1. Whether the star undergoes any change of light, of color, or of motion, on its immediate approach to the edge of the Moon. 2. Whether it appears to be projected on the Moon's disc, and if so, for how long a time. 3. Whether the dark limb of the Moon be distinctly visible, and well defined, at the time of the phenomenon. 4. Whether the star, on its emersion, appears on the Moon's disc, or emerges quite clear of the Moon's border. Between August 1829 and July 1830 six occultations of this star were observed in Boston, and in each of them, when the Immersion or Emersion took place on the dark side of the Moon, it appeared to be instantaneous, and when on the enlightened side, the star usually became so tremulous, near the Moon's edge, as to cause an uncertainty of one or two seconds in the time of its taking place; but in two instances, viz. on the 28th of March last and 16th of July, the star did actually appear projected for the space of between one and two seconds on the lunar disc, or as if about to pass between the Moon and the Earth; its red color remained unchanged, but its light was very much more brilliant than usual. It will be noticed, that the occultations of the planets and of the stars a 8 and a (excepting that of a 8 on the 9th of June, or day of new Moon) which take place whilst the Sun is above the horizon, only have been computed; the occultations of the other stars have been neglected, as they will be rendered invisible by the light of the Sun. In the computation of the occultations last year, the diameter of the Moon was reduced four seconds for inflexion, according to the theory of Duséjour; this year, this reduction has been altogether neglected. The elements of the eclipses, in the Appendix, are given for mean solar time of the meridian of Greenwich, and of the occultations in mean solar of that of Berlin, which is Oh. 53m. 33.6s. East of Greenwich, or 6h. 1m. 15.6s. East of Washington. When it is desired to convert mean into apparent time, the equation, in these elements, must be applied with the sign prefixed to it; but when apparent is to be converted into mean, the sign must be reversed. No sign is prefixed to the hourly motion of the Moon in Longitude, of the Sun in Longitude or Right Ascension (A. R.), or of Sidereal time, as it is always +. For the accurate calculation of the time of the phases of a solar eclipse or occultation, at any place, the latitude of the place, and the equatorial parallax of the Moon, must be diminished for the ellipticity, or flattening at the poles, of the Earth; which, though not precisely determined, is generally supposed to be about one three-hundredth; the reductions for this quantity will be found in the 38th table of the sixth stereotype edition of the "New American Practical Navigator," or they may be computed by the following formulæ. Let L be the latitude and R the reduction to the geocentric latitude, then log. cotang. (L-R) = 0,0029001 + log. cotang. L. The reduction of equatorial parallax (57' for example) may be found thus, 5.7"-5.7" cos. L. The reduction of the latitude is nothing at the Equator and the Poles, and greatest in latitude 45°, where it is - 11' 28.7". The reduction of the parallax is also nothing at the Equator, but greatest at the Poles, where it is one three-hundredth of the whole parallax; in Lat. 45° it is half that quantity. The elements of the eclipses, with the exception of that of February 12th, and of the occultations, with the exception of the places of the stars, were computed from the Berlin Astronomisches Jahrbuch (Astronomical Year 36 ECLIPSES OF THE SATELLITES OF JUPITER. Book) for 1831, edited by the celebrated Encke, a work far superior, both as to matter and arrangement, to any thing of the kind hitherto published. The places of all the stars, but a 8 and a , were computed from Mr. Baily's Catalogue of Zodaical Stars, which was taken from the Catalogue recently published by the Astronomical Society of London, and were also carefully compared with their places in the Catalogue in the Appendix to the first volume of Dr. Pearson's Practical Astronomy. The longitude and latitude of Aldebaran and Regulus are the mean of the determinations at the astronomical observatories of Greenwich and Konigsberg. Prof. Bessel's determination of the Obliquity of the Ecliptic, and the correction of the mean place of the Sun and Stars for the Aberration of Light and Nutation of the Earth's axis, have been invariably used. The aberration of the planets was calculated by the formulæ in Vol. III, p. 106, of Delambre's Astronomy. ECLIPSES OF THE SATELLITES OF JUPITER IN 1831, Visible throughout, or in some part of, the United States; the Phases of which are expressed in Mean Solar Time for the Meridian of Washington, reckoned according to the manner of Astronomers; who begin the Day at the Noon of the Civil Day, and count the Hours up to 24, or to the succeeding Noon, when another Day is commenced. The eclipses before the opposition of Jupiter on the 10th of August will take place on the west side of the planet, and afterwards on the east. The Immersions only, of the first and second Satellites, will be visible before the opposition, and the Emersions only, afterwards; but both the phenomena of the same eclipse of the two outer Satellites can sometimes be seen. The eclipses take place farthest from the body of Jupiter when he is in quadrature, and nearest when in opposition or conjunction; but for some weeks before and after he is in the latter position, the eclipses cannot be observed, the Planet and his Satellites being rendered invisible by the light of the Sun. Eclipses of these Satellites, of the first and second especially, are very useful for determining to a very considerable degree of accuracy the longitude of any place; which, although not so exact as that obtained by an observed occultation of a star by the Moon, is deduced without the long and fatiguing calculation necessary for obtaining it by the latter method. They have likewise the additional advantage of being of very frequent