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An Attempt to Prove the Motion of the Earth by Observations
(Continued)

I propounded therefore to them the several ways that it was possible to be performed, and what method was to be observed in every one of them, and somewhat of the conveniencies in each of them; for having seriously meditated upon the Inquiry, I quickly thought of many expedients for the doing thereof. As first, I had thoughts of making use of some very great and massy Tower or Wall that were well setled, or of some large Rock or Hill whereunto I might fix my Glasses, so as to take the exact altitude of some eminent Star near the Pole of the Ecliptik, when at its greatest height, at two differing times of the year; to wit, about the Summer and Winter Solstice, to see if possibly I could discover any difference of altitude between the first and second observation.

But to accomplish this (besides the vast difficulty there would have been to have measured such an Angle to the accurateness requisite, if at least it were desired to have the Angle of a latitude to Minutes and Seconds, which ought also to have been repeated as oft as any observation had been made for fear of setling or swelling, &c.)

I was destitute of such a convenience near my habitation; besides, had I had my wish, I sound that 'twas lyable to an inconvenience that would wholly overthrow my whole design, which I knew not well how to avoid: Namely, to that which hath hitherto made even the very best observations of Parallaxes ineffectual and uncertain, the refraction of the Air or Atmosphere, which though it could have been but very little at the greatest altitude of the Pole of the Ecliptick, yet it might have been enough plausibly to have spoiled the whole observation, and to have given the Anticopernicans an opportunity of evading the Arguments taken from it, especially upon the account of the differing constitution of the Atmosphere in June and
December, which might have caused so much a greater refraction of the same altitude at one time then another, as would have been sufficient to have made this observation ineffectual for what it was designed.

Adde to this, that it would have been no easie matter to have set the Glasses or Telescope exactly against the Meridian, so as to see the highest altitude of any Star near the Pole of the Ecliptick distinctly to a Second. The like difficulties I found if observations were made of the greatest altitude of the Pole of the Ecliptick in June and December, or the least altitude of the same in December and June.

For besides all the uncertainties that the Instruments, be they what they will, are liable to, the grand inconvenience of the refraction of the Air, which is enough to spoil all observations if it be intermixed with uncertainty, in the former is considerable, and in the later intolerable. Having therefore examined the wayes and Instruments for all manner of Astronomical observations hitherto made use of, and considered of the inconveniencies and imperfections of them; and having also duly weighed the great accurateness and certainty that this observation necessarily required: I did next contrive a way of making observations that might be free from all the former inconveniencies and exceptions, and as near as might be, fortified against any other that could be invented or raised against it.

This way then was to observe by the passing of some considerable Star near the Zenith of Gresham Colledge, whether it did not at one time of the year pass nearer to it, and at another further from it: for if the Earth did move in an Orb about the Sun, and that this Orb had any sensible Parallax amongst the fixt Stars; this must necessarily happen, especially to those fixt Stars which were nearest the Pole of the Ecliptick.

And that this is so, any one may plainly perceive if he consider the annexed Scheme, Fig.1. where let S represent the Sun placed as it were in the center of the Planetary Orbs, ABCD an imaginary Orb of the fixt Stars of the first magnitude, whose center for demonstration sake we will suppose the Sun. Let ; represent the Orb in which the Earth is supposed to move about the Sun, obliquely projected on the Paper.

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