The 7-foot equatorial sextant was made for the first occupant of the Observatory, John Flamsteed. It was his main instrument until 1689 when it was superseded by his newly constructed Mural Arc. In all, it was used for more than 20,000 observations. Its whereabouts was last recorded in 1720. No surviving parts are known.
Designed largely by Flamsteed himself, the equatorial sextant was erected in a specially constructed ‘Sextant House’ at the bottom of his garden. It was used to measure the positions of the fixed stars – a prerequisite of eventually being able to find longitude at sea by the method of lunar distances for which the Observatory was founded. The unknown coordinates of one star were determined from the known coordinates of another by measuring the angular distance between them.
In essence the instrument consisted of two telescopic sights fitted onto to a 60º arc. One telescope was fixed to the ‘zero degrees’ end (the extreme right in the left hand image) whilst the other was free to move though the 60º angle of the arc. The sextant was mounted on a polar axis. Once the first of the two stars had been centred with the ‘fixed’ telescope, it could be tracked by pushing the telescope to follow the Earth’s rotation (the advantage of an equatorial mount) while the second telescope was manoeuvred to fix on the second star. As originally conceived, the intermutual distance as Flamsteed called it was then determined by counting the number of turns of the worm wheel that had been necessary to ‘open’ the telescopes to the required separation and converting this by means of a table into an angle. The first published observation was made on 19 September 1766 (Saturn) and the last on 3 September 1689 (Mars). The instrument needed three people to operate it, two of whom needed to be skilled in the art of observing.
The continual winding of the worm caused the teeth against which it turned to wear rapidly and introduce errors. As a result, at the end of 1677 Flamsteed had the instrument taken down and with his own hands, he divided the brass limb diagonally as Tycho and Hevelius had done before with their instruments. By doing this, Flamsteed now had two independent means of measuring the angular separation of the stars – one a cross-check the other. When the two measures differed by more than a minute he always rejected the observation, (except where the corrections were obvious,) and repeated it at the first opportunity..
Flamsteed’s observations with the instrument were published in volume 1 of Historia Coelestis Britannica. Click here to view the published observations.
The following account, which is in Flamsteed’s own words, is taken from An account of the Revd. John Flamsteed by Francis Bailey (London, 1835)
Whilst I lived at Sir Jonas Moore’s [Flamsteed’s patron at the Tower of London, where Flamsteed lived following his appointment, but before the Observatory was built], I contrived the large sextant of six feet nine inches radius: but, because the workmen might mistake the draught, Sir Jonas caused it to be formed in wood, with its axles and semicircles, by the Tower smiths at his own charge. The chief workman dying before the semicircles were formed, I could not make his successor to understand how the bigger of them might, and was to be moved by a perpetual screw: but forced to suffer him to move it by wheel work. But, before the lesser semicircle, whereby the plane of the sextant was to be inclined, was ready to be applied, he understood me, and wrought it off so that it performed very well. So that I was the less concerned for his misapprehensions of my first design: since the wheel-work performed well enough, except in some particular cases, which could seldom happen, and might be remedied, when they did, by care and caution.
The frame of the sextant being finished, with the axis and semicircle, the brass limb was fitted on it, with the telescopes; and the limb screwed to carry the moveable index gently upon the limb, by Mr. Tompion. But this being the first instrument that was contrived of this bigness and manner, the cells for bearing the glasses and threads were not so neatly nor conveniently formed, as they have been since in some lesser instruments of the same sort that I have contrived for my friends and ingenious acquaintance.
Mr. Hook pretended to have been the inventor of this way of screwing the limbs of instruments, which he published as his own in his animadversions on Hevelius's Machina Ccelestis: though 'tis evident, from the preface to Tycho Brahe's Historia Coelestis, that the Emperor Ferdinand was the first contriver of it, and that Mr. Hook learnt it from the said preface. He contending then that the account by the revolves of the screw, and parts numbered, would give the arcs observed much more accurately than any diagonal divisions could do, Sir Jonas was persuaded by him to suffer no other divisions, but the revolves of the screw, to be inscribed and numbered on the limb. I was sensible that screws would wear, and that the weight of the index, as the plane of the instrument happened to be inclined, would alter the parts of the revolves considerably: therefore I inscribed them close to the edge of the limb, leaving room enough for diagonal divisions within, nearer the centre. And now our next business was to find how many revolves, and parts of the screw, would answer to any number of degrees, minutes, and seconds.
Close by the foot of the hill, on which the Observatory stands, there is a terrace [in front of the Queen’s House], that had been found very plain and even when the walks were made. At the west end of this I caused a frame to be built, parallel to the plane of the walk, whereon to lay the sextant, with contrivances on it to fix the instrument very firm: from whose centre, to the other end of the walk, by the help of strong pikes I measured the distance 8762 inches precise. At this distance I placed a long, strong, flat rail, well fixed and supported: and having computed how many feet answered to a degree, marked them off on this rule; which was made so long as to subtend an angle of five degrees. Then bringing the threads in the fixed telescope to the beginning of the divisions, and firming it close, I moved the screw; and carrying the moveable index along the limb, noted what revolves and parts were marked by it when the threads covered the divisions of the five next degrees. And repeating these trials oft, and carefully, made at last the table for turning the revolves and parts observed, into degrees, minutes, and seconds, which I made use of in all my observations of this, and the next following year.
The sextant was fitted in its place in September, 1676; but had then only the revolutions of the screw figured and numbered upon its limb: to which I had found what degrees and minutes answered by observations made at land angles. This is the reason why, in all the distances taken from this time, till the end of the year 1677, you find the revolutions and parts only noted, with the degrees, minutes, and seconds answering them. But now I found a fault committed by the screw, and therefore in the Christmas holidays following, I caused the instrument to be taken down; and with my own hands divided the limb diagonally, as Tycho and Hevelius before me had done their instruments. Whereby I both avoided the fear of error in my measures, and hi transcribing them: for I continued to note the parts and revolves as formerly; and if at any time I found the equipollent degrees, minutes, and seconds, to differ much more than a minute from the measure numbered by the diagonals, I always rejected such observations, (except the corrections were obvious,) till I could repeat them again: which, if possible, I took care to do, the first following opportunity.
The sextant is an instrument which cannot be managed with less than three persons, of which the two observers ought to be skilful in the business; for the third, any indifferent person, of a strong able body, may serve.
Flamsteed, John, Historia Coelestis Britannica, Volume 3 (London, 1725).
This account is in Latin. The description of the sextant starts at the bottom of page 102. The accompanying plate was normally bound into volume 3 with the preface, but sometimes (as in this online edition) it was bound into volume 1. Click here to view it.
An English translation is available in National Maritime Museum monograph no.52: Flamsteed, John, and Allan Chapman. The Preface to John Flamsteed's Historia Coelestis Britannica, or, British Catalogue of the Heavens (1725) (London: Trustees of the National Maritime Museum, 1982). p.113.