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VI.—Some remarks on Theories of Cometary Physics

Published online by Cambridge University Press:  17 January 2013

C. Piazzi Smyth Esq.
C. Piazzi Smyth Esq.
Affiliation:
Astronomer Royal for Scotland, and Professor of Practical Astronomy in theUniversity of Edinburgh.
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Extract

While the physical appearances of comets have ever excited such intense curiosity and interest, all the theories concerning them are generally confessed to be insufficient to explain them; and, certainly, if we may judge from the various views advocated by different writers, and the anomalous forces gratuitously brought in to support the different hypotheses—it is so.

This unsatisfactory state of things, so different from that in which is the theory of the motions of comets,—seems to be owing partly to the difficulty of making the necessary observations by reason of the undefinable nature of the bodies themselves, and partly from the untoward circumstances under which the observations must be made, as well as the rareness of any opportunities offering. Hence, theories are built upon accounts handed down from old astrological times when men's prejudices would have prevented them, even if their means had been ample, which they were not, from giving any satisfactory and trustworthy accounts of the phenomena displayed by the heavens of their day.

Type
Transactions
Information
Earth and Environmental Science Transactions of The Royal Society of Edinburgh , Volume 20 , Issue 1 , 1853 , pp. 131 - 145
Copyright
Copyright © Royal Society of Edinburgh 1853

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References

page 141 note * This is not stated with perfect correctness, at least with regard to the comet of 1843, which might have had a tail of that length some days after the perihelion passage, when it had grown with the rapid increase of its radius vector; but the first day after the perihelion passage, the tail was observed to be only double the sun's diameter (excluding inclination), and its distance must then have been 100 times greater than at the perihelion; so that if the sesquiplicate ratio holds good, we shall have for that epoch a size not very different from planetary bodies. (A curious meeting this must be of the molecules brought for an instant into such close proximity, after having been separated for ages by distances so vast and inconceivable as they must be at the aphelion; and when separating for their diverse orbits, what speculations on their next meeting, not in thunder, lightning, and in rain, but in light and heat unspeakable. On the last occasion, February 1843, the heat was equal (according to Sir J. Herschel) to 47,000 of our suns, 1900 whereof are sufficient to melt the very rocks. Such, at least, must have been the heat, if the comet travelled at that part of its orbit only at the mean rate of the earth; but the velocity was really vastly greater, and the heat much modified thereby. The degree to which velocity in the heavenly spaces may modify distance, in respect of heat, is one still open for inquiry; and the result, in the case of our own earth, as far as it may have been very imperfectly examined into, would lead us to expect that the above proportion would be greatly reduced.)