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Chambers, Ephraim, 1680 (ca.)-1740 / Cyclopædia, or, An universal dictionary of arts and sciences : containing the definitions of the terms, and accounts of the things signify'd thereby, in the several arts, both liberal and mechanical, and the several sciences, human and divine : the figures, kinds, properties, productions, preparations, and uses, of things natural and artificial : the rise, progress, and state of things ecclesiastical, civil, military, and commercial : with the several systems, sects, opinions, &c : among philosophers, divines, mathematicians, physicians, antiquaries, criticks, &c : the whole intended as a course of antient and modern learning

Miserere - Moon,   pp. 560-579 PDF (18.6 MB)

Page 576

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*f Elongation E L; or which is the fame thing, the Angle
S T E is nearly equal to the Angle M LO; as is demon-
firated by Geometers. See ELONGATION
To delineate the Moon's Pbafes for any time. Let the
Circle C 0 B P (fig. 14.) reprefent the Moon's Disk turn'd to-
wards the Earth, and let OP be the Lines in which the
Semi-circle 0 M P is projeaed, which fuppofe cut at right
Angles by the Diameter B C; then making L P 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, defcribe the Semi-Ellipfis B F C; this Ellipfis
will cut off from the Moon's Disk the Portion B E C P of
the illumin'd Face vifible on the Earth.
As the Moon illumines the Earth by a Light refleaed
from the Sun, fo is fhe reciprocally illumin'd by the Earth,
which refleas the Sun's Rays to the Surface of the Moon,
and that more abundantly than {he receives them from the
Alfoon. For the Surface of the Earth is above i5 times
greater than that of the Moon i and therefore fuppofing the
Texture of each Body alike, as to the Power of Reflecting;
the Earth muft return x5 times more Light to the Moon,
than (he receives from it. In New Mtions, 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
Nioon ; and then the Lunar Inhabitants (if fuch there be)
will have a fall Earth, as we, in a fimilar Pofition, have a
full AIoon: 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 obscurely. When the Moon comes to be
in oppofition to the Sun, the Earth feen from the Moon
will appear in Conjunfion with him, and its dark Side will
be turn'd towards the .Moon ; in which Pofition the Earth
will difappear to the Moon, as that does to us at the time
of the New Moon, or in her Conjundtion with the Sun.
After this, the Lunar Inhabitants will fee the Earth 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. Hook, accounting for the Reafon why the Moon's Light
affords no vifible Heat, obferves, that the Quantity of
Ligtht which falls on the Hemifphere of the full Moon, is
rarify'd into a Sphere z88 times greater in Diameter than the
Noon, o'er it arrive at us; andconfequently that the Moon's
Light is I04368 weaker than that of the Sun. It would
therefore require 104368 fuU 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 Periodical Month, fhe is longer in
paffing from one Conjunaion to another; which Space we
cadi a Synodical Month, or a Lunation. See MONTH and
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 almoPt a whole Sign towards tne Eaft;
fo that the Point of the Orbit, which in the former Pofition
was in a right Line paffing the Centres 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 Conjundion with the Sun; nor will the Luna-
tion be compleated in lefs than 29 Days and a half. See
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 jufi the fame with
that of the Sun; with this only difference, that the Sun
would be found to defcribe his Circle in the fpace of a
Year, and the Moon hers in a Month: but this is not the
Cafe; for the two Planes cut each other in a right Line,
pafling thro the Centre of the Earth, and are inclin'd to
each other in an Angle of about five Degrees. See INCLI-
Suppofe, e.g. AB (fig. J 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: from the fame Centre T, in
the Plane of the Ecliptic, defcribe another CD G D H,
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 interfea each other in a Line
DC, pafling thro the Centre of the Earth. Confequently,
C E D, one half of the Orbit of the Moon, will be raifed
