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the Motions of the Planets in their elliptic Orbits are not equable, by reafon the Sun is not in their Centers but their
Focus. Hence they move, fometimes fatter and fometimes
flower, as they, are nearer or further from the Sun ; but yet thefe Irregularities are all certain, and follow according to
an immutable Law. Thus; fuppofe the Ellipfis BEP,
&c. (Tab. Aflronomv, Fig. 61.) the Orbit of a Planet ; and the Focus S, the Sun's Place ; A P the Axis of the Elhplis, is called the Line of the Apfides ; the Point A the higher Ap- fis or Aphelion ; P the lower Apfu or Perihelion, S C. the Eccentricity ; and E S the mean Diftance of the Planet from the Sun. See Aps i s, Aph el I ON, P er 1 H el 1 o N, r>c Now the Motion of the Planet in its Perihelion, is Iwitt- eft; in its Aphelion, flowed; at E the Motion as well as the Diftance is mean, i. e. fuch as would defcribe the whole Orbit in the lame time it is really defcribed in.
The Law whereby the Motion is regulated in every Pomt of the Orbit, is, that a Line, or Radius-, drawn from the Cen-
and thus carried
Planetary Syjlem, is the Syftem, or Afferivblage of the Planets, primary and fecundary, moving in their refpe- ftive Orbits, round their common Center, the Sun. See Solar System. Planetary Hours in Chronology. See Hoo R.
PLANIMETRY, Planimetria, that Part of Geometry which conlidets Lines and plain Figures ; with- out any Confideration of Heights or Depths. See Geome- try', fee alfo Line and Figure.
The Word is particularly reftrained to the Menfuration of Planes, or Surfaces ; in oppofition to Stereometry, or the Menfuration of Solids. See Measuring.
PLANISPHERE, a Projeaion of the Sphere and the feveral Circles thereof, on a Plane ; as, Upon Paper, &c. See Plane, Sphere, and Projection.
In this Senfe, Maps of the Heavens and the Earth, where- in are exhibited the Meridians, and other Circles of the Sphere, are called Planifpheres. See Map. ,
Planisphere is 1 fometimes conlider'd as an Aftrono-
ter of the Sun to the Center of the Planet, and thus earned ' L A N I s P H E R 1 1 is lomenmes connaer a as an niuono-
nlen, withal Zguar Motion does always 'defcribe an Elliptic .meal Inftrument, tiled in obferving the Motions ; of the hea Area proportional to the T,me. Suppofe, e.gr. the Planet venty Bod.es-, confifhng ot a F oietaol^ he Celet a
1 propo,,.*.,,,.* .„ ...» - u ,. - u
in A, and thence in a certain Time to proceed to B; th! Space or Area the Ray S A defcribes, is the Triangle A S B ; when, at length the Planet arrives at P, if from the Center of the Sun S there be drawn SD, in fuch manner as that the elliptic Area P S D is equal to that A S B ; the Planet ■will here move thro' the Arch P D, in the fame time where- in it moved thro' the Arch A B -, which Arches are unequal,
Celeffial Sphere upon a Plane, reprefenting the Stars, Conftella- tions, &c. in their proper Situations, Diftances, &c. See Star and Constellation.
Such is the Aftrolabe, which is a common Name for all fuch Projections. See Astrolabe.
In all Planifpheres, the Eye is fuppofed to be a Point viewing all the Circles of the Sphere, and referring th e m to
1L 111UVCQ imO inC mill U u , vvuu.ii "^"" "'- "'"2 ' nil 1^1 ■ ■ fl .. T. t-1 ■
dnearly in a reciprocal Proportion to their Diftance from a Plane whereon the Sphere is as it were flatten d.-This
r. ' ~. /• ' .1 t- ____i..: -r *.!-.. \~„~* ;r Crt!l«i.. c 'bnp it; rail H th^ Pl<rtip at thr I'ro'rttinn.
the Sun. For from the Equalities of the Areas it follows, that the Arch P D muft exceed A B as much as S A exceeds S P.
This Law was firft demonftrated by Kepler, from Ob- fervation; and is fince accounted for from Phylicks: And to this all Aftronomers, now, fubferibe, as of all others that which beft folves the Planetary Phenomena.
Computation of. a Planet'* Motion and Place.
or the'
Plane is call'd the Plane of the Projection.
A Perfpeftive Plane is only a Plane of Projection placed be- tween the Eye and the Object, fo as to contain all the Points' which the feveral Rays drawn from the Objeft to the Eye imprefs thereon. (See Perspective Plane.') — But in Plani- fpheres, or Artrolabes, the Plane of the Projection is placed beyond the Object-, which is the Sphere.
The Plane of the Projection is always forae of the Circles of the Sphere. SeeCiRCLE.
