ESSAY I.

PART II.

OF THE PHENOMENA OF THE HEAVENS, AS SEEN 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

seasons.

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 proaches the sun from day to day, till at last it will

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disappear, being effaced by his light, though but few days before it was at a sufficient distance from him. That it is the sun which approaches the stars, and not the stars the sun, is plain, for this reason: the 'stars always rise and set every day at the same points of the horizon, opposite to the same terrestrial objects, and are always at the same distance from each other; whereas the sun is continually changing both the place of its rising and setting, and its distance from the stars.

The sun advances nearly one degree every day, moving from west to east; so that in 365 days we see the same star near the setting sun, as was observed to be near him on the same day in the preceding year. In other words, the sun has returned to the place from whence he set out, or made what we call his annual revolution.

We cannot indeed observe the sun's motion among the fixed stars, because he darkens the heavens by his splendour, and effaces the feeble light of those stars that are in his neighbourhood; but we can observe the instant of his coming to the meridian altitude; we can also compute what part of the starry heaven comes to the same meridian, at the same time, and with the same altitude. The sun must be at that point of the starry heavens thus discovered. Or we can observe that point in the heavens, which comes to the meridian at midnight, with a declination as far from the equator on one side, as the sun's is on the other side; and it is evident, the sun must be in that part of the heavens, which is diametrically op

posite to this point. By either of these methods we can ascertain a series of points in the heavens, through which the sun passes, forming a circle called the ecliptic.*

OF THE APPARENT PHENOMENA OF THE MOON.

The motion of the moon through the heavens, and her appearance therein, are still more remarkable than those of the sun; she engages the attention "by the nightly changes in her circling orb". At the new moon, or when she first becomes visible, she is seen in the western part of the heavens, at no great distance from the sun. She increases every night in size, and removes to a greater distance from the sun, till at last she appears in the eastern part of the horizon, when the sun is disappearing in the western; she then appears with a full round face, and we say it is full moon. After this, she gradually removes

further and further eastward, till at last she seems to approach the sun as nearly in the east as she did before in the west, and rises a little before him in the morning; whereas in the first part of her course she sets in the west, long after him. All these different appearances happen in the space of a month, after which they re-commence in the same manner; "sometimes half-restoring day with her waxing brightness; sometimes waning into dimness, and scarcely scattering the nocturnal gloom."

*The conformity of this definition of the ecliptic, with that given in page 13, will be seen hereafter.

There is sometimes an irregularity in these appearances, particularly in harvest-time, when the moon appears for several days to be stationary in the heavens, and to preserve nearly the same distance from the sun; in consequence of which, she rises at that season of the year nearly at the same hour for several nights.

OF THE APPARENT MOTION OF THE STARS.

In contemplating the Stars, it is observed that some among them have the singular property of neither rising in the east, nor setting in the west; but seem to turn round one immoveable point, near which is placed a single star, called the pole, or polar star.

This point is more or less elevated, according to the part of the earth from which it is viewed. Thus to the inhabitants of Lapland it is much more vertical, or elevated above the horizon, than with us: we see it more elevated than the inhabitants of Spain: and these again see it more elevated than those of Barbary. By continually travelling southward, we should at last see the pole star depressed to the horizon, and the other pole would appear in the south part of the horizon, round which the stars in that part would revolve. There is, however, no star in the southern hemisphere so near the pole as that in the northern hemisphere. Supposing us still to travel southward, the north pole would entirely disappear, and the whole hemisphere would seem to