MERCURY,

whose disc Can scarce be caught by philosophic eye, Lost in the near effulgence of his blaze.

We now proceed to the consideration of the bodies which revolve round the Sun, and shall treat of them in the order of their distances, the nearest first.

We should remark, before proceeding further, that the word planet signifies a wanderer; an appellation given to those heavenly bodies, which seemed to move in circles in the heavens. Hence this distinction may refer both to the planets usually so called, and to their moons; the former of which are styled primary, the latter secondary planets. Again the primary planets are distinguished into interior or inferior; and exterior or superior :-the first pair of terms pertaining to the two planets whose orbits are between the Sun and the orbit of the Earth; the second pair referring to the planets more distant from the Sun than the Earth.

The planet nearest to the Sun, as far as we know for certain, is Mercury; which is somewhat more than 36,000,000 of miles distant therefrom, and is about 3,123 miles in diameter. He revolves on his axis in about 24 hours 5 minutes, being a little more than the time occupied by the Earth in revolving on her axis. He moves round the Sun in an elliptical orbit, (as we said do all the other planets,) and when we speak of the distance between him and the Sun, it must be understood that we refer to the mean distance. The time which he takes to travel round the Sun is almost 88 days; a period which constitutes his year; for we must bear in mind, as will be hereafter more particularly shown, that our year is nothing more than the length of time which the Earth occupies in going once round the Sun. Hence when we speak of a year as connected with the motion of Mercury, we must remember that his year is different from ours, the latter being rather more than four times as long as the former.

As he revolves round his orbit of 72 millions of miles in diameter in about 88 days, he has a velocity of motion of about 100,000 miles an hour; a rate of which we can form some conception by considering that 100,000 miles is about four times the circumference of the earth.

As Mercury is so much nearer the Sun than the Earth, it follows that the amount of light and heat received by this planet is much greater than that received by the Earth; and it has been estimated that it is about seven times greater. Another effect of the comparative proximity of Mercury is, that he never appears so far removed from the Sun as the other planets. At no time is he more than 30° distant from the Sun; that is, if, at any time we draw a line from the Earth to the Sun, and another from the Earth to Mercury, those two lines will never form a larger angle than 30°. It is, in part, for this reason that we do not usually speak of Mercury as a morning or an evening star; he never rises much before the Sun, nor does he set much after the Sun: and, in the next place, owing to the proximity of the Sun, by means of which the rays of this planet are, as it were, drowned, we seldom see him at any time. Not but that, in very clear weather, this planet may be seen just before sunrise in the morning, or just after sun-set in the evening, when it appears a good way off from the Sun. His appearance is brighter than that of Venus, and has a light blue tint about it. He subtends an angle of from 5" to 12" of a degree, according to his position in respect of the Earth.

There is a circumstance which satisfactorily proves that Mercury does not shine by virtue of any light of his own, but merely by light reflected from the Sun. This circumstance, which also applies to some of the other planets, is the existence of phases, similar to those which are periodically observed in our moon. When Mercury appears at his greatest distance or elongation from the Sun, his illuminated surface has nearly the form of a half circle, more or less, according to the position of the Earth. But when he is passing round on the opposite side of the Sun to that at which the Earth is situated, the illuminated portion becomes more than a semicircle, and assumes that form which is called gibbous; which phase may be represented by placing a semicircle and a semioval with their flat edges in contact. When Mercury is at his greatest distance from the Earth, the Sun is then between them, and shuts out the view of Mercury altogether, so that the whole of the illuminated disc of Mercury can never be seen from the Earth; but when

This is a word from the Greek, and signifies appearances.

he emerges on the other side of the Sun, he becomes again visible, and his illuminated surface approaches more and more to the form of a semicircle, as he travels on to the position in which he forms a right angle with the Sun and the Earth. As he proceeds in his orbit, he approaches nearer to the Earth, and his semicircular phase becomes diminished to a crescent, which attains its smallest dimensions, when Mercury is either exactly over, or exactly under the Sun; in which position only a few of the Sun's rays, reflected from the surface of Mercury, can reach the earth, and even those few are rendered almost imperceptible by the superior flood of light, which proceeds directly from the Sun to the Earth. These observations apply likewise to the planet Venus.

