WHether the Earth move or stand still hath been a Problem , that since Copernicus revived it , hath much exercised the Wits of our best modern Astronomers and Philosophers , amongst which notwithstanding there hath not been any one who hath found out a certain manifestation either of the one or the other Doctrine . The more knowing and judicious have for many plausible reasons adhered to the Copernican Hypothesis : But the generality of others , either out of ignorance or prejudice , have rejected it as a most extravagant opinion . To those indeed who understand not the grounds and principles of Astronomy , the prejudice of common converse doth make it seem so absurd , that a man shall as soon perswade them that the Sun doth not shine , as that it doth not move ; and as easily move the Earth as make them believe that it do's so already . For such Persons I cannot suppose that they should understand the cogency of the Reasons here presented , drawn from the following observations of Parallax , much less therefore can I expect their belief and assent thereunto ; to them I have only this to say , 'T is not here my business to instruct them in the first principles of Astronomy , there being already Introductions enough for that purpose : But rather to furnish the Learned with an experimentum crucis to determine between the Tychonick and Copernican Hypotheses . That which hath hitherto continued the dispute hath been the plausibleness of some Arguments alledged by the one and the other party , with such who have been by nature or education prejudiced to this or that way . For to one that hath been conversant only with illiterate persons , or such as understand not the principles of Astronomy and Geometry , and have had no true notion of the vastness of the Universe , and the exceeding minuteness of the Globe of the Earth in comparison therewith , who have confined their imaginations & fancies only within the compass and pale of their own walk and prospect , who can scarce imagine that the Earth is globous , but rather like some of old , imagine it to be a round plain covered with the Sky as with a Hemisphere , and the Sun , Moon , and Stars to be holes through it by which the Light of Heaven comes down ; that suppose themselves in the center of this plain , and that the Sky doth touch that plain round the edges , supported in part by the Mountains ; that suppose the Sun as big as a Sieve , and the Moon as a Chedder Cheese , and hardly a mile off . That wonder why the Sun , Moon , and Stars do not fall down like Hail-stones ; and that will be martyr'd rather then grant that there may be Antipodes , believing it absolutely impossible , since they must necessarily fall down into the Abyss below them : For how can they go with their feet towards ours , and their heads downwards , without making their brains addle . To one I say , thus prejudiced with these and a thousand other fancies and opinions more ridiculous and absurd to knowing men , who can ever imagine that the uniformity and harmony of the Celestial bodies and motions , should be an Argument prevalent to perswade that the Earth moves about the Sun : Whereas that Hypothesis which shews how to salve the appearances by the rest of the Earth and the motion of the Heavens , seems generally so plausible that none of these can resist it .

Now though it may be said , 'T is not only those but great Geometricians , Astronomers and Philosophers have also adhered to that side , yet generally the reason is the very same . For most of those , when young , have been imbued with principles as gross and rude as those of the Vulgar , especially as to the frame and fabrick of the world , which leave so deep an impression upon the fancy , that they are not without great pain and trouble obliterated : Others , as a further confirmation in their childish opinion , have been instructed in the Ptolomaick or Tichonick System , and by the Authority of their Tutors , over-awed into a belief , if not a veneration thereof : Whence for the most part such persons will not indure to hear Arguments against it , and if they do , 't is only to find Answers to confute them .

On the other side , some out of a contradicting nature to their Tutors ; others , by as great a prejudice of institution ; and some few others upon better reasoned grounds , from the proportion and harmony of the World , cannot but imbrace the Copernican Arguments , as demonstration ; that the Earth moves , and that the Sun and Stars stand still .

