« PředchozíPokračovat »
the capitol at Washington, the University of Virginia, Philadelphia, or Boston, the distance of which from the meridian of Greenwich is supposed to be correctly known. The longitude of the Capitol is the mean of the results, deduced from the observations on the annular eclipses of 1791, 1811, and 1831, and has recently been confirmed by the editor, by comparing it by chronometers with the University of Virginia and the city of Philadelphia. The unfortunate adoption, in the construction of several maps of this country, of the longitude of the Capitol (5h. 7' 42"), reported by an individual acting under authority of a Resolve of Congress, has caused an error of 6 minutes of a degree therein. Since this table went to press, the position of several places in Massachusetts and New York has been determined by the editor, the publication of which must be deferred until another year.
In the arrangement of the Calendar pages there is no alteration from that in the Almanac for 1833.
In the computation of the rising and setting of the Sun, two corrections have been introduced into the Almanac for this year, for the first time. These corrections are, Ist, for the effect of refraction in causing him to appear above the sensible horizon sooner in the morning and later in the afternoon, than he actually is, and 2dly, for the interval between the rising or setting of his centre and of his highest point; the instant of the appearance or disappearance of this point, and not (as heretofore) of his centre, being considered the time of his rising or setting. So that at the time indicated in the Calendar pages, as that of sunrise or sunset, his centre is 90° 50' from the zenith; the semidiameter being about 16' and the horizontal refraction 34'.
The amount of these corrections varies at every place, with the season of the year, and is different in different latitudes. At Boston, when greatest, they lengthen the interval between sunrise and sunset about 12 minutes; at New Orleans, nearly 9.
The setting of the Moon is given from new moon to full, and the rising from full moon to new; the letters M. A. m. a., found in these columns and in other parts of the Almanac, are used to denote Morning and Afternoon.
The time of the Phases of the Moon is computed for the meridian of Washington, but may be readily reduced to that for any other meridian, by adding or substracting the difference of the longitude, according as the same is east or west of that city. The time of the moon's southing is computed for the same meridian. The variation, however, even in a remote part of the United States, will be inconsiderable.
The time of High Water is corrected for the difference of the Right Ascension of the Sun and Moon, and the distance of the Moon from the Earth. The time of the tide immediately preceding the southing of the moon, only, having been given, it should be corrected by the addition of half the difference when the time of the other tide is required.
The Planets are placed in the order in which they pass the meridian on the first day of each month, and their declinations are computed for the moment of their passage over the meridian of Washington.
The Ephemeris of the Sun (pages 52 to 57) is partly taken from the celebrated Almanac of Professor Encke and partly from the English Nautical Almanac; now for the first time truly an“ Astronomical Ephemeris,” and worthy of the great nation under whose auspices it appears.
In ours, will be found, the Sun's Semidiameter, Horizontal Parallax, and Declination, the time (mean, which, by the addition of 0.19", will be converted into sidereal) occupied by the Semidiameter in culminating or passing the meridian, the Equation or reduction of apparent to mean time, to be applied to apparent time in the manner indicated, the Side. real time, and the Obliquity of the Ecliptic. The epoch of all is noon, mean time, of the meridian of Greenwich.
The Table of Refractions (pages 58, 59) is that computed on principles explained by Dr. Young, and is recommended by its great simplicity ; moreover, it is said to agree as closely as any other with the latest observations; nevertheless, had not Professor Bessel's new Table required the use of logarithms, it would have been preferred. The elements of the
eclipses (page 60) were computed from the Berlin Jahrbuch, and reduced to the meridian of Greenwich by considering the Longitude of Berlin 53m. 35.5s. The solar elements were corrected for the second differences; those of the Moon, at the time of the eclipses of January 9th, June 7th and 21st, and December 15th, for the second and third; but in those of the eclipse of the 30th of November, corrections were introduced for the differences of the fourth order.
The Tables used by the computers of the Jahrbuch, are Bessel's, for the Sun, and Burckhardt's, for the Moon.
All the calculations in this Almanac have been adapted to mean solar time, or that time which should be indicated by a well regulated clock. On account of the eccentricity of the Earth's orbit and the inclination of the Ecliptic to the Equator, the motion of the Earth in Right Ascension is not uniform, and consequently the solar* days are not equal, about half being more, and about as many less, than 24 hours, and requiring a clock indicating apparent or solar time, to be frequently adjusted. To avoid this inconvenience, the fiction of mean time has been invented; which has already come into very general use and probably will soon supersede the other. It derives its name from the circumstance, that the length of a mean solar day, hour, &c , is the mean or average length of all the apparent solar days, hours, &c., in a tropical year.
The greatest difference between Mean and Apparent Time occurs on the 3d of November, viz. 16m. 16.s., and the equation then being subtractive from apparent time, the instant the Sun's centre is on the meridian or bears exactly south, a clock regulated to mean time should indicate 11h. 43m 43.4s. On the 11th of February is the greatest additive equation, when the time of noon by the clock should be 14m. 34s. after 12.
But mean time can be easily reduced to apparent, by applying the equation (pages 52 to 57) on the day in question, in a manner directly the reverse of that indicated therein.
The most interesting of the Astronomical phenomena happening in the year 1835, and visible in the United States, are the Occultation of Jupiter in April, the return of Halley's Comet (otherwise called the Comet of 1759) to its perihelion, on the 4th of November, and, on the 7th of the same month, the transit of Mercury over the disc of the Sun.
A communication of any observations that may be made on the total eclipse of the Sun of November of the coming year, together with the correct Latitude of the place of observation, will be thankfully received by the Editor of the Astronomical department, and the Longitude of the place be thence deduced.
R. T. PAINE. 16 Newton Place, Boston,
September 25th, 1833.
* A solar day is the interval between the instant his centre is on the meridian of any place, to the instant of his return to the same situation.