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CAL DEPARTMENT.

1

The most remarkable of the phenomena that this year (1834) will happen, is the eclipse of the Sun, on Sunday the thirtieth of November. This is the third of the very uncommon series of five large eclipses, visible to us, in the short term of seven years; the fourth of this series will take place May 15th, 1836, and the last, September 18th, 1838.

The eclipse of the present year will doubtless receive great attention throughout our country. In those places where its magnitude will not exceed eleven digits, much diminution of the light is not to be expected, even at the time of the greatest obscuration; perhaps, however, it may be sufficient to render visible the planet Venus, then about 30 degrees E. S. E. of the Sun, and much nearer the Earth, than usual: nor will the obscuration be very great where the eclipse is almost tatal; since it has been observed, on former occasions, that the uneclipsed part, even when reduced to a mere point, sheds sufficient light to render small objects distinctly visible, and invisible the brightest of the stars. Indeed, on account of the refraction of the Sun's rays by the atmosphere of the Earth, the darkness can hardly with strictness be considered total, even where the Sun is completely shut out from the sight. In the great and remarkable eclipse of June 16th, 1806, when the Sun was totally obscured, at Boston,

for five minutes, as much light remained as is given by the Moon when full; and greater darkness will not probably be experienced, in any place, on the present occasion.

Throughout the United States, however, a great depression of the thermometer, if placed in the sun, will probably be noticed ; and, for some minutes before and after the moment of greatest obscuration, the power of a lens to produce combustion, by condensing the solar rays, will be quite, if not entirely, destroyed. At the time of the Annular eclipse of February 12th, 1831, it was observed by the Editor, that the thermometer in the sun, fell from 72 to 29, and that during the continuance of the ring, no sensible effect was produced by placing its blackened bulb in the focus of a powerful burning-glass.

This Eclipse, it will be seen on tracing the path of the centre, will be total in a small part of the Territory of Arkansas, and of the States of Mississippi, Alabama, Georgia, and South Carolina. The principal places, in which the obscurity will probably be complete, are Charleston, Beaufort, S. C., Savannah, Milledgeville, Tuscaloosa, and Little Rock. The greatest duration of total darkness in any place, will be about 1m. 56s.; at Tuscaloosa, it will be about 1m. 53s. and at Beaufort, lin. 46s.; these places lying very near the central path. At Charleston and Savannah, the duration will be considerably less, the former being situate about forty miles north of this path, the latter about thirty south. The width of the line of total darkness varies in its passage across the Earth, but in the United States will be about one hundred miles. Those of the inhabitants of the Atlantic States, who desire to behold this rare spectacle, the most magnificent and sublime of the phenomena of nature, compared with which even Niagara sinks into mediocrity, will find Beaufort the most eligible place in which to make their observations; and they will not neglect this opportunity when they reflect, that the Moon's shadow will not again, for the space of thirty-five years, pass over any part of the inhabited portion of the United States, or until August 7th, 1869.

As, at the time of the Eclipse of Feb., 1831, much inconvenience

and even injury was sustained from want of care in looking at the Sun without any protection for the eye, or through glass not sufficiently colored, it may be proper to remark, that should the sky, during the continuance of this Eclipse, be clear, one of the very darkest green or red glasses of a sextant, and in default of this, a piece of common window glass, free from veins, and rendered quite black by the smoke of a lamp, only, can be used with safety. If the lustre of the Sun should be diminished by intervening clouds, a lighter shade will be sufficient.

In the computation of the phases of this Eclipse for some of the principal places in the United States (see pages 10 – 15), the semidiameters of the Sun and Moon were reduced 5li for irradiation and inflexion; the quantity indicated by all the observations on the Eclipse of Feb., 1831.

The total eclipse of the Moon of June 21st, and that of Dec. 15th, will be more interesting to the public generally, than to the astronomer.

Occultations of the planets and of stars of not less than the fourth magnitude, will this year be rare. Jupiter will be eclipsed in the morning of September 24th, and Venus, in the southern extremity of the United States, in the afternoon of the 2d of December.

The moment of the Immersion or Emersion of any star, however small, behind, or from, the dark side of the Moon, can be determined with precision ; but if the star is small, great difficulty is experienced in satisfactorily ascertaining it, when the phenomenon takes place on the side that is enlightened: Indeed, it has been found by Professor Struve, even with the assistance of the celebrated telescope in his possession, by Fraunhofer, so nearly impossible, that he recommends measuring with a micrometer the star's distance from the limb of the Moon, some minutes before or after the moment of contact, and when its light is, comparatively, but little diminished by her superior lustre. Those conjunctions, however, of the Moon with stars of less than the fourth magnitude, which may be occultations in some part of the United States, are noted in the Calendar pages by an asterisk, instead of the usual symbol of conjunction.

The catalogue of the eclipses of the Satellites of Jupiter (pages 17 and 18) contains only those visible in some part of the United States. The eclipses before the planet comes into conjunction with the Sun, on the 9th of May, will happen on the east side, then, until the opposition, on the 29th of November, on the west, and afterwards again on the east: between the 9th of May and 29th of November, the Immersions only of the first and second satellites will be visible, and during the remainder of the year, the Emersions only; but both the Immersion and Emersion of the iwo outer satellites can sometimes be seen.

The fourth satellite will not, however, be eclipsed this year, its Latitude, at every opposition, being greater than the planet's semidiameter.

The eclipses take place farthest from the body of Jupiter when 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 satellites being rendered invisible by the superior light of the Sun. As these eclipses appear to take place at the same moment of absolute time in every part of the Earth where they are visible, to determine the approximate time, at which any one in the catalogue will happen in any place in the United States, it is necessary merely to subtract the estimated Longitude of that place from the time of Immersion or Emersion at Greenwich.

In the table of Latitude and Longitude of some of the principal places in the United States (page 24, &c.), will be found the latitude of several, as determined by the editor, by recent observations made by himself; also the longitude of a few, deduced by him from observations made by others on the annular eclipse of February 1831, or as ascer. tained by comparison of the place in question, by chronometers, with 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 Sidereal 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 Jalırbuch, are Bessel's, for the Sun, and Burckhardt's, for the Moon.

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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.s. 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.

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