two successive ascents above or descents below the ecliptic, such will be its periodic time.

be

253. It will be easily seen that the place of a planet, seen from the earth, will in general differ greatly from its place as seen from the sun, but not always. The planet x, as seen from the earth y, or from the sun, at s, will in either case be referred to the point v. The same will be the case with a planet when at m, which will be referred both by the earth and sun to ~, cause in both instances the three bodies are in one and the same straight line. But the planet, when at b, will be referred by the inhabitants of the earth at y, to 10° of the sign; by the sun, to 0° of the same sign: the former is termed its GEOCENTRIC LONGITUDE, the latter its HELIOCENTRIC LONGITUDE.

Should it so chance that an inferior planet is in either its ascending or descending node when between the earth and the sun, as at a, it will be observed to pass like a dark spot across the disc of the sun. Such appearances are called TRANSITS; and they are of the utmost importance in determining certain calculations, which could not be done without their aid. They are of unfrequent occurrence, and when they happen are watched by astronomers with the greatest anxiety. Indeed, one of the principal objects of our government in sending out Captain Cook on one of his expeditions was to make remarks on the transit of Venus in the southern hemisphere. They afford a most irrefragable proof of the planets Mercury and Venus being nearer to the sun than we are.

254. A few remarks on the movements of a superior planet will conclude the subject of apparent motions.

Let n be the position of the earth, and p that of Mars, at a particular instant: if the earth were stationary, the planet would move round it according to the order of the signs; but the earth's motion is more rapid than that of Mars, from its being nearer to the sun (see § 200); hence it will describe the arc ny while Mars describes px. At the part nearest p, the planet Mars will appear to move according to the order of the signs, except that for a short period, as the earth approaches him, and they travel on for a time together, he will be apparently stationary. The earth, however, will soon outstrip and pass by him at or about x. In some part of his orbit between x and c, which we will suppose to be performed in the same time that the earth takes to describe the arc yd, he will appear slowly to retrograde; as when one vessel outsails another, the slower will appear to move backward when referred to fixed objects on shore.

That part of the earth's orbit between e and n we will suppose to be described in the same time as Mars takes to pass from b to m: here his motion will be direct through, N, and m.

After the earth leaves n, and Mars m, there will be another stationary point not far from the direction of a line joining n m.

The same explanation will apply to the movements of all the superior planets.

If y be

255. Two heavenly bodies seen in the same quarter of the heavens are said to be in CONJUNCTION. the earth, s the sun, and m Mars, the sun and Mars would be in conjunction. An inferior planet would be in its inferior conjunction with the sun at a, and in its superior conjunction at z.

If two heavenly bodies are in opposite quarters of the heavens, or distant from each other 180°, they are said to be in OPPOSITION. Thus a planet at x, and the sun at s, would be in opposition to each other, if viewed from the earth at y.

The inferior planets cannot ever be in opposition to the sun, as the diagram will shew.

The term SYZYGIES applies to either of these last two definitions.

256. Should a heavenly body pass over and obscure the light of another, such an occurrence is termed an OCCULTATION. The moon frequently is observed to pass over and hide from view those stars which lie in the neighbourhood of her orbit. The times of such occurrences may be found in the Nautical Almanac, under the head of "Occultations of the Fixed Stars by the Moon."

257. From what has been said of the movements of the planets, and the position of the earth's orbit with regard to them, it is clear that their apparent paths are exceedingly different from their real. The student who wishes to trace these paths among the stars for any length of time, would do well to procure the Nautical Almanac, in which the Right Ascensions and Declinations of the planets are given. Let him mark, with a brush full of vermilion, upon a celestial globe, the points which they occupy, at intervals of three or four days during several months; and, by joining these points with a fine line, he will be able to remark their direct and retrograde movements, and their stationary positions he will thus acquire in a few hours a more correct idea of their apparent motions than can be ob

tained by any other means. Or the same may be done with a black-lead pencil on maps of the stars. In globes the zodiac is divided by fine lines into degrees, for the purpose of facilitating this delineation of the apparent paths of the heavenly bodies. A damp sponge will obliterate the colour without the slightest injury to the globe.

258. When Copernicus first promulgated the true system of astronomy, which assumes the sun to be the centre, in opposition to that of Ptolemy, who considered the earth to be immovably fixed, while the planets and the sun revolve round it, he was fully aware that the planet Venus ought, in point of fact, to present phases similar to those of the moon. What must have been the gratification of the followers of that philosopher, when the telescope of Galileo, directed to that planet, shewed his reasoning to be based on truth, and that the Copernican system alone could account for the appearances of the heavens! By referring again to fig. 16, Plate III., it will be at once seen that any planet included within the earth's orbit must present phases similar to those of the moon. Let the earth be at y, and a h gf, &c., the orbit of an inferior planet, either Venus or Mercury at the portion of the planet enlightened by the sun-namely, its whole disc-will be turned towards the earth; at a, on the contrary, only the dark side will be so directed. At g and v, half of its illuminated disc will be seen; while between ƒ and and x and 29 the planet will appear gibbous, or more than half enlightened. At h and w a crescent only will be presented to the earth.

2,

259. The planet Mars, exterior to the earth, will

sometimes appear gibbous, and at others circular; at x and m, for instance, the whole of his illuminated disc will be directed towards the earth, while at b only about eight-tenths of it will be seen. The planets Jupiter and Saturn are so far distant, that no variation in their discs is perceptible.

In the Nautical Almanac will be found a "Table shewing the Illuminated Portion of the Discs of Venus and Mars" for every month in the

year.

When Venus appears to the eastward of the sun, she shines in the evening after sunset; when west of him, she rises before him. In the former case she is termed the evening, in the latter the morning star.

SECTION VI.

ON DETERMINING THE DISTANCES AND DIAMETERS OF THE SUN, MOON, AND PLANETS.

PARALLAX

HORIZONTAL PARALLAX PARALLAX IN ALTITUDENONE IN THE ZENITH-TRANSITS OF MERCURY AND VENUSMANNER OF DETERMINING THE PARALLAX OF THE SUN FROM A TRANSIT OF VENUS-METHOD OF MEASURING THE DISTANCE OF AN INFERIOR PLANET FROM ITS GREATEST ELONGATION-OF A SUPERIOR PLANET FROM ITS ARC OF RETROGRADATION.

260. In the pages immediately following, it will be our endeavour to present to the reader such considerations as may enable him to form some idea of the method which astronomers adopt in measuring the distances of