above the Plane of the Circle C G 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 Angles C and D the Nodes: whereof, that
where the Moon afcends above the Plane of the Ecliptic,
Northwards, is call'd the Afcendinx Node, and the Head of
the Dragon; and the other D, the Defcending Node, aznd the
Dragon's Tail. See NODE. Avtd the Interval of Time be-
tween the Moon's going from the afcending Node, and re-
turning to it, a Dracontic Month. See D&AGoN'sIeadc.
DR ACONTI C Month, i$c.
If the Line of the Nodes Were immoveable, that is, if
it had no other Motion, but that whereby it iscarr)'d round
the Sun, it would fill look towards the fame Point of the
Ecliptic, i.e. would always keep parallel to itfejf; but it is
found by Obfervation, that the Line of the Nodes conflantly
changes place, and Jhifts its Situation from Eaft to Wet
contrary to the Order of the Signs, and by a Retrograde
Motion, finiffies its Circuit in about 19 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 lhe is in the
Nodes: throughout the reft of her Courfe fhe deviates
from it, being nearer or further from the Ecliptic, as file
is nearer or further from the Nodes. In the Points F and
E, She is at her greatell Ditiance from the Nodes i which
Points are call'd her Limits. See LIMITS.
The Mvoon's Diflance from the Nodes, or rather from the
Ecliptic, is call'd her Latitude, which is meafur'd by an
Arch of a Circle drawn tbro the Moon perpendicular to the
Ecliptic, and intercepted between the Moon and tho Ecit,-
tic. The Moon's Latitude, when at the greatefi, as in Ie
or F, never exceeds 5 Degrees, and about i8 Minutes;
which Latitude is the Meafure of the Angles at the Nodes.
It appears by Obfervation, that the Moon's Diflance from
the Earth is continually changing; and that fhe is always
either drawing nearer, or going further from us. The rea-
fon is this, that the Moon does nor move in a circular Or-
bit, which has the Earth for its Centre; but in an Elliptic
Orbit (fuch as is represented in Fig. il.) one of whofe Foci,
is the Ctnrer of the Earth; A P reprefents the greater Axis
of the Eiiipfis, and the Line of the Apfides i and T C, 'the
Eccentricity: the Point A, which is the higheff Apfis, is
call'd the Apogee of the Aloon; and P, the lowef Apfis, is
the M oon's Peregee, or the Point wherein flhe comes ncare&l
the Earth. See APOGEE and PERIGEE.
The Space of Time wherein the Moon, going from the
Apogee, returns to it again, is call'd the Inomali/fic Month.
If the Moon's Orbit had no other Motion, but that
wherewith it is carry'd round the Sun, it would fill retain
a Pofition parallel to itfelf, and always point the fame way,
and be obferv'e in the fame Point of the Ecliptic; but
the Line of the Apfides is likewife obferv'd to be move-
able, and to have an angular Motion round the Earth from
Wefi to Ealt, according to the Order of the Signs, re-
turning to the fame Situation in the fpace of about nine
Years. See ANGUL kR 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, r. When the Earth is
in her Aphelion, the Moon is in her Aphelion likewife; in
which cafe Ihe quickens her Pace, and performs her Cir-
cuit in a (horter 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 Conjunaion or Oppofition; fhe moves fwifter,
caeteris Paribas, than when in the Quadratures. See SY-
Further, 3. According to the different Diflance of the
Moon from the Syzygies, i.e. from Oppofition or Conjunc-
tion, fhe changes her Motion: In the firft Quarter, that
is, from the Conjunalion to her firfi Quadrature, the abates
fomewbat of her Velocity; which, in the fecond Quarter,
lhe recovers: In the third Quarter, fhe again lofes; and
in the laft, again recovers. This Inequality was firfm
difcover'd by Tycho Brasbe, who call'd it the Mdoon'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 Earth, round which
She defcribes Areas proportionable to the Times, as the
primary Planets do round the Sun; whence her Motion in
the Perigee mufd be quickeft, and floweft in the Apo-
5. The very Orbit of the Moon is changeable, and does
not always perfevere in the fame Figure; its Eccentricity
being fometimes increas'd, and fometimes diminil'h'd ;
greatef, when the Line of Apfides coincides with that of
the Syzygies; and leaft, when the Line of Apfides cuts the
other at right Angles. See ORs NT.
6. Nor

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