transit can occur, except when the planet crosses the visual ray drawn from the eye of the observer to the sun's centre, it is manifest that the planet Mercury, during a central transit, must actually pass through the ecliptic from one side of this plane to the other. This point of passage through the plane of the sun's apparent orbit is called the node of the planet's orbit. There are, of course, two such points. The planet passes its descending node in moving from the north to the south side of the ecliptic, and its ascending node on its return from the south to the north side.

It is thus seen that in order to produce a transit of Mercury there must be a conjunction of the planet, its node, and the sun. Whenever this conjunction is absolute, Mercury will pass across the sun's centre. When it is only approximate, the planet will transit a small portion of the sun's disc, or possibly pass without contact at all.

An attentive examination of the places of the planet, before and after a transit, led to a pretty accurate determination of the angle under which the plane of the planet's orbit is inclined to the plane of the ecliptic. This angle was approximately determined by the ancients, while modern science fixed it at the commencement of the present century at 7° 00' 10".

The motion of Mercury in its orbit is more rapid than that of any of the planets thus far discovered, travelling, as it does, more than one hundred thousand miles an hour, and performing its entire revolution about the sun in about eighty-eight of our days. In case this world has the same variety of seasons which mark the surface of our own earth, these will follow each other in such rapid succession that the longest of them will consist of only about three of our weeks. It is not difficult to compute the intensity of solar light and heat which falls upon the surface of the planet Mercury, in case these be subjected to the same modifying influences which exist upon the earth. But as we remain in ignorance of the circumstances which surround

this distant planet, it is vain to speculate upon the physical constitution of a world whose close proximity to the sun has thus far shut it out from the reach of telescopic examination.

The distance of the planet Mercury from the sun may be readily determined, in certain portions of its orbit, in case we know first the earth's distance from the same orb. For example, conceive a visual ray to be drawn from the earth, tangent to the orbit of Mercury (supposed, for the present, to be circular); place the planet at the point of contact, and join the centre of the planet with the centre of the sun; also join the centres of the earth and sun—the triangle thus formed, having the earth, Mercury, and the sun as the vertices of its three angles, is right-angled at Mercury, while the angle at the earth is readily measured, and is nothing more, indeed, than the elongation, for the time being, of that planet. Hence, in the right-angled triangle, we know the angles and the longest side, extending from the earth to the sun, and by the simplest principles of trigonometry, we can compute the remaining parts-namely, the distance of Mercury from the sun and from the earth. By this, and by other methods more accurate, it is found that Mercury revolves in an orbit around the sun, and at a mean distance of about thirty-six millions of miles.

As the entire orbit of this planet lies within the limits already assigned, it follows that the planet can never be seen in a quarter of the heavens opposite to the sun, or can never be in opposition. When nearest the earth, and on the right line joining the sun and earth, Mercury is said to be in inferior conjunction. When 180° distant from this place, it is on the other side of the sun, with respect to the earth, and is then in its superior conjunction.

The telescope has demonstrated that this planet passes through changes like those presented by the moon. When in superior conjunction, the planet will be seen nearly round, as in that position nearly the whole of the illuminated surface is turned toward the eye of the observer

on the earth. As the planet comes round to its inferior conjunction, the light gradually wanes, until at inferior conjunction a slender crescent of great delicacy and beauty is revealed to the eye, provided the planet does not lose its light entirely in the passage across the sun's disc. These phases of Mercury prove, beyond question, the fact that the planet does not shine by its own light, but that its brilliancy is derived from reflecting the light of the solar orb.

The degree of precision reached in predicting the transits of Mercury indicates, with wonderful force, the progress of modern astronomy. The first predicted transit which was actually observed, occurred in 1631, when the limits of possible error were fixed by the computer at four days; and hence the watch commenced two entire days before the predicted time.

