Though the orbits of the planets in the frontispiece are circles, yet they are not concentric, that is, have not the same centre The centre of each orbit is placed out of the centre of the sun at a distance equal to the eccentricity of its true orbit. Each planet is placed in its aphelion.

The relative distances of the primary planets from the sun could not be well preserved in this figure, but are represented in the margin.

9. The sun and all the planets, primary and secondary, are globular, though not perfect globes. This is known of all

, except the earth, by their always appearing nearly round to the naked eye, or through a telescope. known of the earth, by its shadow on the moon in an eclipse, which is always circular. Pl. 1, fig. 4, represents the relative magnitudes of seven of the primary planets and the moon, together with the ring of Saturn, which will be described hereafter. The diameter of the sun in relation to that of the planets, as here represented, is about one foot.

10. The sun and the primary and secondary planets, as far as astronomers have means and opportunity of ascertaining, turn on imaginary lines passing through their centres, which are called axes. The time, in which the heavenly bodies turn on their axes, is various ; but generally the largest turn quickest. A wire passing through the centre of an apple properly represents the aris of a planet. The extremities of an axis are called POLES.

11. If the earth were seen from the sun, (Pl. I, . fig. 1,) it would appear to describe a circle among the stars, while it revolves in its orbit. For while it is passing from A to B, it would be seen to move among the stars from a to b. And in like manner through its whole orbit. While the earth, viewed from the sun, would describe this circle among the stars, the sun, to us on earth, appears to describe precisely the same circle, only beginning at the opposite point, For while

Sect. II. Of Day and Night
SECT. III. Art. 1.-Aberration of light
Art. 2.-The Seasons
Art. 3.—Equation of time
Art. 4.--Of the Harvest Moon
Sect. IV. Of phenomena arising from the Earth's

atmosphere Sect. V. Of Parallax

56 59 60 63 69

72 79

BOOK II.

Attraction

87 Sect. I. Of the motion of heavenly bodies in their orbits

90 Sect. II. Of the retrograde motion of the Moon's nodes

94 Sect. III. Of Irregular Motions

95 Sect. IV. Of the spheroidal figure of the planets 101 Sect. V. Of the precession of the equinoxes

103 Sect. VI. Of the Tides

105

APPENDIX.

Sect. I. Of Meteors

109 Sect. II. Of the different Systems

115 SECT. III. Of Leap Year

117 Sect. IV. Of Old and New Style

118 Sect. V. Of Cycles

118 SECT. VI. Of the Dominical Letter

121 SECT. VII. Of Epact

125 SECT. VIII. Problems Art. 1.- Problems to be solved by the Terrestrial Globe

126 Art. 2.-Problems to be solved by the Celestial Globe 134 Questions

137

INTRODUCTION.

1. The first change in nati re, which the eye, just opened upon the things of this world, notices, is that of light and shade, from day to yht and from night to day. The beams of the morning awake the infant from the slumber of the cradle, and call him forth to activity and life; the twilight of the evening insensibly disengages his attention from objects of sight, and the darkness of night finds him again in repose.

He soon walks forth under the heavens. He notices, that when darkness gives way to light, the sun becomes visible in the east; and that when the sun has passed through the heavens, he disappears in the west, and light gradually yields to darkness. At the same time that these changes are noticed by the eye, he feels that warmth or heat increases and decreases, much in the same manner, and in the same degree, that light does; and that at the same time that darkness steals one object after another from his sight, a sensation of cold pervades his frame. He soon comes to this conclusion, that the sun is the grand dispenser

of heat and light; that the day is caused by his presence; and that the coldness and darkness of night are nothing but his absence.

2. Besides the changes of heat and cold which a single day exhibits, he will pass but a small part of the ordinary age of man, before he becomes sensible of other changes. He observes a long succession of days and nights, during which the atmosphere is warm and comfortable to himself and all other animals, and the earth puts forth her thousand forms of vegetable life. By degrees the atmosphere is divested of its heat, the vegetable kingdom is stript of its foliage, and cold and snow succeed agreeable temperature and verdure. After some length of time, he beholds the earth again renovated, and' nature again rejoicing in genial warmth. During these changes, the sun appears to move northward, and southward. By witnessing a few of these changes, he understands what is meant by the seasons, Summer and Winter, Spring and Autumn.

3. When the sun has apparently retired from creation, night presents its countless multitude of shining bodies. The most careless observer cannot long withhold attention to the ever varying phases of the moon. At one time, it is seen just after sunset, like half a ring. Gradually this ring fills up, or thickens, till, in about a week, it becomes a semicircular surface. It continues to increase, till the surface becomes perfectly circular. It then decreases, as it had before increased, and for a short time is invisible; when it appears again as a part of a ring.