Flamsteed made the great majority of his published observations in three distinct locations: the Octagon Room (1676-1719), the Sextant House (1676-1690), and the Quadrant House (1689-1719) which is where he installed the Mural Arc in 1689. He may also have made a small number of observations of solar eclipses in the eastern Summer House (and possibly the western one). He also made a few attempts to observe from the bottom of the Well Telescope (not published) and with his long refractors from the roof of Flamsteed House and from the garden. The Octagon Room, the Sextant House and the Quadrant (Arc) House were each equipped with their own clock or clocks in the case of the Octagon Room. Flamsteed also had a portable clock.
Until the introduction of atomic time in 1967, the rotating Earth and the length of the day were the basis of our timekeeping system. When the Observatory was founded precision timekeeping was still very much in its infancy. It had long been known that natural days varied slightly in length and that the difference between apparent and mean solar time varied during the course of the year. Many astronomers also believed that the Earth was rotating at a steady rate (i.e, that it was isochronal) – but nobody had yet be able to show it, not least because prior to the invention of the pendulum clock by Christiaan Huygens in 1656, nobody had a sufficiently accurate timekeeper to do so.
In 1721, the Observatory acquired its first Transit Telescope – a telescope mounted between two piers in such a way that it could only point higher of lower in the plane of the meridian. By 1750 and until 1871, the Transit Clock (i.e. the one used with the Transit Telescope) was the Observatory's de facto sidereal standard – the clock to which all others (including those set to mean solar time) were ultimately referred. The transit clock itself was checked by comparing it with the rate of the Earth's rotation using the telescope. Certain of the brighter stars, whose positions had been refined by repeated observation over a long period of years, were used as ‘clock stars’ to determine the errors of the Observatory’s clocks and hence the local time at Greenwich. In 1851 for example, some 67 clock stars were kept under observation for this purpose. With the help of a good clock, astronomers were able to reverse the process and determine the right ascensions of stars from their times of meridian transit.
When Flamsteed became astronomer royal, the best available star catalogue was that of 1,004 stars that had been produced by Tyco in 1598 before telescopes, pendulum clocks, logarithms and the micrometer screw were invented. Although Flamsteed made frequent reference to the catalogue, even if he had had a transit telescope, it would not have been much use for determining accurate times as the catalogue was not sufficiently accurate ... and it was, after all, partly to improve this catalogue, that the Observatory had been founded. Given the absence of any established 'clock stars', Flamsteed needed to find a different way of compiling his catalogue to that used by his successors.
The difference between apparent and mean solar time is today known as the equation of time. In Flamsteed's era, it was sometimes referred to as the equation(s) of natural(l) day(e)s. In 1672, three years before he became Astronomer Royal and before he even owned a clock, Flamsteed wrote a paper on the equation of time – De temporio aequatione diatriba. It first appeared in 1673 as an appendix to John Wallis's Jeremiae Horrocci opera poshuma (which was reprinted in a different order in 1678).
One of Flamsteed's first investigations at Greenwich was to check if the Earth was indeed iscochronal. By 1678, with the help of a fixed telescope with which to observe the dog star Sirius that he had set up in 1677 and rhe Octgagon Room clocks, he had been able to satisfy himself that it was. Somewhat surprisingly Flamsteed did not publish a paper announcing his findings. From the time of the Observatory's founding in 1675 until his death in 1719, Flamsteed used mean solar time and/or apparent solar time for all his observations including those that he made with his mural instruments – a practice that was discontinued by his successors.
Famsteed's observations were published in Latin in two editions. The first, Historiae Coelestis Libri Duo, appeared in 1712. The story of how it came to be published is both complex, and acrimonious and resulted in Flamsteed having a lifelong feud with both Halley and Newton. Flamsteed thoroughly disapproved of it. Following the death of Queen Anne in 1714 and Newton’s patron, the Earl of Halifax, in 1715, he was able to acquire 300 of the 400 copies that had been printed. After extracting those pages of which he approved for reuse in a new edition (those that had been printed before the end of 1707), apart from a few copies, he burnt the rest.
The second edition was published posthumously in three volumes in 1725:
Historia Coelestis Britannica Volume 1
Historia Coelestis Britannica Volume 2
Historia Coelestis Britannica Volume 3
The pages extracted pages from the 1712 edition make up the bulk of Volume 1 which covers the years 1676-1689, whilst the observations in Volume 2 cover the period 1689-1719. Volume 3 contains amongst other things contains his star catalogue as well as a lengthy preface that runs to some 164 pages of text.
The 1712 edition also had a preface, but this one was written by Halley. When talking about the Mural Arc he wrote:
' ..both the perfect levelness of the instrument and its perfect position on the plane of the meridian were put in the place so that true differences of right ascensions are obtained from intervals of time; a somewhat daring hypothesis, it seems.'
The preface to the 1725 Historia includes an extensive description of Flamsteed's major instruments as well as the means by which he calibrated them (p.101-113). Of the clocks he says virtually nothing. The only ones he mentions are the three that he had when he entered the Observatory in 1676 ( p.103) and of these, he only tells us their going times ('duobus pendulis Oscilatoriis annuis' & 'Pendulo hebdomadario'). None of the other four are mentioned, nor does he state where where any of the clocks were deployed.
