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This is plain and direct Matter of Obfervation; and depends only on the Comparison of the former Places of Planets noted by Aftronomers, to the fixed Stars, to the Sun, or to their Primaries, with the latter; and the eafy Computation of the Number of Years, Days, Hours, and Minutes, between fuch Obfervations. Thus, for Inftance, we first know, by grofs Obfervation, that the Moon is above 27 Days in revolving round us, from any fixed Star to the fame again; we then compare feveral Months at once, and find that in 12 Months one with another, the Moon is almost eight Hours above 27 Days in fuch a Revolution. And then, laftly, we compare the like Revolutions for many Years together, and fo difcover that just 27 Days, 7 Hours, and 43 Minutes, is its true Periodical Time.

N. B. The ordinary or Diurnal Parallax of any Celestial Body, is the fmall Angle that is made in a plain Triangle compofed of three ftrait Lines; the One from the Center of the Body, to a Spectator on the Earth's Surface: The Second from that Center to the Earth's Center: The Third from the Spectator to the Earth's Center; or that Angle which is ever over against the Earth's Semidiameter.

N. B. If the Reader be unacquainted with plain Trigonometry, he is to obferve, that in every plain Triangle, if two Angles be known, the third is of Course known; and the Proportion of the Sides is known alfo: This he may try himfelf in any Triangles equiangular to one another, and may, by meafuring the Sides, fee that their Proportion is perpetually the fame.

(3.) The

Pag. 50.

true Place

3.true Place

0.3.Vifible Place

Otric Place

Diurnal Parallaxes

Path of the Sun

Apparent

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Zenith

По

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the Rational Horizon

Parallax

the Forth

Menftrual Parallax

Moon

Sun

Line shens the same Star to be out of the Zenith when viewd in December
Annual Parallax

47.

this Line shens the Star to be in the Zenith when view from the Earth in lune.

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・Orbis·

This Plate contains the Schemes of the Diurnal Parallaxes of the Ju in the Ze=

the Menstrual Parallax of the Jun; and the annual of a Fixed nith The laft Peculiar to our Famous D.Hook and M. Flamsteed.

and Moon;

It exhibits the Parallactick Triangle in every Cafe: And in the DiurnalParallax of the Sun and Moon in their Greatit quantity at the Horizon their mean quantity. at a confiderable elevation above the Horizon, and their vanishing away to nothing at the Zenith The Greatest Angle of Parallax is here still set fown according to it's real quantity, found by Observation; the Diurnalofthe Moon 57, of the Sun 10,, The Menstrual of the fun about 10, and the Annual of the Fixed Star about 4" The time of the menftrual Parallax is only at the Quadrature of the Moon, when the limit dividing the Light and Dark part appears a right line,

The Triangle of the Diurnal Parallaxes will give the Distances of the Moon 60 Semidia= meters of the Earth or 240,000 miles; of the Sun 20000 Semidiameters or 81,000,000, miles, equal to & Menstrual of 337 times & diftance of the Moon: That of the Annual 4300 tims the Diameter of the Earths Orbit or,700,000,000,000 miles.

I Senex fculpt.

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(3.) The real mean Distances of the Primary Planets, and the known Comets, from the Sun, as alfo of the Secondaries from their Primaries, have been by me thus determined.

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