If the transit had taken place in the night-time, the opportunity for verification would have been lost. Fortunately this was not the case, and the toil and zeal of Gassendi were rewarded with the first view of Mercury projected on the solar disc ever witnessed by mortal man. Nearly two hundred years later, at the beginning of the nineteenth century, the French astronomers ventured to assert that their predictions could not be in error more than forty minutes. The transit which occurred on the 8th November, 1802, verified this assertion very nearly. By a more careful study of the causes affecting the place of the planet, forty-three years later, the discrepancy between computation and observation was reduced to only sixteen seconds of time, a quantity very minute, when we take into account the variety of causes affecting the resolution of the problem. The transits of Mercury recur at certain regular intervals, repeating themselves after a cycle of 217 years, falling for the present in the months of May and November.

Having learned the distance of Mercury from the earth, and having measured the angle subtended by its diameter, we find its actual magnitude to be much smaller than that

of the earth. Its diameter is but 3,140 miles, and its vo lume is but 0.063, the earth's volume being counted as unity.

In comparison with the vast proportions of the sun, this little planet sinks into absolute insignificance; for if the sun be divided into a million equal parts, Mercury would not weigh as much as the half of one of these parts.

CHAPTER III.

VENUS, THE SECOND PLANET IN THE ORDER OF DISTANCE FROM THE SUN.

The First Planet discovered.-Mode of its Discovery.-Her Elongations.Morning and Evening Star.-A Satellite of the Sun.-Her Superior and Inferior Conjunctions.-Her Stations.-Direct and Retrograde Motions -These Phenomena indicate a Motion of the Earth.-Transits of Venus. -Inclination of the Orbit of Venus to the Ecliptic.-Her Nodes.-Intervals of her Transits.-Knowledge of the Ancients.-Phases of Venus.Her Elongations unequal.-No Satellite yet discovered.-Sun's Light and Heat at Venus.-Her Atmosphere.

THIS planet is the second in order of distance from the sun, and as it is the most brilliant of all the orbs, with the exception of the sun and moon, it was undoubtedly the first discovered of all the planets. The movements of the sun and moon among the fixed stars must have claimed the attention of the observers of celestial phenomena in the earliest ages of the world. In marking the rising and setting sun, and in noting the stars which were the last to fade out in the morning twilight and the first to appear in the evening after the setting of the sun, the brilliancy of Venus could not fail to have attracted the attention of the very first observer of celestial phenomena. A star of unusual brightness was noticed in comparative proximity to the sun in the early evening. The sun's place, with reference to this object, having been carefully marked, for a few consecutive nights, it was found that the distance between them was rapidly diminishing. It was readily seen that this diminution of distance was due to the fact that the bright star was

approaching the sun, for by comparing its place ainong the fixed stars with what it was a few nights previous, this star was found to have changed its position among the group in which it happened to be located, and was evidently advancing rapidly toward the sun.

We are thus presented with the exact facts which must have marked the discovery of the first planet or wandering star ever revealed to the eye of man. We know not the name of the discoverer, nor the age or nation to which he belonged, but we are satisfied that the facts as above stated did undoubtedly occur; and we find not only profane authors, but one of the Hebrew prophets,* referring to this planet more than two thousand five hundred years ago. The student who desires may easily re-discover the planet Venus. She will be readily recognized as the largest and brightest of all the stars, and will be found never to recede from the sun more than about 47°. From this distance, which she reaches at her greatest elongation, the planet will be found, at first slowly, but afterwards more rapidly, to approach the solar orb. She will finally be lost. in the superior effulgence of the sun; and when the unaided eye ceases to follow her in her approach to the sun, telescopic power will enable the observer to continue his observations, until, finally, the sun's direct beams mingling with those of the planet, she ceases to be visible, and is now lost for a greater or less period, until she emerges from the solar rays, appearing just before the sun in the gray morning twilight. She now recedes from her central orb, finally reaches her greatest elongation upon the opposite side, stops in her career, returns again, and thus oscillates backward and forward, never passing certain prescribed limits.

As already stated, the fact that Venus was a planet or wandering star must have become known among the very first of astronomical discoveries; but it required, doubtless, a long series of observations to determine the truth

Isaiah xiv. 12.

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