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one half of its path on one side, and the other half on the opposite side of the standard path or orbit. Astronomers generally assume the earth's orbit as a standard from which to compute the inclination of the others, and call it the ecliptic. The points where the orbits intersect each other, are called the nodes.
This inclination of the orbits to each other may be rendered more familiar to the imagination,* by taking as many hoops as there are planets, with a wire thrust through each, and thereby joined to that hoop which represents the ecliptic; the other hoops may be then set more or less obliquely to the representative of the ecliptic.
The several orbits do not cross or intersect the ecliptic in the same point, or at the same angles; but their nodes or intersections, are at different parts of the ecliptic.
It should, however, be observed here, that in speaking of the orbits of the planets, nothing more is meant by this term, than the paths they pass through in the open space in which they move, and in which they are retained by a celestial but continuous mechanism.
OF THE MOTION OF THE PLANETS ROUND THEIR
By attentively considering, with a telescope, the surface of the primary planets, our solar observer will find that some parts or spots are more obscure than others. By continued observation he will find,
* Pr. IValti's Astronomy.
that these spots change their places, and move from one side of the planet to the other; then disappear for a certain space of time; after which they again, for a while, become visible on the side where they were first seen, always 'continuing the same motion nearly in an uniform manner. The distance between the spots grows wider as they advance from the edge towards the middle of the planet, and then grows narrow again as they pass from the middle to the other edge. The time they are seen on the planets disc is somewhat less than the time of their disappearance.
From these circumstances he will conclude, first, that these spots adhere to the body of the planet; and secondly, that each planet is a globe turning on its axis, and has consequently two motions; one whereby it is moved round its axis in a short time, the other whereby it revolves round the sun. These motions may be easily conceived, by only imagining a small ball to roll round a large sphere. The first of these motions, or that of a planet round its axis, is called the diurnal motion; and the second, or its revolution round the sun, is called the annual motion. The tutor may in some measure realize to his
pupil the foregoing heliocentric phenomena, by plate 1, fig. 1, of the solar system; or still much better by means of a planetarium : for, by supposing himself on the brass ball which represents the sun, he will see that all the planets move round him in a beautiful and harmonious order. If on account of their distance, he refers their motions to the fixed stars,
he will see how readily the periods of their revolutions
may be obtained, by observing the time that elapses between their setting out from any fixed point, or star, and their returning to the same again. He will also see, that if the paths of the planets were in one plane, as in the instrument, they would all be transferred to one circle in the heavens.
When he understands these particulars, the tutor may proceed to shew him that the motions, which are so regular when viewed from the sun, become intricate and perplexed when viewed from the earth; and infer from hence, that whenever “we examine the works of the Deity at a proper point of distance, so as to take in the whole of his design, we see nothing but uniformity, beauty, and precision.” Thus the heavens present us with a plan, which, though inexpressibly magnificent, is yet regular beyond the power of invention; and the volume of the universe will be found to be as perfect as its AUTHOR, containing mines of truth for ever opening, fountains of good for ever flowing, an endless succession of bright and still brighter exhibitions of the glorious Godhead, answering to the nature and idea of infinite fulness and perfection.
OF THE PHENOMENA OF THE HEAVENS,
FROM THE EARTH.
The various appearances of the celestial bodies as seen from the earth, are the facts which lay the foundation of all astronomical knowledge. To account for, and explain them, is its principal business; a true idea of these phenomena is therefore a necessary step to a knowledge of astronomy. Let us therefore suppose ourselves in the open air, contemplating the appearances
that occur in the heavens.
OF THE APPARENT MOTION OF THE SUN.
The first and most obvious phenomenon 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. These appearances give rise to what is called the apparent diurnal motion of the heavens.
This cannot be long observed, before we must also perceive, that the sun does not always rise exactly at the same point of the heavens, his motions
deviating considerably at particular seasons from those they perform at other times. Sometimes we perceive him very high in the heavens, as if he would come directly over our heads; at other times he is almost sunk in the southern part of the heavens. If we commence our observations of the sun, for instance, in the beginning of March, we shall find him
appear to rise more to the northward every day, to continue longer over the horizon, to be more vertical, or higher, at mid-day; this continues till towards the end of June, when he moves backward in the same manner, and continues this retrograde motion till near the end' of December, when he begins to. move forwards, and so on.
It is this change in the sun's place that occasions him to rise and set in the different parts of the horizon, at different times of the year. It is from hence that his height is so much greater in summer, than in winter. In a word, the change of the sun's place in the heavens is the cause of the different lengths in the days and nights, and the vicissitudes of the
As the knowledge of the sun's apparent motion is of great importance, and a proper conception of it absolutely necessary, in order to form a true idea of the phenomena of the heavens the reader will excuse my dwelling something longer upon it. If on an evening we take notice of some fixed star near the place where the sun sets, and observe it for several successive evenings, we shall find that it
approaches the sun from day to day, till at last it will