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opposite points. These points are called the planet's nodes.* These nodes are all in different parts of the ecliptic; and therefore if the planetary tracts remained visible in the heavens, they would in some measure resemble the different ruts of waggon-wheels on a road, after crossing each other, but never going far asunder.

29. While the primary planets are performing their revolutions round the sun, and the secondaries round their primaries, they have all a motion from west to east, round an imaginary line passing through their centre, called their aris. In some of the planets this axis is nearly perpendicular to the plane of its orbit; and in others it is inclined to the plane of its orbit. On this depends the change of seasons in the planet; for the smaller the angle which the axis of any planet makes with the plane of its orbit, the greater is the variety of its seasons.

30. The extremities of the axis is called the poles; and that which points towards the northern part of the heavens is called the north pole; and the other, pointing towards the southern part, is called the south pole.

31. As it is necessary to have some method by which the position of any celestial body may be determined at any time, or its distance from some known point, astronomers have fixed on the ecliptic for this purpose, as well as for reckoning from it, the inclination of the planetary orbits.

The line of the equinoxes, or that line which joins the equinoctial points, being always in the plane of the ecliptic, must mark out two points of that line traced among the fixed stars; and from one of these points astronomers reckon distances on the ecliptic. This point is called the vernal equinox, because the sun appears in it in the spring, about the 20th of March; its opposite is called the autumnal equinox, because the sun arrives at it about the middle of autumn, or the 23d of September.

32. The Ecliptic is supposed to be divided into twelve equal parts, called signs, each of which occupy a space of 30 degrees.

33. The Longitude of all the celestial bodies is reckoned eastward, on the ecliptic, from the vernal equinox quite round the heavens, and consequently may amount to nearly 360 degrees.

The Latitudes of the celestial bodies is reckoned from the ecliptic, north and south; but their declination is reckoned from the equinoctial, in a similar

manner.

34. The Right Ascension of the celestial bodies is reckoned on the equinoctial, from the vernal equinox, eastward, quite round the heavens.

35. Instead of the Vernal Equinox, astronomers sometimes find it convenient to count the distance of a planet from its aphelian, (as from A, in the foregoing figure.) This distance is called the true anomaly of the planets.

36. An imaginary line, drawn from the sun to a planet, in any point of its orbit, as S B, is called the radius vector, or great radius. This line has the property of describing equal areas of the orbit, in equal portions of time.t

37. The Elongation of a Planet is its angular distance‡ from the sun, as seen from the earth.

* The node, where the orbit ascends above the ecliptic, is called the ascending node; and the other the descending node.

+ This was discovered by Kepler, and is called his second law.

By angular distance is meant any distance in degrees, minutes, &c.

38. The Opposition of two celestial bodies takes place when they are in opposite points of the heavens, as viewed from the earth; and their Conjunction when they appear in the same point. These points are also sometimes termed the syzygies, especially when the bodies are the sun and moon; And when a planet is between the sun and the earth, at the time of conjunction, it is called its inferior conjunction; but when the sun is between the earth and the planet, it is called its superior conjunction.

39. The Geocentric Place of a Planet, means its place as seen from the earth, and its Heliocentric place as seen from the sun.

40. The Direct Motion of a Planet is its motion in the order of the signs, as from Aries to Taurus, &c.; when it moves in a contrary direction, it is said to have a retrograde motion.

41. The Occultation of a Star or Planet is its obscuration by the moon, or other planet, coming between it and the earth.

42. By the Siderial Revolution of a Planet is meant the time it requires to move from any fixed star to the same again.

43. The Disc of the Sun, or a Planet, is its face, which appears flat on account of its immense distance.*

44. Aberration is an apparent motion or change of place in the heavenly bodies, occasioned by the progressive motion of light, combined with the earth's annual motion in its orbit.

45. Nutation, or Deviation, is a small inequality which has been observed in the precession of the equinoxes, which makes the fixed stars appear to change their positions about 9" of a degree.

46. Evection is an inequality in the motion of the moon, by which at or near the time of her first and third quarter she is not in the line drawn through the centres of the earth and sun, as she is at the time of new and full moon. This inequality sometimes amounts to 2° 51'.

47. Nebula are clusters of small stars, which have been chiefly discovered by the telescope. They have received this appellation from their cloudy appearance.

Macula are dark spots which are frequently seen upon the disc of the

sun.

48. Facula are certain bright spots frequently seen on the disc of the sun. 49. Centrefugal Force is that force by which a revolving body endeavours to recede or fly off from the orbit or path in which it revolves.

