ASTRONOMY.

3i> 6

of the Earth 's dusk, and consequently is the middle of flic genera!eclipse : the point x is that where the conjunction of the Sun and"Moon is, according to equal time by the. fairies ; and the point yis the- ecliptic conjunction of the Sun and Moon. I ake the Moon 'strue horary motion from the Sun , 27' P'-j, in your compasses, from■the scale C'A, (every divi-ion of which is a minute of a degree)and with that extent make murks along the path of the penumbra'sCentre, and divide each space, frdm mark to mark, into 00 equalparts or.horary minuter, by dots; and set the hours to-entry60th minute in such a manner, that the dot signifying the instantOf New Moon by the Tables, may fall into tin; point r, halfway between the axis of the Moon 's orbit and tin- axis of theecliptic; and then the vest of the dots will shew the points on theEarth 's di-t where the centre of the penumbra is at the instantsdenoted bv them, in its transit over the Earth . Apply one sideof a square to the line of the penumbra's path, and move thesquare oackvvards and forwards, until the other side of it cutsthe same hour and minute (as at in and in') both in the path ofLondon , and iu the path of the penumbra’s centre: and theparticular minute or instant which the square cuts at the samelime in both paths, will be the in-tunl ot the visible conjunctionof tlie Sun and Moon, or middle of the eclipse, at the placefor which tiie construction is made, namely, London , in the pre-sent example ; and this middle time is at 38 minutes past onein the afternoon, according to the diagram, 'l ake the Sun’ssemidiameter, 13' in your compasses, from the schle CA,and setting one foot in the path of London at m, namely at 38minutes past one, with the other foot describe the circle UZ,which represents the Sun’s disk as seen Iron) London at themiddle ot the eclipse. Then take tile Moon ’s semidiameterIT 43", in your compasses from the same scale; and selling onetool in the path of the penumbra’s centre at in, with the otherfoot describe the circle TY for the Moon ’s disk, as seen fromLondon , at tl e middle of the eclipse, or time.of greatest ob-scuration ; and tjie portion of the Sun 's disk which is hid or cutoil' by the Moon ’s will shew the quantity of the eclipse atthat tune ; which quantity may be measured on a line equal tothe yun’s diameter, and divided into twelve equal parts for digits.Lastly, take the semidiameter of the penumbra, 30’ 38', from thescale CA in your compasses, and setting one foot in the line oftiie penumbra’s central path, on the left hand from the axis ofthe ecliptic, direct the other foot toward the path of London ;and carry that extent backwards and forwards, till both the pointsof the compasses fall into the same instants in both paths: andthese instants -will denote the time when the eclipse begins atLondon . Then, do the like on the right hand of the axis of the.ecliptic ; and where the points of the compasses tall into tiie sameinstants in both the paths, they will shew at what time the eclipsevends at London . These trials give 10} minutes past twelve’, noon,for the beginning of the eclipse, 38 minutes least one, for the mid-dle of the eclipse, and 5 minutes past three for the time when theeclipse; ends. The following are the particulars of the* eclipse col-lected into one view, both tor mean aiul apparent time :

© eclipsed, Thursday, Sept. 7, 182,), N. S. in the afternoon.

Kelipfic conjunctionEnd of the eclipse

Mean T ime.

Apparent T

It. m.

h. in.

ipse....

.13 10}

12 12}

1 402.

.1 46j(

1 *•}<*

3 ‘}

Digits eclipsed 10) on Q’s north limb.Sect. IX. Remarks ox Eclipses ix general.

