be uniform, I have preferred to use the mean E from the

several determinations. The next reliable time observations are on February 20, 1854, which gave E 4h 42m 51.4 at 7h 56m (mean time); the daily rate is accordingly — 0.42 (gaining), and the chronometer error on mean time for the several occultations becomes:

That the rate of the pocket chronometer during the above times was uniform is proved by the comparisons with the other chronometers; between December 12 and January 8, 2143 had a gaining rate on the pocket chronometer of 18.9, between the latter date and February 5 it was 2.3, and between the last date and February 13 it was 2.2. The true rate of 2143 is, therefore, — 2.2-0.4 —— 2o.6. The following table contains the observed times of the several phases of the occultations of Saturn and ring:

The method of reduction used is that of finding the time of true conjunction in right ascension, and the notation the same as given in vol. II. of Sawitsch's Treatise on Practical Astronomy (German edition, by Dr. W. C. Goetze, Hamburg, 1851). Hansen's interpolation formula was used. The tabular Nautical Almanac places and data have been corrected, when practicable, from the Greenwich observations.

LONGITUDE FROM THE OBSERVATIONS OF THE OCCULTATION OF SATURN'S RING. December 12, 1853.

From the Greenwich Nautical Almanac, as corrected from the Greenwich observations :—

The following quantities were computed for the purpose of plotting the phenomenon to obtain the points of contact on the ring, and the required radial correction to the moon's semidiameter to refer the point on the ring to the centre of Saturn.

.+ 9 31.6

At emersion. + 4' 12'.5 +14 23.5

Relative approximate difference in AR (reduced to a great circle) -11' 06".3

From the Greenwich N. A.

By means of the above numbers, the moon's path relative to Saturn was plotted, and the correction to refer the points of contact to the centre was obtained graphically. See above diagram, showing the position of the planet and ring, and contacts of the moon's limb.