Page:Cyclopaedia, Chambers - Volume 1.djvu/642

 ECL

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ECL

To determine the Duration of a Solar Eclipfe.

Find the Horary Motion of the Moon from the Sun, For one Hour before the Conjunction, and another Hour after : Then fay, as the former Horary Motion is to the Seconds in an Hour, fo are the Scruples of half Duration to the Time of Immcrfion i And as the latter Horary Motion is to the fame Seconds, fo are the fame Scruples of half Duration to the Time of Emerfion. Laftly, adding the Time of Immerfion to that of Emerfion ) the Aggre- gate is Total Duration.

To determine the "Beginning, Middle, anil End of a Solar Eclipfe.

From the Moon's Latitude, for the Time of Conjunflioii, ; find the Arch I L, (.Fig. if.) or the Diflance of the greateft Obfcurity. Then fay, as the Horary Motion of the Moon from the Sun, before the Conjunction, is to 5600 Seconds of an Hour ; fo is the Diflance of the greateft Darknefs, to the Interval of Time between the greateft Darknefs and the Conjunction.

Subflraa this Interval, in the firft and third Quarter of the Anomoly, from the Time of the Conjunct ion 5 and in the other Quarters, add it to the fame ; the Refult is the Time of the greateft Darknefs. Laftly, from the Time of the greateft Darknefs, fubftraft the Time of Incidence, and add "it to the Time of Emerfion ; the Difference, in the firft Cafe, will be the Beginning; and the Sum, in the latter Cafe, the End of the Eclipfe.

Indeed, as the Interval between the Conjunction, and the greateft Obfcurity is very fmall, and exceedingly precarious ; 'tis fcarce worth while to be fo very precifc ; and, ac- cordingly, many Authors ufe the Time of the apparent Conjunction for that of the greateft Darknefs.

3«' 59"

% 3<?

E. gr. in our Cafe, Time of Conjunction Time of Incidence

Beginning of Eclipfe 20 34 2 5

Or S h Morn. 34 23

Time of Conjunction 21 36 59

Time of Emergence 1 5 47

4*

4 5

End of Eclipfe 22* 42

Or 10 Morn. 42

Or, if you wou'd determine it with more Accuracy, fub- ftract about two Minutes for the Diflance between the Conjunction feen, and the Time of the greateft Obfcurity.

T'o find the Moon's apparent Latitude at the Be- ginning and End of an Eclipfe,

From the Argument of the Moon's Latitude, computed for the Time of the apparent Conjunction, SubftracT: the Scruples of half Duration, together with the Motion of the Sun, anfwering to the Time of Incidence : The Remainder, is the Argument of Latitude, at the Begin- ning of the Eclipfe. , ■ To the fame Sum add the fame Scruples, together with the Sun's Motion anfwering to the Time of Emergence : The Aggregate is the Argument of Latitude, at the End of the ^Eclipfe. The Argument of the Latitude given ; the Moon's true Latitude is found after the common Man- ner. (See Latitude.) And from the true Latitude, the apparent one.

The apparent Latitude of the Moon, at the Beginning and End of a Solar Eclipfe, given ; to draw a Type or Figure thereof.

This is perform'd, as already taught for Eclipfes of the Moon.

1*o calculate an Eclipse of the Sun.

1. Find the mean New Moon, and thence the true one 5 together with the Place of the Luminaries for the apparent Time of the true one.

2. For the apparent Time of the true new Moon, com- pute the apparent Time of the new Moon obferved.

3. For the apparent Time of the new Moon feen, com- pute the Latitude feen.

4. Thence, determine the Digits Eclipfed.

5. Find the Time of the greateft Darknefs, Immerfion, and Emerfion.

6. Thence determine the Beginning, and Ending of the

From the preceding Problems, it is evident, that all the Trouble, and Fatigue of the Calculus, ariies from the Parallaxes of Longitude and Latitude - y without which, the Calculation of Solar Eclipfes would be the fame with that of Lunar ones.

Eclipses of the Satellites. See Satellites of Jupiter. • -

The chief Circumftances here obierved are,

1. That the Satellites of Jupiter undergo two or three Kinds of Eclipfes 5 whereof; the firft are proper, being fuch as happen when Jupiter's Body is directly interpofeS between them and the Sun : Thefe happen almoft every Day. Mr. Flamfteed and Caffmi, give us Tables, wherein their Immerfions into Jupiter's Shadow, and Emerikms again, are computed to "Hours and Minutes.

