Page:Cyclopaedia, Chambers - Volume 2.djvu/225

 MOO

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MOO

of Elongation E L; or which is the fame thing, the Angle S T E is nearly equal to the Angle MLO; as is demou- llrated by Geometers. See Elongation.

To delineate the Moon's Tbafes for any time. Let the Circle C O B P (rig. 14.) reprefent the Moon's Disk turn'd to- wards the Earth, and let OP be the Lines in which the Semi-circle O M P is projefled, which fuppofe cut at right Angles by the Diameter B C; then making LP the Ra- dius, take L F equal to the Co-fine of the Elongation of the Moon; and upon B C, as the greater Axis, and L I, the lefs, defame the Semi-Ellipfis B F C; this Ellipfis will cut off from the Moon's Disk the Portion BECP of the illumin'd Face vifible on the Earth.

As the Moon illumines the Earth by a Light reflected from the Sun, fo is fhe reciprocally illumin'd by the Earth, which refleas the Sun's Rays to the Surface ot the Moo«, and that more abundantly than (lie receives them from the Moon. For the Surface of the Earth is above 15 times greater than that of the Moon; and therefore fuppofing the Texture of each Body alike, as to the Power of Reflecting; the Earth mull return 1 5 times more Light to the Moon than fhe receives from it. In Nev Moons, the illumined Side of the Earth is turn'd fully towards the Moon, and will therefore at that time illumine the dark Side of the Moon; and then the L»B.ir Inhabitants (if fuch there be J will have a full Earth, as we, in a fimilar Pofition, have a full Moon : And hence arifes that dim Light obferv'd in the Old and New Moons; whereby, befides the bright Horns, we perceive fomewhat more of her Body behind them, tho very obfcurely. When the Moon comes to be in opposition to the Sun, the Earth feen from the Moon will appear in Conjunction with him, and its dark Side will be turn'd towards the Moon; in which Pofition the Earth will difappear to the Mson, as that does to us at the time of the New Moon, or in her Conjunction with the Sun. After this, the Lunar Inhabitants will fee the Eirth in a horned Figure. In fine, the Earth will prefent all the fame Tbafes to the Moon, as the Moon does to the Earth.

Dr. Hooi, accounting for the Reafon why the Moon's Light affords no vifible Pleat, obferves, that the Quantity of Light which falls on the Hemifphere of the full Moon, is rarify'd into a Sphere 288 times greater in Diameterthar. the Moon, e'er it arrive at us; andconfequently that the Moon's Li"ht is 104568 weaker than that of the Sun. It would therefore require 104348 full Moons to give a Light and Heat equal to that of the Sun at Noon. See Sun, Heat,

Motion of the Moon.

Tho' the Moon finifhes its .Courfe in 27 Days, 7 Hours, which Interval we call a Tenodical Month, fhe is longer in patting from one Conjunction to another; which Space we call a Synodical Month, or a Lunation. See Month and Lunation.

The reafon is, that while the Moon is performing its Courfe round the Earth in its own Orbit, the Earth with its Attendant is making its Progrefs round the Sun, and both are advanced almoft a whole Sign towards the Ealt; fothat the Point of the Orbit, which in the former Pofition was in a right Line patting the Centtes of the Earth and Sun, is now more weflerly than the Sun : and therefore when the Moon is arrived again at that Point, it will not be yet feen in Conjunction with the Sun; nor will the Luna- tion be compleated in lefs than 29 Days and a half. See Periodical, Synodical, t>e.

Were the Plane of the Moon's Orbit coincident with the Plane of the Ecliptic, i. e. were the Earth and Moon both moved in the fame Plane, the Moon's Way in the Heavens, view'd from the Earth, would appear juft the fame with that of the Sun; with this only difference, that the Sun would be found to defctibe his Circle in the fpace of a Tear, and the Moon hers in a Month : but this is not the Cafe; for the two Planes cut each other in a right Line, patting thro the Centre of the Earth, and are inclin'd to each other in an Angle of about five Degtees, See Incli- nation.

Suppofe, e.g. A B (fig. 1 5.) a Portion of the Earth s Or- bit; T the Earth; and C E D F the Moon's Orbit, wherein is the Centre of the Earth : ftom the fame Centre T, in the Plane of the Ecliptic, defcribe another CDGDH, whofe Semi-diameter is equal to that of the Moon's Orbit : Now, thefe two Circles being in feveral Planes, and ha- ving 'the fame Centre T, will interfedf each other in a Line DC, patting thro the Centre of the Earth. Confequently, C ED, one half of the Orbit of the Moon, will be raifed above the Plane of the Circle CG H, towards the North; and D F C, the other half, will be funk below it towards the South. The right Line DC, wherein the two Circles interfect each other', is call'd the Line of the Nodes, and the Points of the Anoles C and D the Nodes: whereof, that where the Moon afcends above the Plane of the Ecliptic, Northwards, is call'd the AfcaiMl Node, and the Head of

the Dragon; and the other D, the Defending Node, and the Dragon's Tail. See Node. And the Interval of Time be- tween the Moon's going from the afcending Node, and re- turning to if, a Dracontic Month. See Va.\coN'sHead,^ c% Dracontic Month, i$c.