50. Centripetal Force is that force by which a body is drawn towards the centre of the path or orbit it describes, and prevents it from flying off.

51. Penumbra, a faint shadow which borders the dark shade produced by an eclipse.

52. Digit, the twelfth part of the sun or moon's diameter.

*When any planet appears on the face of the sun, it is said to transite his disc.

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GENERAL APPEARANCE OF THE HEAVENS TO
THE NAKED EYE.

We, tho' from Heav'n remote, to Heav'n will move
With strength of mind, and tread the abyss above:
And penetrate, with an interior light,

Those upper depths, which nature hid from sight.
Pleas'd we will be to walk along the sphere
Of shining stars, and travel with the year.
To leave this heavy earth, and scale the height
Of Atlas who supports the heav'nly weight:
To look from upper light, and thence survey
Mistaken mortals wand'ring from the way.

Ovid's Metamorphosis, Book xv.

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THE first and most obvious phenomenon of the heavens is the daily rising of the Sun in the east, and his setting in the west; after which the moon and stars appear still keeping the same westerly course, till we lose sight of them altogether. By attending to these

appearances, it must soon be perceived that neither the sun nor the moon rises or sets in the same place. If we begin to observe the sun about the middle of March, he will appear to rise every day sensibly more to the northward than he did the day before, to continue longer above the horizon, and to be more elevated at mid-day. This continues to be the case till the 20th of June, when he begins to move backward in the same manner; and this retrograde motion continues till the 22d of December, when he begins again to move forward, and so on.

The motion of the Moon through the heavens, and her appearance at different times, are still more remarkable. When the moon first becomes visible to us, she is called new moon, and appears in the western part of the heavens, at no great distance from the sun. Every night she increases in size, but removes to a greater distance from the sun, till at last she appears in the eastern part of the heavens, just at the time he disappears in the western.

The western sun withdraws: meanwhile the moon,
Full orb'd, and breaking thro' the scatter'd clouds,
Shews her broad visage in the crimson'd east.

THOMSON.

After this, she gradually moves farther to the eastward, and, therefore, rises every night later than the preceding night; till at last she seems to approach the sun as nearly in the east as she did in the west, and rises only a little before him in the morning, as in the first part of her course she set in the west not long after him.

All these appearances take place in the space of a month; after which they begin again in the same order as before. They are not, however, at all times regular; for at some seasons of the year the moon appears to differ little in the time of her rising for several nights together, and at others the difference is very considerable. But this will be more fully stated, and the cause explained, when describing what is called the harvest moon.

In contemplating the stars in a clear night, they all appear at the same distance from us, and therefore seem to be situated in the concave surface of a sphere, having the eye for its centre. But when we view them for a considerable time, we find that they change their positions relatively to the objects on the earth, although they still retain their positions relatively to one another, and that they all seem to make progress towards the west, where some of them disappear under the horizon, while others again appear to come above that circle in the east. It will also soon be observed, that many of the stars never go below the horizon at all, but seem to turn round an immoveable point, near which is placed a single star, called the Pole Star. This point is more or less elevated according to the different parts of the earth from which it is viewed. The inhabitants of Lap

This is only the case in the northern hemisphere, which is the one here alluded to.

+ For an explanation of Astronomical terms, see page 8.

land, for example, see it more elevated above their horizon than the inhabitants of Great Britain; they see it more elevated than the inhabitants of Spain and Italy; and they see it still more elevated than the inhabitants of the West India Islands. By continually proceeding southward, this star will seem depressed to the horizon, and at last lost sight of altogether; but another point will appear directly opposite to it, round which the stars in the southern hemisphere will appear to turn. There is, however, no star so near this point as in the northern hemisphere; neither are the stars so numerous in the southern as in the northern half of the heavens.

The general appearance of the heavens is, therefore, that of a vast concave sphere, turning round two fixed points once in twenty-four hours.

These are Thy glorious works, Parent of Good!
Almighty! Thine this universal frame,

Thus wondrous fair! Thyself how wondrous then!
Unspeakable! who sit'st above the heavens,

To us invisible, or dimly seen

In these Thy lowest works; yet these declare

Thy goodness beyond thought, and power divine!

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Of all the celestial bodies, the Sun is certainly the most wonderful. It is the fountain of light which illuminates the world; it is the cause of that heat which maintains the productive power of nature, and makes the earth a fit habitation for man; it is the central body of the planetary system, and is so far superior in lustre to all other celestial bodies, that they disappear in its presence.

No stars besides their radiance can display
In Phoebus' presence, the dread lord of day;
Ev'n Cynthia's self, tho' regent of the night,
Is quite obscur'd by his emergent light.

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