I confess there is somewhat of reason on both sides , but there is also something of prejudice even on that side that seems the most rational . For by way of objection , what way of demonstration have we that the frame and constitution of the World is so harmonious according to our notion of its harmony , as we suppose ? Is there not a possibility that the things may be otherwise ? nay , is there not something of probability ? may not the Sun move as Ticho supposes , and the Planets make their Revolutions about it whilst the Earth stands still , and by its magnetism attracts the Sun , and so keeps him moving about it , whilst at the same time ☿ and ♀ move about the Sun , after the same manner as ♄ and ♃ move about the Sun whilst the Satellites move about them ? especially since it is not demonstrated without much art and difficulty , and taking many things for granted which are hard to be proved , that there is any body in the Universe more considerable then the Earth we tread on . Is there not much reason for the Hypothesis of Ticho at least , when he with all the accurateness that he arrived to with his vast Instruments or Riccioli , who pretends much to out-strip him , were not able to find any sensible Parallax of the Earths Orb among the fixt Stars , especially if the observations upon which they ground their assertions , were made to the accurateness of some few Seconds ? What then , though we have a Chimera or Idea of perfection and harmony in that Hypothesis we pitch upon , may there not be a much greater harmony and proportion in the constitution it self which we know not , though it be quite differing from what we fancy ? Probable Arguments might thus have been urged both on the one and the other side to the Worlds end ; but there never was nor could have been any determination of the Controversie , without some positive observation for determining whether there were a Parallax or no of the Orb of the Earth ; This Ticho and Riccioli affirm in the Negative , that there is none at all : But I do affirm there is no one that can either prove that there is , or that there is not any Parallax of that Orb amongst the fixt Stars from the Suppellex of observations yet made either by Ticho , Riccioli , or any other Writer that I have yet met with from the beginning of writing to this day . For all Observators having hitherto made use of the naked eye for determining the exact place of the object , and the eye being unable to distinguish any angle less then a minute , and an observation requisite to determine this requiring a much greater exactness then to a minute , it doth necessarily follow that this experimentum crucis was not in their power , whatever either Ticho or Riccioli have said to the contrary , and would thence overthrow the Copernican System , and establish their own . We are not therefore wholly to acquiess in their determination , since if we examine more nicely into the observations made by them , together with their Instruments and wayes of using them , we shall find that their performances thereby were far otherwise then what they would seem to make us believe . The Controversie therefore notwithstanding all that hath been said either by the one or by the other Party , remains yet undetermined , Whether the Earth move above the Sun , or the Sun about the Earth ; and all the Arguments alledged either on this or that side , are but probabilities at best , and admit not of a necessary and positive conclusion . Nor is there indeed any other means left for humane industry to determine it , save this one which I have endeavoured to make ; and the unquestionable certainty thereof is a most undenyable Argument of the truth of the Copernican Systeme ; and the want thereof hath been the principal Argument that hath hitherto somewhat detained me from declaring absolutely for that Hypothesis , for though it doth in every particular almost seem to solve the appearances more naturally and easily , and to afford an exceeding harmonious constitution of the great bodies of the World compared one with another , as to their magnitudes , motions , and distances , yet this objection was alwayes very plausible to most men , that it is affirmed by such as have written more particularly of this subject , that there never was any sensible Parallax discovered by the best observations of this supposed annual motion of the Earth about the Sun as its center , though moved in an Orb whose Diameter is by the greatest number of Astronomers reckoned between 11 and 12 hundred Diameters of the Earth : Though some others make it between 3 and 4 thousand ; others between 7 and 8 ; and others between 14 and 15 thousands ; and I am apt to believe it may be yet much more , each Diameter of the Earth being supposed to be between 7 and 8 thousand English miles , and consequently the whole being reduced into miles , if we reckon with the most , amounting to 120 millions of English miles . It cannot , I confess , but seem very uncouth and strange to such as have been used to confine the World with less dimensions , that this annual Orb of the Earth of so vast a magnitude , should have no sensible Parallax amongst the fixt Stars , and therefore 't was in vain to indeavour to answer that objection . For it is unreasonable to expect that the fancies of most men should be so far streined beyond their narrow dimensions , as to make them believe the extent of the Universe so immensly great as they must have granted it to be , supposing no Parallax could have been found .

