Nevil Maskelyne's parallax proposal
The Royal Society having come to a resolution to send persons of ability to proper places, in order to observe the approaching passage of Venus over the Sun, the 6th of June next year; (which phenomenon was first proposed to this Society, by the late excellent Dr Halley, forty-four years ago, as a proper means of determining the Sun's parallax, to a great degree of exactness) I take this opportunity of recommending to the consideration of this learned body another very important object in astronomy, which, I apprehend, may be cleared up at the same time, by the astronomers sent to one of those places, which will probably be judged convenient for the observation of Venus's transit; I mean the island of St Helena. The astronomical object, in favour of which I desire to engage your attention, is no less than the determination of the annual parallax of the Orbis Magnus; the finding out of which, from observation, would be the fullest and directest proof of the Copernican system, as the want of this proof, hitherto, has been the strongest argument made use of by those, who have withheld their assent to an hypothesis, which so fully, and yet simply satisfies all the other phenomena. No one, indeed, at this time of day, will venture to assert, that, even if no annual parallax could be found, after the greatest exertion of human art and industry, the Copernican system was not, therefore, true; since the quantity of this parallax may be so small, as to escape the reach of our fight, though assisted to the utmost. But though the defect of it would be no just argument against the Copernican system, yet the actual demonstration of it, from observation, would be a direct and convincing proof of the truth of that system. It. remains then to be considered, what hope there is now left, after astronomy has been brought to such a great degree of perfection, of being able to find out an annual parallax in any of the fixed stars. I am sensible I may here seem to be presumptuous, in venturing to treat upon this subject, after the many accurate observations made by the learned Dr Bradley, with an instrument constructed for this very purpose. No one can have a higher veneration than I have for the discernment and exactness of this illustrious astronomer, whose merit I have the greater opportunity of being acquainted with, by having the honour of calling him my friend. I would just beg leave to take notice, that the stars, which this astronomer observed, were such only as lay within a few degrees of his zenith: and though his observations do not seem to show a sensible parallax in any of them, yet we cannot thence absolutely conclude, that, amongst the great number of visible stars, there are none in which it may be perceptible, till they have all of them, especially those of the greatest lustre, been observed in proper places, near the zenith, with the like care and accuracy, which he has used: for, as Dr Bradley has himself remarked, where any stars are remote from the zenith, the uncertainties of refraction) and the irregular motions of the air, become so great, as to take away from us all hopes of observing them to an equal degree of exactness The particular star, which I propose, should be carefully observed, with a view of discovering its annual parallax, if sensible, is Sirius, the brightest of all the stars in the firmament, and which is therefore, probably, the nearest to us of them all. With us, this star passes the meridian, at the altitude of 22, where the refractions are too irregular to admit of our discovering a very minute quantity, by observation: but, at the island of St Helena, Sirius passes only half a degree south of the zenith; and, on this account, I have, for some time, looked upon that as the most proper place to make observations at, for this purpose.
JOC/EFR March 2014
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