District of Pennsylvania, to wit:

Be it remembered, That on the thirteenth day of February, in the thirtieth year of the Independence of the United States of America, A. D. 1806, Mathew Carey, of the said District, hath deposited in this office, the title of a book, the right whereof he claims as Proprietor, in the words following, to wit:

"Astronomy explained upon Sir Isaac Newton's Principles, and made easy to those who have not studied Mathematics. To which are added, a plain method of finding the distances of all the planets from the sun, by the transit of Venus over the sun's disc, in the year 1761: an account of Mr. Horrox's observation of the transit of Venus, in the year 1639: and of the distances of all the planets from the sun, as deduced from observations of the transit in year 1761. By James Ferguson, F. R. S.

Heb. xi. 8. The worlds were framed by the Word of God.

Job xxvi. 7. He hangeth the earth upon nothing.

13. By his Spirit he hath garnished the heavens.

The first American edition, from the last London edition; revised, corrected, and improved, by Robert Patterson, Professor of Mathematics, and Teacher of Natural Philosophy, in the University of Pennsylvania."

In conformity to the Act of the Congress of the United States, intituled, "An Act for the Encouragement of Learning, by securing the Copies of Maps, Charts, and Books, to the Authors, and Proprietors of such Copies, during the Times therein mentioned," And also to the Act, entitled "An Act supplementary to an Act, entitled, 'An Act for the Encouragement of Learning, by securing the Copies of Maps, Charts, and Books, to the Authors and Proprietors of such Copies, during the Times therein mentioned,' and extending the Benefits thereof to the Art of designing, engraving, and etching, historical and other Prints."

(L. S.)

D. CALDWELL,

Clerk of the District of Pennsylvania.

PREFACE

QB42 F4 1809

TO THE FIRST AMERICAN EDITION.

THE well-established reputation of Ferguson's Astronomy, renders any particular encomiums on the work, at this time, altogether unnecessary.

The numerous editions through which this Treatise has passed, and the increasing demand for it, bear ample testimony to its merit.

The Publisher submits to the candid acceptance of his fellow-citizens, this correct American Edition; for which he solicits, and flatters himself he shall obtain, their liberal patronage.

No cost or pains have been spared to render it worthy of this patronage. In the text, a number of typographical errors, and grammatical inaccuracies, have been corrected; and a variety of notes, explanatory or corrective of the text, which the numerous discoveries since our author's time had rendered necessary, have been occasionally subjoined.

Besides, to this edition alone there is prefixed a copious explanation of all the principal terms in astronomy, chronology, and astronomical geography, occurring in the

11482!

iv

work, arranged in alphabetical order; with such remarks and examples interspersed, as were judged necessary for illustration: together with Tables of the periodical times, distances, magnitudes, and other elements, of all the planets, both primary and secondary, in the solar system; according to the latest observations.

This, it is presumed, cannot fail to be considered as a valuable appendage to the work-especially by the young student of astronomy: as the glossary will tend greatly to facilitate his progress, and the tables will present him with a comprehensive view of the whole science-the result of the observations and researches both of past and present times.

Philadelphia, Feb. 14th, 1806.

Explanation of the principal Terms relating to Astronomy, Chronology, and the astronomical parts of Geography; with occasional Illustrations and Remarks.

40"

= 40"

Aberration of a star, is a small apparent motion, occasioned by a sensible proportion between the velocity of light and that of the earth in its annual orbit. From this cause, every star will, in the course of a year, appear to describe a smail ellipsis in the heavens, whose greater axis = and its lesser axis, perpendicular to the ecliptic, X cos. of star's lat. (to radius 1.) In astronomical calcu lations, where great accuracy is required, and the place of a star concerned, a correction on account of aberration, as well as on other accounts, ought to be applied to the star's place as found in the tables. This correction may readily be found by the following theorems; in which A the star's right ascension, D its declination, and S = the Sun's longitude.

=

Theorem 1. (-1.272 cos. (AS)) cos. (A + 5)) cos. Daberr. in R.

time.

cos. D + (0.055. A. in seconds of

Theorem 2-20 cos. A. sin. S. sin. D + 18.346 sin. A. cos. S. sin D-7.964 cos. S. cos. D aberr. in dec. in seconds of a degree; observing that the sine, cosine, &c. of all arches between 90° and 270° are to be considered as negative, and those of all other arches as affirmative.

When the star has south declination, let the sign of the last term in the 2d theorem be changed.

Acceleration (diurnal) of a fixed star, is the difference be tween the sidereal and the mean solar day, which = 3' 55.9 or 3′ 56′′ of mean time nearly; and so much sooner will any fixed star rise, culminate, or set, every day, than on the preceding day. A planet is said to be accelerated in its motion, when its velocity, in any part of its orbit, exceeds its mean velocity; and this will always be the case when its distance from the Sun is less than its mean distance.

Era, or epoch, any noted point of time, in chronology, from which events are reckoned, or computations made. Different nations or people make use of different epochs: as the Jews, that of the creation of the world; the christian nations, that of the nativity of Christ, A. M. 4007; the Mahometans, that of the Hegira, or flight of Mahomet from Mecca, A. D. 622; the ancient Greeks, that of the Olympiads, commencing B. C. 775: the Romans, that of the building of Rome, B. C. 752; the ancient Persians and Assyrians, that of Nabonasser, &c.

Altitude of a celestial body, is its elevation above the horizon, measured on the arch of an azimuth-circle intercepted between the body and the horizon. The apparent altitude, or that measured by an instrument, re uires to be corrected in order to obtain the true altitude-1. by subtracting the refraction; 2. by adding the parallax; 3. by subtracting the dip corresponding to the height of the observer's eye above the surface of the earth; and 4. when the lower or upper limb of the sun or moon is observed, by adding or subtracting the apparent semidiameter. Altitude, meridian, is that of a body when on the meridian. Amplitude of a celestial body, is an arch of the horizon intercepted between the east or west points thereof, and that point where the body rises or sets. The true amplitude of a body may be found by the following proportion:

Rad: cos. lat. sin. dec.: sin. amp. which will be of the same name (north or south) with the declination.

The difference between the true, and the magnetic amplitude of a body, or that observed by a compass furnished with a magnetic needle, will be the variation of the com

pass.

Angle is the inclination of two converging lines meeting in a point, called the angular point. A plane angle is that drawn on a plane surface. The measure of a plane angle is the arch of a circle comprehended between the lines including the angle, the angular point being the centre. A spheric angle is that formed by the intersection of two great circles on the surface of a sphere. The measure of a spheric angle is the arch of a great circle comprehended between the two arches including the angle, the angular point being its pole. A right angle is one whose measure is an arch of 90°. An acute angle is one less than 90°. An obtuse angle, one greater than 90o. Anomaly is the angular distance of a planet from its aphelion. It is distinguished into true, excentric, and mean. True anomaly of a planet, is the angle at the sun or focus of the elliptical orbit, formed by the line of apses and radius vec