ASTRONOMY.

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his astronomical tables from the observations of Tycho Brahe , ven-tured to predict the times when Venus and Mercury ' would passbefore the Sun. He even announced a transit of Mercury for1631, and two transits of Venus , the one for 1631, and the otherfor 1761, in an advertisement to Astronomers , published at Leip-sic in 1639, entitled, “ Admonitio ad Astronomos Rerumquecoelestium studiosos de miris rarisque Anni 1631 Pharnomenis,Veneris puta et Mercurii in Solem Incursu.” Kepler had notgiven to his tables that degree of perfection sufficient to announceinfallibly the return of these pha’nomena, the calculations of whichdepend upon arcs so very minute. However, the two transitswhich he predicted took place, though not exactly as he foretoldThat of Mercury' was observed eight days before Kepler ’s deathwhich happened on Nov. 15, 1631, A’. .S’. Venus returns to herinferior conjunction once in one year and 219 days; and werethe plane of the orbit of Venus coincident with that of the Earth ,we should have a central transit of this planet every synodic revo-lution ; but this not being the case, it is clear that at the time of atransit Venus must not only be in her inferior conjunction, butalso so near her node that her geocentric latitude may not exceedthe semidiameter of the Sun , which is about 16 minutes of a degree.The same may be said of Mercury whose synodic revolution is116 days at a mean rate. The first observation that was ever madeof a transit, was by Gassendi, who saw Mercury on the Sun on thejnorningof the 7th of November, 1631. lie has given an accountof it in a letter addressed to Schickardus the same year, and whichis found at the end of his “ Institutio Astronomica,” under thefollowing title: “ Mercurius in Sole visus.” Since his time therehave occurred 17 other transits of this planet, the last of which wasat his ascending node on the 9th of November, 1802. In orderto calculate or observe a transit of Venus or Mercury thoroughly ,the following particulars ought to be carefully ascertained ; viz.the place of the Sun and planet at the ecliptic conjunction ; thelongitude of the planet’s ascending or descending node, and theSuirs distance from it at the inferior conjunction ; the angle of theplanet’s visible path with the ecliptic ; the planet’s relative horarymotion ; the semidiameter of the Sun and planet at the time of thetransit; the planet’s heliocentric and geocentric place at the eclipticconjunction; the apparent or mean time of the ecliptic conjunction ;the time of the nearest approach of the centres of the Sun andplanet; the duration of the transit; and the apparent and geocen-tric nearest approach of the centres. The nature of the above par-ticulars will be best seen by an inspection of the annexed example:

Particulars of the Transit of Mercury of Nov. 9, 1769, (A'. S.)carefully observed at Philadelphia . Eat. of the place 39'56'54"N. Lon. from a mean of a great number of observations on theeclipses of Jupiter ’s satellites, 73° P 45” W. of Greenwich.

The place of O and $ at ecliptic c!.m,17 c 50'41"

The longitude of g’s .8 15 35 29

0’s distance from %’s Q, . 2 f5 12

The d. of g’s visible path with ecliptic. 3 21 18

The horary motion of $ upon ©’s disk. 5 55 j

The semidiameter of © on the day of the transit... 16 10 T ' 5

The semidiameter of $ at the same time. 41g

^’s heliocentric latitude N. at ecliptic . 16 34j^

^’s geocentric latitude N. at the same time. 7 39^%,

Apparent time of ecliptic d at Philadelphia . 5h. 12m.46|s.

Time of the nearestapproach of the cent, of 0 and g 5 1 30

Central semidur. of transit betw een the ext. contacts 2 25 21

Ditto betw een the internal contacts. 2 23 44

Apparent time of external contact at Philadelphia .. 2 36 9

Ditto.internal contact.ditto. 2 37 30

Apparentnearestdistancesof the centres of © and £.455" T | T

Geocentric nearestapproach of the centres of 0 and$ ....451

From observations of these transits the celebrated Dr. Halley liascalculated the different periods in which these phenomena may beexpected to happen. We insert the following periods of Dr. Ilalley,as corrected by M. De La Lande from his N ew Tables of Mercury .The shortest period for the transits of this planet at the ascendingnode, in the month of November, is that of 6 years, 8 days, 18h..39m. or one day more when the year of the preceding transit isbissextile, or the - first after bissextile. When there is a bissextileomitted according to the rule of the Gregorian calendar, as in1800 or 1900 , we must add an extra day. Phis rule must he alsoobserved in the other periods of which we shall speak presently,titer this interval of six years, Mercury will pass 31' 26' more to