The Inquisitive Jesuit Riccioli has taken great pains by 77 Arguments to overthrow the Copernican Hypothesis , and is therein so earnest and zealous , that though otherwise a very learned man and good Astronomer , he seems to believe his own Arguments ; but all his other 76 Arguments might have been spared as to most men , if upon making observations as I have done , he could have proved there had been no sensible Parallax this way discoverable , as I believe this one Discovery will answer them , and 77 more , if so many can be thought of and produced against it . Though yet I confess had I fail'd in discovering a Parallax this way , as to my own thoughts and perswasion , the almost infinite extension of the Universe had not to me seem'd altogether so great an absurdity to be believed as the Generality do esteem it ; for since 't is confessedly granted on all hands the distance of the fixt Stars is meerly hypothetical , and not founded on any other ground or reason but fancy and supposition , and that there never was hitherto any Parallax observed , nor any other considerable Argument to prove the distances supposed by such as have been most curious and inquisitive in that particular , I see no Argument drawn from the nature of the thing that can have any necessary force in it to determine that the said distance cannot be more then this or that , whatever it be that is assigned . For the same God that did make this World that we would thus limit and bound , could as easily make it millions of millions of times bigger , as of that quantity we imagine ; and all the other appearances except this of Parallax would be the very same that now they are . To me indeed the Universe seems to be vastly bigger then 't is hitherto asserted by any Writer , when I consider the many differing magnitudes of the fixt Stars , and the continual increase of their number according as they are looked after with better and longer Telescopes . And could we certainly determine and measure their Diameters , and distinguish what part of their appearing magnitude were to be attributed to their bulk , and what to their brightness , I am apt to believe we should make another distribution of their magnitudes , then what is already made by Ptolomy , Ticho , Kepler , Bayer , Clavius , Grienbergerus , Piff , Hevelius and others .

For supposing all the fixt Stars as so many Suns , and each of them to have a Sphere of activity or expansion proportionate to their solidity and activity , and a bigger and brighter bodied Star to have a proportionate bigger space or expansion belonging to it , we should from the knowledge of their Diameters and brightnesses be better able to judge of their distances , and consequently assign divers of them other magnitudes then those already stated : Especially since we now find by observations , that of those which are accounted single Stars , divers prove a congeries of many Stars , though from their near appearing to each other , the naked eye cannot distinguish them ; Such as those Stars which are called Nebulous , and those in Orion Sword , and that in the head of Aries , and a multitude of others the Telescope doth now detect . And possibly we may find that those twenty magnitudes of Stars now discovered by a fifteen foot Glass , may be found to increase the magnitude of the Semidiameter of the visible World , fourty times bigger then the Copernicans now suppose it between the Sun and the fixt Stars , and consequently sixty four thousand times in bulk . And if a Telescope of double or treble the goodness of one of fifteen should discover double or treble the said number of magnitudes , would it not be an Argument of doubling or trebling the former Diameter , and of increasing the bulk eight or twenty seven times . Especially if their apparent Diameters shall be found reciprocal to their Distances ( for the determination of which I did make some observations , and design to compleat with what speed I am able . ) But to digress no further , This grand objection of the Anti-copernicans , which to most men seem'd so plausible , that it was in vain to oppose it , though , I say , it kept me from declaring absolutely for the Copernican Hypothesis , yet I never found any absurdity or impossibility that followed thereupon : And I alwayes suspected that though some great Astronomers had asserted that there was no Parallax to be found by their observations , though made with great accurateness , there might yet be a possibility that they might be mistaken ; which made me alwayes look upon it as an inquiry well worth examining : first , Whether the wayes they had already attempted were not subject and lyable to great errors and uncertainties : and secondly , Whether there might not be some other wayes found out which should be free from all the exceptions the former were incumbred with , and be so far advanced beyond the former in certainty and accurateness , as that from the diligent and curious use thereof , not only all the objections against the former might be removed , but all other whatsoever that were material to prove the ineffectualness thereof for this purpose .