It is by such evidence as this that we know that a planet does not appear luminous by any inherent light of its own, but that we see it, in consequence of its reflecting the solar light to the Earth. There is every reason to believe that the Earth reflects light from its surface, just as we have described with respect to Mercury; and that if there be inhabitants in any of the other planets, they see the Earth under the various forms of crescent, semicircular, and gibbous. To this conclusion we arrive both by analogy, and by certain appearances of the Moon during a solar eclipse, to which we shall hereafter refer more particularly.

A line drawn from the Earth to the Sun is said to be in the plane of the ecliptic. The ecliptic may be imagined to be a large oval flat surface, which passes through the Sun, and round the edge of which the Earth moves in the space of a year. Now if we suppose a similar, but smaller, oval surface passing through the Sun, and that Mercury passes round the edge of that oval in his year of eighty-eight days, it may be asked whether these two oval surfaces coincide, or whether one slopes with respect to the other? We have to reply that they do not coincide; but that although both of them pass through the sun, they are inclined the one to the other. It will perhaps be desirable here to state that each planet may be conceived to have an oval plane belonging to itself, round the edge of which it passes in the course of its year; and further, that no two of these planes exactly coincide, all of them being inclined one to another. Now the mode of expressing the amount of the inclination, or obliquity of two such planes, is by the angle included between them. Two such planes are said to cut each other in an imaginary line passing through the Sun, which line therefore may be considered as a kind of hinge, connecting the two planes; and the angle, at which these planes are inclined to each other, is measured with reference to that line.

It will now be understood what is meant when we say that the plane of the orbit of Mercury is inclined at an angle of about seven degrees to the plane of the ecliptic, the latter term being always reserved for the plane of the Earth's orbit. If the plane of the orbit of Mercury coincided with the ecliptic, it would follow that Mercury, when at its shortest distance from the earth, would be exactly between the centre of the Sun and the centre of the Earth; and that we should therefore see him as a black spot on the central part of the Sun, which circumstance was first observed by Gassendi in the year 1631. As, however, the planes of the two orbits do not coincide, and yet the planet Mercury is occasionally seen as a black spot on the Sun's disc, the determination of the circumstance, whether or not Mercury will appear on the face of the Sun, depends on the position of the line, which joins the two planes. If Mercury happen to be at his nearest point to the Earth, when he reaches that line, then we shall see him cross the centre of the Sun's disc, in which case the line connecting the two planes will pass through the centres of the Earth, Mercury, and the Sun. But if that line be in any other direction, when Mercury gets between the Earth and the Sun, then he will be either a little above or a little below the centre of the Sun, or more probably, exterior to the disc of the Sun altogether.

From these circumstances it follows that Mercury may revolve many times round the sun, without getting exactly on the line joining the Earth and the Sun. When, however, such an occurrence does take place, it is called a transit t of the planet. At such a time Mercury appears as a small round black spot, because his illuminated surface is directed wholly away from the Earth. He crosses the Sun's disc from east to west, in consequence of his motion from west by south to east. Such a transit as this occurred about the middle of the day on May 5th, 1832. These transits of the

(† From the Latin transitus, a passing over.

planets show that they are opaque bodies, having no light | being considered with reference to the smaller or greater of their own.

Fig. 10.

Conjunction

Superior Conjunction.

Inferior Conjunction.

Opposition.

It is obvious therefore, that the splendour of her light, and her apparent size must suffer considerable modifications, as well from her varying distance, as from the phases which she exhibits. This is well illustrated in the different figures or phases of this planet, as given above. The first phase, which is the gibbous, shows Venus, just after having passed from superior conjunction, and when at almost its greatest distance from the earth; it not being seen when in a line with, or too close to, the sun. The second phase corresponds to our half-moon, and is the aspect of Venus when forming a right angle with the sun and the earth; it is then at its greatest elongation. The third phase, or crescent, is observed just before and after the planet's inferior conjunction, and when at about its nearest distance to the earth. Hence it has been noticed, as shown in the figure above, that the apparent size of Venus is indicated as greatest by the crescent form, and that her light is then most brilliant and abundant; and that her apparent size is least, when in the gibbous state, and her light proportionably diminished :all depending upon her distance from the earth. When Venus gets near to the earth she appears upwards of thirty times larger than when farthest off; and her light about this time is so intense as to project a real shadow upon a white ground, such as clean writing-paper. She may even be seen in the day-time, near, and soon after her greatest elongation. The phases of Venus were first pointed out by Galileo; this being one of his earliest discoveries with the telescope.

