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we shall soon see, is but an infinitesimal portion of the universe of God, one unit among the unnumbered millions which fill the crowded regions of space.Standing at the verge of the planetary system, we find ourselves surrounded by a multitude of shining orbs, some radiant with splendor, others faintly gleaming with beauty. The smallest telescopic aid suffices to increase their number in an incredible degree, while with the full power of the grand instruments now in use, the scenes presented in the starry heavens become actually so magnificent as to stun the imagination and overwhelm the reason.
Worlds and systems, and schemes and clusters, and universes, rise in sublime perspective, fading away in the unfathomable regions of space, until even thought itself fails in its efforts to plunge across the gulf by which we are separated from these wonderful objects.
In our measurements within the limits of the solar system, the radius of the earth's orbit has sufficed for a unit with which to exhibit the distances of the planets and comets. Great as is this unit, measuring no less than 95,000,000 of miles, we shall soon find it far too minute and insignificant to serve in our researches with reference to the grand scale of the visible universe. To obtain comprehensible ideas with reference to the interstellar spaces, we shall be obliged to call to our aid a unit, not exactly of distance, but of velocity; and before entering on the full exhibition of the main object of this lecture, permit me to direct your attention to a remarkable discovery, by which the important fact has been revealed, that light does not pass instantly from a luminous body to any remote object on which it may fall, but with a progressive notion, whose actual velocity has been ascer. tained. The important bearing of this discovery will become apparent as we advance in our examination of the sidereal heavens.
After the motions of the four moons of Jupiter had been sufficiently observed to construct tables of their movements, with a view to predict their eclipses, some unaccountable phenomena presented themselves, which, for a long time, baffled all efforts to explain them. It should be remembered, that the orbit of Jupiter encloses that of the earth, and when the two planets happen to be on the same side of the sun, and in a straight line passing through that orb, they are then at their least distance from each other, and are said to be in conjunction. Now suppose Jupiter to remain stationary, at the end of half a year the earth will have reached the opposite point of her orbit, and will now be more distant from Jupiter by an amount equal to the diameter of her orbit, or nearly 200,000,000 of miles. Retaining carefully these positions in the mind, we shall follow the facts about to be presented with the greatest ease.
It was found that those eclipses of Jupiter's satellites, which occurred while the earth and planet were at their least distance from each other, always came on sooner than the time predicted by the tables; while, on the contrary, those which took place when the planets were most remote from each other, occurred later than the computed time. A still more extended examination of these remarkable phenomena demonstrated the fact, that the discrepancies depended evi. dently on the absolute increase and decrease of distance which marked the relative position of the planets in their revolutions around the sun.
For a long time, no explanation of these undeniable truths could be found, until the mystery was finally solved by Roemer, a Danish astronomer, who, with admirable sagacity, traced these irregularities to their truo source, and found that they arose from the fact tha light traveled rough space with a finite and meas urable velocity.
The explanation is simple. When Jupiter and the earth are at their least distance from each other, the stream of light flowing from the satellite of the great planet traverses a shorter space to reach the eye of the observer on the earth, by nearly 200,000,000 of miles, than when the planets are most remote from each other. In case, therefore, this stream is in any way cut off, it will run out sooner in the first than in the second position, by the time required to pass over the diameter of the earth's orbit. The stream of light is actually shorter, by 200,000,000 of miles, in the first than in the second position of the planets.
Now the satellites of Jupiter receive their light from the sun ;-they reflect this light to the earth, and when the body of their primary is interposed between them and their source of light, they are eclipsed; their light is cut off ; its flow is interrupted; and when the stream of light starting from them at the instant the supply is cut off shall have run out, then, and not till then, does the satellite become invisible. This ex. planation accounted for all the phenomena in the most beautiful manner.
The tables had been constructed from the mean of a great number of observed eclipses. Hence, those which took place while Jupiter and the earth were near to each other, would happen earlier than prediction; while those taking place when th: planets were at their greatest distance, would occur later than the time given by the tables. But the velocity with which this mysterious, subtle, intangible substance called light, flew through the regions of space, as determined by this wonderful theory, was so great as to startle the minds of even its strongest advocates, and to demand the most positive testimony to induce the belief of those disposed to scepticism. It was found to traverse a distance equal to the entire diameter of the earth's orbit, or 190,000,000 of miles, in about 16 minutes !-giving a velocity of 12,000,000 of miles per minute, or 192,000 miles in each second of time !
It is not our purpose to enter into any investigation as to the true theory of light, whether it be an actual emanation from a luminous body of material particles, or whether it be a mere vibratory or undulating motion produced by luminous bodies on some ethereal medium. My only object, at this time, is to assert the undoubted fact, that in case a luminous body were to be suddenly called into being, and located in space at the distance of 12,000,000 of miles from the eye of an observer, who was on the look-out for its light, this light would not reach him until one minute after the creation of the object; and should it suddenly be struck from existence, the same observer would behold it for one minute after the extinction.
Should any mind revolt from these statementsshould the difficulty of the investigation, and the incredible velocity of light, demand higher and better evidence, before full faith can be given to the theory, I can only say that this evidence shall be given before we close this discussion, and with a fallness and clearness which shall set all doubts at defiance.
I now proceed to an examination of the great problem of the parallax of the fixed stars, a problem which has taxed the ingenuity of the greatest minds, and which has called into requisition the most admirable skill for a period of more than 300 years. A familiar explanation of the nature of this problem may prepare the way for a rapid sketch of the various means which have been employed in its solution. If | it were possible to measure on the earth's surface a base line of a thousand miles in length, by locating an observer at each extremity of this base, with instruments suitable to fix the moon's place among the fixed stars, the telescopes of those two observers, directed to the moon's centre at the same instant, would incline towards each other, and the visual ray from each of those instruments would meet at the moon's centre, and form an angle with each other. This angle, or opening of the visual rays, is called the parallax, and in case the object under examination were a fixed star, then would the angle in question be called the parallax of the fixed star.
It is readily seen that when the length of the base is known, and the parallactic angle measured, then the length of the visual ray may be at once determined, and the distance of the object is made known by the simplest rules of geometry. Parallax, then, in general, is the apparent change in the place of an object, occasioned by the real change in the place of the spectator.
The whirling of the trees of a forest, produced by the rapid speed of the beholder along a railway, is a