I began therefore first to examine into the matter as it had already been performed by those who had asserted no sensible Parallax of the annual Orb of the Earth , and quickly found that ( whatever they asserted ) they could never determine whether there were any or no Parallax of this annual Orb ; especially if it were less then a minute , which Kepler and Riccioli hypothetically affirm it to be : The former making it about twenty four Seconds , and the latter about ten . For though Ticho , a man of unquestionable truth in his assertions , affirm it possible to observe with large Instruments ; conveniently mounted and furnished with sights contrived by himself ( and now the common ones for Astronomical Instruments ) to the accurateness of ten Seconds ; and though Riccioli and his ingenious and accurate Companion Grimaldi affirm it possible to make observations by their way , with the naked edge to the accurateness of five Seconds ; Yet Kepler did affirm , and that justly , that 't was impossible to be sure to a less Angle then 12 Seconds : And I from my own experience do find it exceeding difficult by any of the common sights yet used to be sure to a minute . I quickly concluded therefore that all their endeavours must have hitherto been ineffectual to this purpose , and that they had not been less imposed on themselves , then they had deceived others by their mistaken observations . And this mistake I found proceeded , from divers inconveniencies their wayes of observations were lyable to . As first from the shrinking and stretching of the materials wherewith their Instruments were made , I conceive a much greater angle then that of a minute may be mistaken in taking an altitude of fifty Degrees . For if the Instruments be made of Wood , 't is manifest that moyst weather will make the frame stretch , and dry weather will make it shrink a much greater quantity then to vary a minute : and if it be Metal , unless it be provided for in the fabrick of the Instrument accordingly , the heat of Summer , when the Summer observations are to be made , will make the Quadrant swell , and the cold of Winter will make it shrink much more then to vary a minute : Both which inconveniencies ought to be removed . Next the bending and warping of an Instrument by its own weight , will make a very considerable alteration . And thirdly , the common way of Division is also lyable to many inconveniencies : And 't is hardly possible to ascertain all the subdivisions of Degrees into minutes for the whole Quadrant , though that be not altogether impossible . But I will suppose that they did foresee , and in some manner prevent all these inconveniencies , especially Ticho and Riccioli , who seem to have been aware thereof . But there was one inconvenience which was worse the all the rest , which they seem not to have been sufficiently sensible of , from whence proceeded all their own mistakes , and their imposing upon others , and that was from their opinion that the sight of the naked eye was able to distinguish the parts of the object as minutely as the limb of the Quadrant ( of what largeness soever ) was capable of Divisions ; whereas 't is hardly possible for any unarmed eye well to distinguish any Angle much smaller then that of a minute : and where two objects are not farther distant then a minute , if they are bright objects , they coaless and appear one , though I confess , if they be dark objects , and a light be interposed , the distance between them shall be visible , though really much less then a Second ; and yet notwithstanding , my first assertion stands good ; for though a bright object , as a candle or light at a distance , or a Star , or the like , can be seen by the eye , though its body do really not subtend an Angle of one third , yet it proceeds from a radiation ( that is , from reflection and refraction together ) in the air and in the eye , whereby the body thereof is represented to the naked eye some hundred times bigger then it really is . That this is so , any one that will but carefully examine will find it true .