Occurrence. Until very recently, it was generally supposed, that these eclipses could not be observed at sea on account of the motion of the vessel; but an officer in the English Navy has lately shown this opinion to be erroneous; he having succeeded in observing their phenomena from a ship, with a very considerable degree of accuracy. To determine the time at which either of the preceding eclipses will take place, on any other meridian than that of Washington, it is merely necessary to add four minutes for every degree of longitude less than 76° 55′ 30′′, 38 POSITION AND MAGNITUDE OF THE RINGS OF SATURN. and subtract the same quantity for every degree greater, and in proportion for a part of a degree. For Boston, add 23m. 25s.; for New York, 11m. 38s. For Charleston, subtract 11m. 30s.; for Cincinnati, 29m. 463.; for New Orleans, 52m. 54s. Position and Magnitude of the Rings of Saturn, according to Bessel and Struve, for every Fortieth Day in the Year. p. Angle between the semiconjugate axis of the ring ellipse, with the circle of declination; positive when east, negative when west. 1. Angle of elevation of the earth above the plane of the rings, as seen from Saturn, positive when north, negative when south. a. Semitransverse axis of the ring ellipse. b. Semiconjugate axis; positive, when the northern surface of the rings is visible; negative, when the southern. It has been recently ascertained, that Saturn is not placed exactly in the centre of the rings. This singular circumstance was first perceived by M. Schwalz, of Dessau; but for some time was considered an optical illusion, occasioned by the shadow of the planet upon the ring. The question was settled by Prof. Struve, with the celebrated telescope by Fraunhofer, at Dorpat; who ascertained that the rings are actually eccentric. This eccentricity cannot, however, be perceived but by the assistance of the very best telescopes. The planet Mercury will set after the Sun until the 27th of January, then rise before him until the 5th of April, then set after him until the 25th of May, then rise before him until the 20th of July, then set after him until the 25th of September, then rise before him until the 13th of November, then set after him to the 11th of January, 1832. This planet cannot be easily seen in any other position than when at, or very near, its greatest elongation from the Sun, or when apparently passing over the Sun's disc, a phenomenon of rare occurrence, but which will actually take place on the 5th of May, 1832. The elongations take place, this year, January 10th (elong. 18° 58'), February 20th (el. 26° 40'), May 3d (el. 21° 3′), June 20th (el. 22° 37'), August 31st (el. 27° 11'), October 12th (el. 18° 5′), December 25th (el. 19° 48'); but, in the present year, the following periods will be the most favorable, in the United States, for observing the planet, as during them it will not only be at or near its greatest apparent distance from the Sun, but will be nearer the elevated pole, and consequently will remain longer above the horizon. Jan. 1st to Jan. 19th, in the evening after sunset, bearing W. 20° S. April 22d to May 16th, 66 66 66 W. 24 N. HEIGHT OF THE GREATEST OR SPRING TIDES IN 1831. 39 66 66 morning bef. sunrise, Oct. 4th to Oct. 25th, E. I S. Dec. 22d to Dec. 31st, W. 23 S. Venus will set after the Sun until the 8th of October, then rise before him until the 28th of July, 1832. Its greatest eastern elongation (elong. 45° 43′). will take place on the 30th of July, and its greatest western (46° 55') on the 19th of December; but it will be brightest as evening star on the 20th of August, and as morning star on the 30th of November, about which times the planet can be readily seen whilst the Sun is above the horizon. Mars will set after the Sun until the 24th of September, then rise before him until November 20th, 1832. Vesta will set after the Sun until the 1st of June, then rise before him during the remainder of the year. Pallas will rise before the Sun until the 17th of July, then set after him through the year. Juno will set after the Sun until the 1st of June, then rise before him through the year. Ceres will rise before the Sun until the 9th of August, then set after him through the year. Jupiter will set after the Sun until the 20th of January, then rise before him until the 10th of August, then set after him until February 24th, 1832. Saturn will rise before the Sun until the 17th of February, then set after him until the 29th of August, then rise before him until March 2d, 1832. Uranus will set after the Sun until the 30th of January, then rise before him until the 5th of August, then set after him until February 4th, 1832. On the 21st of March this planet and Jupiter will come into conjunction; at which time their distance will be very small, Uranus being 6' South of . A conjunction of Jupiter and Uranus is a phenomenon of rare occurrence, happening only once in about fourteen years. The inferior planets, or all but Mercury and Venus, will appear brightest when nearest to the earth, that is, when in opposition to the Sun. The oppositions in 1831 will take place as follows, viz. of Saturn, Feb. 17th; of Pallas, July 22d; of Ceres, Aug. 4th; of Uranus, Aug. 5th; of Jupiter, Aug. 10th; but Mars, Vesta, and Juno will not be in opposition this year. HEIGHT OF THE GREATEST OR SPRING TIDES IN 1831, Computed by the formula of La Place (Mécanique Céleste, vol. II. p. 289.) 27th, 9 A. Feb. 12th, 0 A. 26th, 0 A. March 14th, 1M. 0.94 66 0.87 New Moon July 9th, 9 M. 0.96 66 24th, 4 A. 0.81 Aug. 7th, 5 A. 0.98 0.95 Full 66 66 23d, 5 M. 0.92 1.07 New Full 0.91 Full 28th, 3 M. New April 12th, 11 M. Full 66 26th, 7 A. New 66 May 11th, 7 A. Full 66 26th, 11 M. New 66 June 10th, 2 M. Full 66 25th, 2 M. The unit of altitude, is the altitude of the tide which happens about a day and a half after the time of New or Full Moon, the Sun and Moon being, at the moment of 6 or 8, at their mean distance from the Earth, and in the plane of the equator. 66 19th, 0 M. 1.03 Sept. 6th, 3 M. 0.99 66 66 21st, 5 A. 1.03 66 |