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what is the position of the axis about which it revolves, and is this axis permanent or changeable ? If a motion of translation in space must be adopted, then whither is the earth urging its flight? what the nature of the path described ? the velocity of its movement, and the laws by which it is governed ? These are some of the questions which present themselves in the outset, touching the condition of the earth, on whose surface the astronomer is located, in his re searches of the heavens,
Beyond the limits of the earth, a multitude of objects present themselves for examination: and first of all the sun, the great source of life and light and heat, demands the attention of the student of the heayens. That some inscrutable tie binds it to the earth, or the earth to it, was early recognised in the fact, that whether the sun was moving or at rest, the relative distance of it and the earth never changed by any great amount; and whatever changes did occur, were all obliterated in a short period and the distance by which these bodies are separated was restored to its primitive value, to recommence its cycle of changes in the same precise order.-Here then was a grand problem, to determine the relations existing between the sun and earth; to endue with motion that one of these bodies which did move, and to fix the limits within which the observed changes occurred, both in time and distance.
While the connection between the sun and earth was certain, a mutual dependence between the earth
and the other great source of light, the moon, was · equally manifest. The invariability in the apparent
diameter of the moon, demonstrates the fact, that whether the earth were moving or stationary, the moon never parts company with our planet. In all her wanderings among the fixed stars, in her elonga: tions from the sun, in her wondrous phases and perpetual changes, some invisible hand held her at the same absolute distance from the earth. But to decide whether this power resided in the earth or the moon, or in both, to explain these wondrous changes from the silver crescent of the western sky to the full orb which rose with the setting sun, pouring a flood of light over all the earth, to develop the mysterious connection between the disappearance of the moon and those terrific phenomena, the going out of the sun in dim eclipse—these furnished themes for investigation requiring long centuries of patient watching, of never ending toil.
Passing out from the sun and moon to the inore distant stars, among the brightest of those which gemmed the nocturnal heavens, a few were found differing from all the rest in the fact that they wandered from point to point, and at the end of inter vals widely differing among themselves, swept round the entire heavens, and returned to their starting point, to recommence their ceaseless journies. These were named planets, wanderers, in contradistinction to the host of stars which were fixed in position, unchanged from century to century.
Hence arose a new and profound series of investigations: where were these wandering stars urging their flight? Were their motions real or apparent? Were their distances equal or unequal ? Did any tie bind them to the earth, or to the sun, or to each other? Were their distances from the earth constant or
variable? Were their motions irregular, or gu'ded by law? Did they accomplish their revolutions among the fixed stars in regular curves, or in lawless wanderings ? Among all the moving bodies, sun, moon, and planets, could any principle of association be traced which might bind them together and form them into a common system?
To resolve these profound questions, a critical watch is kept on all the moving bodies. Their pathway is among the stars, and to these ever during points of light their positions are constantly referred. If beyond the limits of the moving bodies a dark veil had been drawn so as to have excluded the light of the stars, at the first glance it might seem that by such a change, simplicity would have been introduced, and the perplexity arising from the motion of the planets among the profusely scattered stars, would have been removed. But let us not judge too hastily. Blot out the stars, and give to the sun, moon and planets a blank heavens in which to move, and the possibility of unraveling their mysterious motions, mutual reations, and common laws, is gone forever.
This will become manifest when we reflect that on such a change, not a fixed point in all the heavens would remain, to which we could refer a moving planet. They must then be referred to each other, and the motion due to the one, would become inextricably involved in that due to the other, and neither could be determined with any precision. Like the ocean islands which guided the early mariners, so God has given to us the stars of heaven as the fixed points to which we can ever refer, in all parts of their
revolutions, the places of the wandering planets, and the swiftly revolving moon.
As the necessity for accuracy in watching the movements of the planets became more apparent, the attention was directed to the acquisition of the means by which this might be accomplished. Hence we find in the earliest ages the astronomer grouping the fixed stars into constellations—breaking up the great sphere of the heavens into fragments, the more easily to study its parts in detail. Not only are the stars of each constellation numbered, their brilliancy noted, but their relative places in the constellation and to each other, are fixed with all the precision which the rude means then in use permitted. Names are fixed to these different groupings, when or where or by whom we know not. Neither history nor tradition lead us back to this first breaking up of the heavens, but the names then bestowed on the fragmentary parts, the richer constellations, have survived the fall of empires, and are fixed forever in the heavens.
Possessing' now a thorough knowledge of the objects among which the planets were moving, and the means of measuring with approximate accuracy, their distance from the stars along their path, it became possible to trace a planet in its career, and to note the changes of its velocity. New and wonderful discoveries were thus made. It was found that all the planets moved with an irregular velocity. Sometimes swiftly advancing among the fixed stars, then slowly relaxing their speed, they actually stopped, turned backward in their career, stopped again, and then, at first slowly but afterwards more rapidly, resumed their onward motion. These strange and anomalous motions, differing from anything remarked in the sun and moon furnished new themes for discussion, new problems for solution. While the phenomena above alluded to became known, the same chain of observations revealed the remarkable fact, that the periods of revolution of the planets, though differing for each one of the group, were identical for any one individual, and moreover, that a simple curve marked out the pathway of sun, moon and planets, among the fixed stars, and that all these wandering bodies were confined to a narrow zone or belt in the heavens.
Centuries had now rolled away, nay, even thousands of years had slowly glided by, since the mind had first given itself to the examination of the heavens, and while discovery after discovery had rewarded the zeal of the observer in every age, yet the grand object of research, the distinction between actual and apparent motion, had thus far eluded the utmost efforts of human genius. But a brighter day was dawning. Each successive effort tore away some petty obstruction which impeded the march of mind upward towards the lofty region of truth. Facts grew and multiplied. Phenomena striking and diversified, were collated and compared. The mind in imagination took leave of the earth as the centre of all these complex movements, inexplicable on its surface, and naturally urged its flight towards the sun.
There it paused and rested, and from this fixed point looked out upon the circling orbs, and lo! the complexity of their movements melted away. The centre was found—the mystery solvedthe ponderous earth rescued from its false position, rolled in its place among the planets; one of the great