If the Line of the Nodes were immoveable, that is, if it had no other Motion, but that whereby it is carry 'd round the Sun, it would ftill look towards the fame Point of the Ecliptic, i.e. would always keep parallel to itfelf; but it is found by Obfervation, that the Line of the Nodes conilantly changes place, and fhifts its Situation from Ealt to Weft contrary to the Otder of the Signs, and by a Retrograde Motion, finifhes its Circuit in about 10 Years; in which time each of the Nodes returns to that Point of the Eclip- tic, whence it before receded. See Cycle.

Hence it follows, that the Moon is never precifely in the Ecliptic, but twice, each Period, viz. when fhe is in the Nodes : throughout the reft of her Cuurfe fhe deviates from it, being nearer or further from the Ecliptic, as fne is nearer or further from the Nodes. In the Points F and E, file is at hct gteatel! Diitance from the Nodes; which Points are call'd her Limits. See Limits.

The Moon's Diitance from the Nodes, or rather from the Ecliptic, is call'd her Latitude, which is meafur'd by an Arch of a Circle drawn thro the Moon perpendicular to the Ecliptic, and intercepted between the Moon and the Eclip- tic. The Moon's Latitude, when at the greateft, as in E or F, never exceeds 5 Degrees, and about 18 Minutes; which Latitude is the Meafure of the Angles at the Nodes. See Latitude.

It appears by Obfervation, that the Moon's Diitance from the Earth is continually changing; and thar fhe is always either drawing nearer, or going further from us. The rea- fon is this, that the Moon does not move in a circular Or- bit, which has the Earth for its Centre; but in an Elliptic Orbit (fuch as is reprcfented in Fig. 16.) one of whofe Foci is the Center of the Earth; A P rcprefents the greater Mis of the Eliiplis, and the Line of the Jpfides, and TC, the Eccentricity: the Point A, which is the higheft Apfis, is call'd the Jfogee of the l"'«m; and P, the loweft Apfis, is the Moon's Perigee, ot the Point wherein fhe comes neareffc the Earth. See Apogee and Perigee.

The Space of 'lime wherein the Moon, going from the Apogee, returns to it again, is call'd the Anomalijlic Month. See Anomalistic.

If the Moon's Orbit had no other Motion, but that wherewith it is carry 'd round the Sun, it would ftill retain a Pofition parallel to itfelf, and always point the fame way, and be obferv'd in the fame Point of the Ecliptic; but the Line of the Apfides is likewife obfetv'd to be move- able, and to have an angular Motion round the Earth from Weft to Eaii, according to the Order of the Signs, re- turning to the fame Situation in the fpace of about nine Years. See Angular Motion and Apsides.

Irregularities in the Moon's Motion.

The Irregularities of the Moon's Motion, and that of her Orbit, are very confiderable : For, 1. When the Earth is in her Aphelion, the Moon is in her Aphelion likewife; in which c-^fc fhe quickens her Pace, and performs her Cir- cuit in a fhorter time : On the contrary, when the Earth is in its Perihelion, the Moon is fo too, and then fhe flac- kens her Motion; and thus revolves round the Earth in a fhorter fpace, when the Earth is in her Aphelion, than when in her Perihelion : So that the Periodical Months are not all equal. See Periodical Month.

2. Again, when the Moon is in her Syzygies, i. e. in the Line that joins the Centers of the Earth and Sun, which is either in her Conjunction or Oppofition; fhe moves fwifter, ceteris paribus, than when in the Quadratures. See Sy- zygies.

Further, 3. According to the different Diftance of the Moon from the Syzygies, i.e. from Oppofition or Conjunc- tion, file changes her Motion : In the firft Quarter, that is, from the Conjunction te her firft Quadrature, fhe abates fomewhat of her Velocity; which, in the fecond Quarter, fhe recovers: In the third Quarter, /lie again lofes; and in the laft, again recovers. This Inequality was firft difcovcr'd by Tycho Brahe, who call'd it the Moon's Varia- tion. See "Variation.

4. Add to this, that the Moon moves in an Ellipfis, one of whofe Foci is in the Centre of the Eatth, round which fhe defcribes Areas proportionable to the Times, as the primary Planets do round the Sun; whence her Motion in the Perigee muft be quickeft, and floweft in the Apo- gee.

5. The very Orbit of fhe Moon is changeable, and does not always perfevere in the fame Figure; its Eccentricity being fometimes increas'd, and fometimes diminifh'd; greateft, when the Line of Apfides coincides with that of the Syzygies; and leaft, when the Line of Apfides cuts the other at tight Angles, See Orbit.

6. Nor