Fig. 11.

on her axis has been determined, but the inclination of that axis to the plane of her orbit has been found to be as much as 75° The inclination of the earth's axis to the ecliptic, or path of the earth round the sun, is about 23°; and this inclination causes the diversity of seasons. How much greater then must be the diversity of seasons on the face of the planet Venus, than on the Earth! Some parts of her surface may enjoy the four seasons twice in the year. The light and heat which she receives from the Sun, is probably about twice as great as that received by the Earth.

We have now to speak of a train of observations, which were productive of important consequences, and in which the planet Venus rendered great service, we mean the correct calculation of the Sun's parallax; and from that, his distance from the Earth, by means of a transit of Venus across his disc. We have stated that Mercury occasionally crosses the Sun's disc, as viewed from the Earth; and it may reasonably be expected that Venus, being, like Mercury, an inferior planet, would likewise cross the Sun's disc at certain times. Such is the case,-as was first observed by Horrox, a student of Cambridge, in the year 1639; but the transits of Venus occur with much less frequency than those of Mercury. The former happening alternately at intervals of eight and a hundred and thirteen years, nearly. The most celebrated of these transits occurred respectively in the years 1761 and 1769;-celebrated, because many of the European governments sent out scientific expeditions, in order that this phenomenon might be observed at different parts of the Earth, with the greatest accuracy, and under the most favourable circumstances. At the time of the transit, Venus must, of course, be at its nearest position in respect of the Earth, and its apparent diameter will be of the largest extent, which is 61", or five times that of Mercury. The mean apparent diameter of Venus is about 17".

Venus, like Mercury, being an inferior planet, cannot, of course, be at any time in opposition to the Sun: indeed, she never recedes farther from him than 47°, which is her greatest elongation, or the angle, which would be formed by two lines drawn to the Earth, one from the Sun and the other from the planet. The orbit of Venus being exterior to that of Mercury, the greatest elongation of Venus, specified by the angle mentioned above, is necessarily greater than that of Mercury; to which circumstance it is owing that Venus is sometimes visible above the horizon for more than three hours before the Sun rises, and upon other occasions for as long a time after the Sun has set:-in the former case she is to the west of the Sun, and is called a morning star, and in the latter she is to the east and is designated an evening star. The elongation of a planet refers, then, to that position in which the planet is seen at its greatest distance from the Sun; at which time the planet appears, as we just said, in the shape of a half-moon-these phases, however, cannot be perceived without the aid of a telescope. The surface of this planet is found to exhibit spots, simi-preciate the distances of the heavenly bodies. lar to those of which we shall hereafter have to speak in the Moon's. By these spots not only the rotation of the planet This is the planet which, rising before the sun, the old poets termed Phosphorus and Lucifer; both words meaning light-bearer. When setting after the Sun, in the evening, they termed it Hesperus or Vesper; which terms have relation to the evening.

The final object, which was more particularly held in view in making these observations was, to determine with greater precision than had been previously done, the distance of the Earth from the Sun; in order that the true magnitude and extent of the whole solar system might be from thence determined. The manner in which this was accomplished does not admit of a full explanation, without entering into more intricate details than are suitable to these papers. We must, however, briefly explain the nature of parallax; -as it was through the medium of this phenomenon that the transit of Venus was rendered available for the object in view; and as the question of parallax is one of the fundamental principles, called into use, in our endeavours to ap

The first step to the accomplishment of the main purpose was to ascertain the parallax of Venus; as a medium point, whereby the solar parallax might be more precisely

obtained.

The term parallax signifies a change produced by passing by. Familiarly speaking, we have parallax constantly