the north than he did in the preceding transit. Thus, in orderthat there may be two transits of Mercury at the same node in theinterval of six years, it is necessary, that in the first transit thisplanet should almost touch the most southern part of the solar disk,so that he may in the consecutive transit appear within the Bun,almost at his most northern extremity. The second period is thatof seven years ; and the time of the conjunction is found bv sub-tracting 7 days and 57 minutes, when there are two intercalations,as from 1769 to 1776; but when there is only one leap-year in theseven years, w hich is the case when the year of the preceding tran-sit is bissextile, then 6 days and 57 minutes must be only sub-tracted. At the end of this period Mercury passes 23' 12”' moreto the south than he did at the beginning. This period as well asthe first can only take place at tlie ascending node. The thirdperiod is that of 13 years, 2 days, 17 hours, and 42 minutes; orone day less when four bissextiles intervene, i. e. w hen the pre-ceding transit, is in the third year after bissextile. At the end ofthis period Mercury moves along the Sun ’s di>k in a path 8'14”more to the north than before. 'I bis period is frequent in bothnodes. The fourth period is that of 46 years and 4h. 42m. whenthere have been 12 intercalations in the interval, wh'ch takes placewhen the first transit is in the second or third year alter loap-vear:otherwise one flay more must be added. Mercury at the end ofthis period parses only 1' 35" more to the north of the first transit’spath. The fifth period is that of 217 years, and Ch. 11m. if thereare 54 intercalations in the interval, i. e. if the preceding transit isin a year w hich precedes bissextile, as from l631tol848. Otherw isea day more must be added. At the end of this period Mercury ’spath over the Sun is only six seconds more to the south Ilian"atthe beginning according to the tables of De La Lande. The sixthperiod is that of 263 Julian years, 1 day, 11 hours, as from 1631to 1894. A day more must be added, it the year of the precedingtransit he bissextile; i.c. when there are only 65 intercalations inthe interval. The omission of two intercalations in 1700 and 1800,according to the Gregorian calendar, will make the period thatbegins w ith the transits of 1605, 1618, or lfc-31, of 263 years, 3 (lavs,11 hours; and the omission of an intercalation in 1900 will makethe period that begins with the transits of 1644, 1651, 1661, See.of 263 years, 4 days, 11 hours. After this interval Mercury passesover the Sun in a path 1'40" more to the north than at his preced-ing transit. According to Dr. Ilalley, the quantity that Mercury is to the north at the end of this period is only 10". In the tran-sits of the month of May, at the descending node, the periods ofsix and seven years cannot take place, because this planet is muchnearer to the Earth than in the other node ; and, consequently,his geocentric latitude at an equal distance from the node will beconsiderably greater ; and there cannot be a transit if the geocen-tric latitude at the inferior conjunction be greater than the solarsemidiameter; hence, if the distance of the planet from the de-scending node be greater than 3 degrees at the time of the inferiorconjunction, there cannot be a transit of Mercury at that node ;but there may be a transit of Mercury at the ascending node, if thedistance exceed not five degrees. The shortest period tor the re-turn of Mercury upon the solar disk, in the month of May, isthat of 13 Julian years, 3 days, 7h. 53m. and a day less if there befour intercalations. Mercury passes more to the south 17'7” atthe end of this period. The second period is that of 33 Julianyears; and the moment of the conjunction is had by subtracting2d. Oh. 52m. or a day more, if the year of the preceding transit bethe third after bissextile, or if the year of the subsequent transit bebissextile, as from 1707 to 1740. At the end of this period Mer-cury passes 13'46" more to the north. The third period is that of46 years with 12 intercalations, and moreoverOh. 40in. Mercury passes 3'I to the south at the end of this period, as from 1661 to1707. The fourth period is that of 217 years and 2 hours, whenthere are 54 intercalations in this interval ; otherwise a day moremust be added. Mercury , at the end of this period is 13” to thenorth, as from 1661 to 1878. The fifth period is that of 263Julian years, one day, and nine hours, with 66 intercalations.At the end of this period, Mercury passes 3 minutes of a degree tothe south. The periods of 217 and 263 years are so exact, as al-most to be sufficient to determine precisely the future transits bvthe means of those calculated in the follow ing table. N. B. '['hosetransits marked with 011 'e asterisk, are those that are only partlyvisible in England; but those marked with two, are visible herefrom beginning to end.

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