(24S.) In eclip-.es of the Moon , even when she is near ihe centreof the Earth ’s shadow, her body is still visible and appears of atarnished copper-colour. This seems to be occasioned by the lavsof hulit which come from the Sun , and which, passing near theEarth , are inllected from their rectilinear course by our atmos-phere ; so that they enter the Earth ’s conical shadow, and thusproduce that faint illumination on the surface of the Moon , whichsome have supposed to be her own native light. In most solarei lipses, the Moon ’s disk is covered with a taint light, which isattributed to the retlection of the light from the illuminated partof the Earth, and in total eclipses, the Moon ’s limb is seen sur-ffjundcd by a pale circle of light; which some astronomers con-

sider as an indication ot a lunar atmosphere; but others as the at-inosplwre of the Sun ; because it is observed to move equally withthe Sun, but not with the Moon . Eclipses have in all ages greatlyi.tlneted the attention of mankind. ’1 he ignorant and upcrsti-tions have viewed them with tenor, and in (ormer ages they wereoften considered as the forerunners of national calamities. '1 heCh:ne~. , even at tiie present day, upon their appearance, performj I lie most absurd <,nd superstitious ceremonies, although tln-\ are soj far acquainted u ith their nature, as to be able la predict them,j But trae philosophy has taught us, that instead of these appear-ances being portentous of evil to mankind, they may, by properobservations upon them, be mrde of great advantage to thescience.-:, and to some of the arts of life. VVe have aheady'shewn,that, by eclipses of Ihe Moon, the Earth is demonstrated to be aglobular liirure. The longitudes of places on the Earth are alsodetermined by observations on solar and lunar eclipses; as willappear by consulting the articles Geography, Eongitiof,, Na-vigation - , Sec. Eclipses- are also of gregt importance in Chko-noi.oi. v, (which sue,) as by them we use enabled to determine ex-actly the time when events recorded in history happened. Wecannot close this section, without observing, that eclipses happenvery frequently to the satellites of Jupiter; and, as they are ofgreat service in determining the longitude of places on the Earth ,astronomers have been at pains to calculate tables for the eclipsesof these satellites, by their primary, for the satellites themselveshave hever been observed to eclipse one another. See Geo-graphy and Longitude.

(249.) The principal instruments for making astronomicalobservations, area pendulum clock, telescopes, micrometers, quad-rants, the astronomical sector, the equatorial, &c Lor an accountof which see those articles.

INDEX.

Etymology and Definition of Astronomy, I.

History op Astronomy, (2 - 37.)

Astronomy 7 , a science of the highest antiquity, 2.

A considerable degree of astronomical knowledge ascribed to Setiiand his posterity by Josephus , 2,3.

The period of 6()0 years, in use among the Antediluvians, a strik-ing proof of it, in the.opinion of Cassini, 4.

The origin of Astronomy and Astrology, fixed by most authors inChaldea or Egypt , 5.

'Die tower of Babel, anti the pyramids of Egypt , supposed to beoriginally intended lor astronomical observatories, ibid.

Ptolemy’s account of the astronomical knowledge ot the Chaldeans,ibid." '

The Chaldean Saros, ibid.

The Astronomy of the Egyptians, ibid.

Transition of Astronomy from Chaldea and Egypt to the Phoeni-cians , 6.

The Greeks chiefly derived their astronomical knowledge from theEgyptians and Pluenicians, ibid.

The table of Atlas, ibid..

Thales , the first among the Greeks, w ho predicted the eclipses ofthe bun and Moon , Ac. See. ibid.

: Anaximander, Anaximenes, Anaxagoras, anil Pythagoras , the suc-I cessors of Thales , considerably enlarge the knowledge of Aslro-I many among the Greeks, 7.

! Philolaus, the disciple of llicetas, 8.

The cycle of 19 years invented by Melon, from whom it wasnamed the Metonic cycle, ibid.

Eudoxus, of Cnidus, first introduced Geometry into the scienceof Astronomy,'ibid.

Calippus, the inventor of the period of 76 years, from him calledthe Calippie period, ibid.

Vitruvius ’s account of the introduction of Astronomy intotireece, 9.

The Astronomy of the Gauls , ibid.

Ptolemy Pliiladelphus, a great encourager of Astronomy, 10.

Aristarchus, a strenuous advocate for the Pythagorean system,ibid.

Eratosthenes, the Cyrenian, skilful in Astronomy , ibid.

Archimedes , a celebrated Astronomer, geometrician, and me-chanic, 11.

Ptolemy’,