The 2 d are Occupations, rather than Obfcurations ; wherein the fame Satellites, coming too near to Jupiter's Body, are loft in his Light : Which Ricciolus calls occidere Zeufiace, fetting Jovially. In which Cafe, Jupiters neareft Satellite exhibits a third Kind of Eclipfe j being obferved like a Macula, or dark round Spot, tranfiting Jupiter's Difk, with a Motion contrary to that of the Satellite : Juft as the Moon's Shadow projected on the Earth, will appear to do, to the Lunar Inhabitants.

The Eclipfes of Jupiter's Satellites furnifh the beft: Means of finding the Longitude at Sea. Thofe, particu- larly, of the firft Satellite, are much furer than the Eclipfes of the Moon - 7 and withal happen much oftener : Betide that, the Manner of applying them is very eafie. See Longitude.

ECLIPTIC, EclipticUs, fdmething belonging to Eclipfes. See Eclii'SE.

All new and full Moons are not Ecliptic, i.e. Eclipfes don't happen every new and full Moon, tho' there be then an Interpofition, either of the Earth between the Sun and Moon, or of the Moon between the Sun and Earth. The Reafon is, that the Interpofition is only as to Longitude, and not as to Latitude. The Sun is always in the Ecliptic, but the Moon is not 5 fhe deviates from it about five Degrees, fometimes on the North Side, and fometim.es on the South. But every five Months, or thereabouts, fhe cuts the Ecliptic ; and 'tis only about thofe Times, that there can be Eclipfes either of the Moon or Sun.

The Places, wherein fhe cuts the Ecliptic, are called the Nodes of the Moon. See Nodes.

Ecliptic Sounds, or Terms, Termini Ecliptici ; is the Space of about 15 Degrees from the Nodes of the Ecliptic 5 within which, if the Moon be, at the Time of a Con- junction or Oppofition with the Sun, there may be an Eclipfe of the Sun, or Moon, tho' fhe be not precifely in the Nodes. See Eclipse.

Ecliptic Digits, T)igiti Ecliptici. See Digits. Ecliptic, in Aftronomy, is particularly ufed for a Line, or Circle, on the Surface ot the Sphere of the World, under which the Centre of the Sun proceeds in its proper Mo- tion : Or a Line, with the Sun's Centre, defcribes in his annual Progiefs. See Circle, Sun, &c.

It has its Name EcliJ>tic, by Reafon all Eclipfes happen when the two Planets are in or near the Nodes, or Inter- feclions of the Ecliptic. See Eclipse.

Ir is alfo called the Sun's Orbit, and Sim's Way, by Reafon the Sun never deviates from it, in his annual Motion from Eaft to Weft. See Orbit.

The North, or afcending Node of the Ecliptic, is called the Tirc-gons-Head, and the South, or defcending Node, the 'Dragons-Tail. See Dragon, KSc.

The Ecliptic is placed obliquely with Refpecc to the Equator, and cuts it in two Points, viz. the Beginnings of dries and Libra, or into two equal Parts ; and, accord- ingly, we find the Sun twice every Year in the Equator: And all the reft ot the Year, either on the North or South Side thereof. See Eqjuinoctial.

The Ecliptic is a great Circle of the Sphere, bifTected by the Horizon ; confequentiy, the Arch of the Ecliptic, intercepted between the Horizon and the Meridian, is a Quadrant. And again, the Solftitial 'Points of the Ecliptic* i. e. thofe moil remote from the Equator, are a Quadrant diftant from the Equinoctial ones. Laftly, as the greateft Declination of the Ecliptic, from the Equator, is an Arch of a great Circle diftant by a Quadrant from the Equi- noctial Points; it will be the Meafure, or Quantity of the Obliquity of the Ecliptic, i. e. of the Angle form'd by the Interferon of the Equator with the Ecliptic. See Sphere.

'The Obliquity of the Ecliptic, or the Angle wherein it cuts the Equator, is ufually fix'd at 23 5c/; which, there- fore, is the greateft Declination of the Ecliptic from the Equator : The Points of which greateft Declination on each Side, are called the Solftitial Points; thro' which are drawn the two Tropicks. See Solstitial and Tropic

The Method of obferving the greateft Declination of the Ecliptic is thus : About the Time of one of the Sol- fticcs obferve the Sun's Meridian Altitude with the utmoft Care, for feveral Days fuccefuvely : From the greareft fubftrad the Height of the Equator, the greateft Declination in the Solftitial

Altitude obferv'd, The Remainder is Point.

Ricciolus, e. gr.

, at 'Boulogne, in the Tear 16 tf, obfer- ved the Sun's Meridian Altitude, on the 20 th of 4tm, to

be