It was , I doubt not , their extraordinary desire and care to be exact , that caused them to make their Instruments so large , and to subdivide them to such an exactness , as to distinguish , if possible , to Seconds ; And I question not but that they used their utmost indeavour in directing the sight to the object : but since the naked eye cannot distinguish an Angle much smaller then a minute , and very few to a whole minute , all their charge and trouble in making and managing large Instruments , and in calculating and deducing from them , was as to this use in vain . Hence I judged that whatever mens eyes were in the younger age of the World , our eyes in this old age of it needed Spectacles ; and therefore I resolved to assist my eyes with a very large and good Telescope , instead of the common sights , whereby I can with ease distinguish the parts of an object to Seconds : and I question not but that this way may be yet made capable of distinguishing much more curiously , possibly even to some few Thirds . This invention removed that grand inconvenience which all former observations were spoiled with : but there remained yet further this difficulty , How to make an Instrument large enough for this purpose , that I might be assured did not shrink , nor warp , nor stretch so much as to vary a Second ; for such is the nature of all Materials that can be made use of for Instruments of the bigness I designed this , that 't is almost impossible to make a moveable Instrument that shall not be subject to a variation of divers Seconds : It was therefore my next inquiry where I might fix this Archimedean Engine that was to move the Earth . For the doing of which , I knew 't was in vain to consult with any Writer or Astronomer , having never then heard of any person that had ever before that time had any thoughts thereof : and when I first propounded it to the Royal Society , 't was look'd upon as a new thought , and somewhat extravagant , and hardly practicable , until upon hearing my explication , and the various wayes how it might be reduced into practise , it was at length judged possible , and desirable to be tryed . 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 and inconveniencies 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 altitude 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 found that 't was 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 Anti-copernicans 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. I. where let S represent the Sun placed as it were in the center of the Planetary Orbs , A B C D 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 s●pposed to move about the Sun , obliquely projected on the Paper . Let ♑ represent the Earth in Capricorn , and ♋ the Earth in Cancer , let 1 2. 1 2. represent the imaginary Axis of the Earth , keeping continually a parallelism to its self , and let ♑ AICD ♋ represent an imaginary Plain passing through the center of the Star at D in the Solstitial Colure , and the two centers of the Earth in ♑ and ♋ , and C represent the Zenith point of Gresham Colledge at noon , when the Earth is in Cancer , and A the Zenith point of the said Colledge at midnight in the aforesaid Orb A B C D when the Earth is in Capricorn , 't is manifest therefore that since the Poles of the Earth , the Poles of the Ecliptick , and the Zenith points of the Earth at noon , when in Cancer , and at midnight , when in Capricorn , are all in the same Plain ; and that the Axis of the Earth keeps alwayes its parallelism , and that the Angles made by the Perpendiculars of Gresham Colledge , with the Axes are alwayes the same , that the aforesaid Perpendiculars of the said Colledge shall be parallel also one to another , and consequently deno●e out two points in the abovesaid Orb A and C as far distant from each other as the parallel Lines A ♑ and C ♋ are , and consequently the point A shall be farther from the Star in D , and the point C shall be nearer to it , when in the Meridian near the Zenith of London , and consequently if the said Star be observed when in the Meridian of the place abovesaid , if there be any such difference considerable , it may be found if convenient Instruments and care be made use of for the observation thereof : and the difference between the Angle A ♑ D , and the Angle C ♋ D , will give the parallactical Angle ♑ D ♋ of the Orb of the Earth to the fixt Star D of the first magnitude . The same demonstration will hold mutatis mutandis , supposing the Star be not in the Meridian or Plain abovesaid , but in some other Meridian , as any one upon well considering the nature of the thing it self may easily prove , if the observation be made when the Zenith passes by the Star at midnight , and at mid-day . But the nearer the Zenith of the place of observation passeth to the Pole-point of the Ecliptick , the better ; The Angle of Parallax being still the more sensible . Therefore the best place to compleat this observation were in some place under the Polar Circles , as in Iseland , where the Zenith of the place at the times abovesaid , must consequently pass at one time to the North side of the Pole of the Ecliptick , and at the other on the South side , and the Zenith of March and Sept. must pass through the very Pole-point it self . Now it falling out so , that there is no considerable Star in that part of the Heavens nearer the above said Plain , and nearer the Zenith point of Gresham Colledge in that Plain , then the Bright Star in the head of the Dragon , I made choice of that Star for the object by which I designed to make this observation , finding the Zenith point of Gresham Colledge to pass within some very few minutes of the Star it self ; the declination thereof according to Riccioli being 51° . 36′ . 7″ . and the Plain the Star and Pole of the World , making an Angle with the aforesaid Plain but of 2° . 52. 36 , the right ascention thereof being according to Riccioli 267° . 7′ . 24″ .