Among the infinite Number of Planifpheres, which the different Planes of Projection, and the different Politions
As to the Periods and Velocities of the Planets, -
Times wherein they perform their Courfes ■, they are found of the Eye, would furnilh ; there are two or three that
to have a wonderful Harmony with their Diftances front have been preferr'd to the reft.— Such are that of Ptolomy,
the Sun, and with one another. The nearer each Planet where the Plane of Projection is parallel to the Equator.—
is to the Sun, the quicker ftill being its Motion ; and its That of Gemma Frifius, where the Plane of Projection is the
Period the fhorter. The great Law they here all immu- Colure, or Solftitial Meridian, and the Eye the Pole of the
tably obferve is, that the Squares of their periodicalTimes are Meridian — That of John dc Royas, a Spaniard, whofe Plane
as the Cubes of their Diftances from the Centre of their Orbits; of Projection is a Meridian, and the Eye placed on the
See Period, Distance, &c. Axis of that Meridian, at an infinite Diftance. This Lift is
This Law' we owe to the Sagacity of Kepler, who found call'd the Analemma. See Analemma
it to obtain in all the primary Planets; as Aftronomers have fince found it to do in the fecundary ones. See Sa- tellite.
Kepler deduced this Law, meerly from Obfervation and Comparifon of the feveral Diftances of the Planets with their Periods : The Glory of inveftigating it from Phyfical Prin- ciples, is due to Sir Ifaac Newton, who has demonftrated that, in the prefent ftate of things, fuch a Law was inevi- table. See Gravitation.
A Planet's Motion or Diftance from it Apogee, is call'd the mean Anomaly of the Planet; and is meafur'd by the Arch, or Area, it defcribes in the Time.— When the Planet arrives at the middle of its Orbit, or the Point G, the Di- ftance or Time is call'd the true Anomaly.— -When the Planet's Motion is reckon'd from the firft Point of Aries, 'tis call'd its Motion in Longitude, which is either mean, viz.. fuch as the Planet would have were it to move uniformly in a Circle •, or true, which is that wherewith the Planet actually defcribes its Orbit, and meafur'd by the Arch of the Ecliptic it defcribes. See Anomaly, Longitude, f>c.
Hence may the Planet's Place in its Orbit for any given Time after it has left the Aphelion, be found — For fuppofe
The common Defeat of all thefe Projections is, th.'t they diftort and alter the Figures of the Conftellations, fo as it is not eafy to compare them with the Heavens; and that the Degrees in ibme Pl.ices are fo fmall, that they af- ford no Room for Operation.
AH thefe Faults bA.de la PJirehiS provided ag.iin ft ina new Projection, or Planifphcre ; where 'tis propofed the Eye fhall be fo placed, as that the Divifions of the Circles pro- jected fhall be fenfiblv eqml in every Part of the Inftru- ment The Plane of his Projection is that of a Meridian.
PLAN O-Concave Clafs, or Lens, is that, one of whofe Surfaces is concave, and the other plain. See Glass.
The Concavity is here fuppoied to be fpherical, unlets the
contrary be exprefs'd. For the Properties, Grinding, crc<
of Piano Concave Lens's, fee Lens ; fee alfo Grinding, &c.
P L a N o-Convex Clap, or Lens, is that, one of whole Surfaces is convex, and the other plain. See Convex.
TheConvexity is fuppofed to be fpherical, unlets the con- trary be exprefs'd. For the Properties, Grinding, &c. of Piano-Convex Len's. See Lens, &c.
PLANT, Planta, an Oiganical Body, cdnfifting of
the Area of the Ellipfis fo divided by the Line SG, that the a Root, effentially, and probably too, a Seed; and produ-
whole elliptic Area may have the fame Proportion to the Area A S G as the whole periodical Time wherein the Pla- net, defcribes its Orbit, has to the Time given : In this Cafe G will be the Planet's Place in its Orbit. See Place.
The Phenomena of 'the inferior Planets, are their Conjun- ctions, Elongations, Stations, Retrogradations, Phafes, and
ciug ufually Leaves, a Stem, Branches, and Flowers. See Root, &c.
Or, a Plant may be defined, in Bocrhaave's manner, to be an Oiganical Body compofed of Vefftls and Juices; to which Body belongs a Root or Part whereby it adheres to fome other Body, and particularly the Earth, from which it
Ecliples. See Conjunction, Elongation, Station, derives the Matter of its Life, and Growth. See Vegetable, Retrogk adation, &c. under their refpeitive Articles. A Plant is diftinguifhed from a Foffil by its being organical,
The Phenomena of the fuperior Planets are the fame with and confifting of Veffels and Juices; (See Fossil.) and
thofe of the inferior ; with an Additional one, we, Oppofition, See Opposition, &c.
The particular Phenomena, Cirumflances, &c. of each Pla- net, fee under the Name of the refpe&ive Planet, &c.
The general Proportions, Diameters, Surfaces, Solidities, Diflanccs, Gravities, Degrees of Light, &c, of the feveral Pla- nets ; fee under the Articles Solar System, Diameter, Se-
MIDIAMETER, &C.
from an Animal, by its adhering to another Body, and de- riving its Nourifhment therefrom. See Animal.
Plant is a general Name, under which are comprized all vegetable Bodies, as Trees, Shrubs, and Herbs. See Tree, Shrub, and Herb.
From the Obfervations of Malpighi, Dr. Grew, M. Re~ neaume, Bradley, and others, there appears a great Simili- tude between the Mechanifm of Plants, and Animals ; the
PLANETARY, fomething that relates to the Planets. Parts of the former feem to beara conftant Analogy to thofe In this Senfe we fay Planetary Worlds, Planetary Inhabi- of the latter ; and the Vegetable and Animal Oeconomy ap-
tants, &c. See Planet. pear both form'd on the fame Model To give an Idea
hereof
