city, which as much exceeds the flowest Motion, as it falls fhort of the swifteft.

The Velocity of the mean Motion of the Earth in its annual Orbit, is computed to be at the Rate of 15 Miles in a Second of Time, 933 in a Minute, 56000 in an Hour: which is more than 100 Times fwifter than the Motion of a Cannon Ball, which is ufually reckoned about 7 Miles in a Minute. Any Point on the Earth's Equator moves, by the diurnal Rotation, at the Rate of about 17 Miles in a Minute, and 1020 in an Hour.*

С Н А Р. III.

Of the Preceffion of the Equinoxes.

'N the laft Chapter we fuppofed the Axis

IN

of the Earth to continue always in one immutable Pofition, or conftantly parallel to itself; but it has been found by the Obfervations of many Years, that this is not precifely the Cafe; but that the Earth's Axis,

according to their Distance from the primary Planets, about which they revolve; the Terms mean Distance and mean Motion are applicable to all of them.

*The Earth's Velocity in its Orbit is indeed greater than what is above affigned, as appears by the Note, Page 47.

tho'

tho' it always preferves the fame Inclination to the Plane of the Ecliptic, yet deviates from its Parallelifm, with a very flow retrogade Motion from East to West, at the Rate of 50 Seconds in a Year, or one Degree in 72 Years; whereby it defcribes a Circle, of 23 Degrees Radius, round the Pole of the Ecliptic, and completes its Revolution in 25920 Years. Suppofe a Circle to be drawn in the Heavens of 47 Degrees Diameter, round the North Pole of the Ecliptic as its Center. Let the Axis of the Earth be produced to that Circle; then the End of the Axis would move round in that Circle, and the Axis itfelf would defcribe the Surface of a Cone, in one complete Revolution, whofe Bafe is that heavenly Circle, and whofe Vertex is at the Center of the Earth. Hence it follows, that all the fixed Stars in that imaginary Circle, will, in their Turns, be Pole-Stars to the Earth; and that which is now the PoleStar, will, in Time, be 47 Degrees diftant from the Pole. The Cafe is the fame as to the South Pole; fo that the Surfaces of two Cones are defcribed, whofe Vertexes meet at the Earth's Center.

Those two Points in the Earth's Orbit, where the Sun is vertical to the Equator, are always the Equinoctial Points: But as the Earth's Axis deviates from its Parallelism, thofe Points in its Orbit must also change their Places. Suppofe the Direction of the

Axis

Axis to be changed 90 Degrees; or that it now points to a fixed Star, in the imaginary Circle, 90 Degrees of the Circle diftant from that which was formerly the Pole-Star: It is evident that the Equinoctial Points are now, where the folfticial Points were then; and that the folfticial Points are now moved into the former Places of the equinoctial Points. When the Axis has moved 180 Degrees, or thro' half the Circle, the vernal Equinox is got into the ancient Place of the autumnal, and the Summer Solstice into the Place where the Winter was formerly. When the former of these two Cafes fhall fall out, viz. when the Axis has moved 90 Degrees, the Summer and the Winter will be of the fame Length; whereas there is now eight Days difference betwixt them: when the latter fhall come to pass, viz. when it has proceeded 180 Degrees, the Summer will be as much shorter than the Winter, as it is now longer; and the Sun will be furtheft off from the Earth in our Winter, and nearest to it in our Sum

mer.

As this Motion of the equinoctial and folfticial Points is backwards, or from Eaft to Weft, contrary to the Order of the Signs of the Zodiac, the fixed Stars will appear to move the contrary Way; which they have done, fince the Colures were firft defcribed, and the Afterifms drawn, by the ancient Aftronomers, (about two Thousand Years ago)

ago) about a whole Sign; fo that the Stars, which were then in that Portion of the Zodiac, which they named Aries, are now got into Taurus; and thofe that were then in Taurus are now in Gemini, &c. But tho' the Afterifms or Conftellations, have thus apparently shifted their Places; we still call the twelve Portions of the Zodiac by the ancient Names; v. g. we ftill call that the first Point of Aries, where the vernal Equinox now happens, tho' it is gone back to the beginning of the Conftellation Pisces; and call that the firft Point of Cancer, where the Summer Solstice now happens, though it is gone back to the Beginning of the Conftellation Gemini.

From this Preceffion of the Equinoxes arifes the Difference betwixt the Siderial and the Tropical Year, which will be taken Notice of Part III. For when the Sun is exactly in one of the Tropics, fuppofe the Tropic of Cancer, the Earth is not yet arrived to the fame Point of the Heavens where it was, when the Sun was in the fame Tropic last Year, by 50 Seconds of a Degree; and it will take 20 Minutes of Time to run thro' thofe 59 Seconds of a Degree, and to arrive at the fame Star, or Point, in the Heavens, where it was at the laft Summer Solftice. Hence the Siderial Year exceeds the Tropical Year by 20 Minutes.

The phyfical Cause of this Preceffion of

the Equinoxes was unknown to all Aftronomers, before the great Sir Ifaac Newton ; who has difcovered, that it arifes from the oblate fphæriodical Figure of the Earth * which is occafioned by its diurnal Rotation.

T

CHA P. IV.

Of the MooN.

HE Moon, which, of all the heavenly Bodies next to the Sun, appears biggest and brightest, because of her Nearnefs to the Earth, on which he is a constant Attendant, is an opake Globe, which has no light of its own, but only reflects the Light of the Sun which shines upon it.

That the Moon fhines only with a borrowed Light, may be concluded from her various Phafes; for if the were a luminous Body, like the Sun, fhe would always shine with a full Orb, as the Sun does.

The Moon's mean Diftance from the Earth is about 240,000 Miles. Its Diameter is 2175 Miles.

One Hemisphere of the Moon is always

i. e. Its Diameter at the Equator is longer than from Pole to Pole; or its Shape is rather like an Orange than a Lemon, or a perfect Globe.

enlightened