The Sextant House Clock (where Flamsteed made most of his observations prior to the arrival of his Mural Arc in 1689) was made by Thomas Tompion in 1675. Prior to being placed in the Sextant House, it was used for a few months in the Octagon Room while the clocks for that room were under construction. The first recorded observations made there were related to the eclipse of 1676, June 11 (June 1 old style civil, May 31 old style astronomical). Details of the clock are scanty. It had a one-second pendulum, but possibly no maintaining power (this is based on a record in Flamsteed's observing Book RGO1/1 that it lost 8 seconds when it was wound on 9 October 1676).
Two year going clocks made by Tompion with 13-foot (four-metre) two-second pendulums (the Great Clocks) were installed in the Octagon Room in the summer of 1676 (probably on 7 July) and were in proper working order for the first time by 24 September. They can be seen to the left of the door in the etching below. Although at first sight, the clocks appear identical, a key difference was in the way the pendulums were suspended. The one closest to the window was suspended from a spring whilst that nearest the door was pivoted on a knife edge. Both were set to show mean solar time, with one presumably acting as a check on the other. One thing we don't know is to what extent they may have interfered with one another (something that Huygens had previously observed in his own clocks and written about in 1673 in his book Horologium Oscillatorium). However, as Flamsteed's pendulums were long and the arc through which they swung was small, any such interference was likely to be undetectable. A third clock with a shorter pendulum (and seemingly designed to show sidereal time) was also planned for the Octagon Room though there are no obvious records of it having been used in practice.
In 1690, Flamsteed bought a new clock for use with the Mural Arc which first came into use at the end of 1689 and was mounted in the former Quadrant House. From September 1689 until the arrival of the new clock on 31 October 1690, the Sextant House Clock was used instead. Like the Sextant House Clock it replaced, the new clock had a seconds pendulum, but little else is known about it. It was probably made by Tompion.
As well as the clocks already mentioned, Flamsteed had a clock made by Tompion in 1691 with a 2/3 second pendulum that was designed to show sidereal time in terms of degrees minutes and seconds rather than in terms of hours minutes and seconds. Now referred to as the Degree Clock, there appear to be virtually no manuscript records (and no published records) of this clock ever being used.
We also know that Flamsteed had a portable clock or horologium ambulatorium. Little is know about the clock, but it does appear in one of the Francis Place etchings of the Observatory produced in about 1677. There, it is shown in the eastern summerhouse where it would presumably have been used to time solar eclipses. It was presumably also used when timings were required with one of the outside telescopes. Presumably spring driven It was probably only wound when needed for observing and would most likely have been set by comparing it to one of the clocks in the Octagon Room. There are sporadic references to it by name in the original observing books (RGO1/1-8).
There are also occasional references in the observing books to a 'horologium supra' and a 'horologium infra' which were presumably alternative names Flamsteed used for two of the clocks already mentioned (one of the Great Room Clocks and the Sextant House Clock?). The Octagon Room Clock was also referred to as the 'Horologium in Camera Superiore'. There are several references to it in both the observing books and the published observations during November and December 1689 it's time was being compared with that of the Sextant House Clock while the Mural Arc was still undergoing the calibration process.
Of the seven clocks owned by Flamsteed, only three survive: the two great clocks (both of which have been modified from their original form and the Degree Clock.
Rather than each location at the Observatory having its own observing book, all the observations were recorded chronologically into a single volume, new volumes being started as and when needed. Some of the observations and notes are recorded in English and some in Latin. Not all of the observing books survive. There is no book of the original entries from 17 November 1702 to 2 January 1712 exclusive. The surviving volumes have the class marks RGO1/1-8. The missing entries would have been preceded by the entries in RGO1/7 and followed by those in RGO1/8. We know from Flamsteed's preface to the 1725 Historia that these First Night Notes (as he referred to them) 'were wrote in 4to Volumes & from them were commonly transcribed correctly into large folios next morning from which the [later] Copies were taken.' (Newton Project RGO1/32C & Baily's Flamsteed, p.80). Those made from the observations made between 1689-1719 (RGO21/4-8) appear to have been transcribed into the volumes that now carry the classmarks RGO1/15-17 and are described by Flamsteed as 'Apographa' and in the current catalogue as 'exact copies'. This is not entirely true as some data that is key to the present investigation was omitted (more on this later). Although he did not comment on this particular omission, when he catalogued the papers in the 1830s. Francis Baily did write:
'In the preceding volume [RGO1/15], and in the present one [RGO1/16] as far as the end of 1698, the zenith distances are copied from the originals with the correction of the instrument applied; which does not always accord with that which Flamsteed ultimately adopted. I would also remark that I have occasionally met with entries of observations which are not to be found in the original MSS: so that it will be necessary to examine these copies as well as the originals, in case of any revision of the observations.'
There is also at least one incidence of a whole day's observations being omitted, for example those that took place on 24 July 1690, seemingly after some issue with the clock at the start of the day's observing.
There is nothing in the archive catalogue to suggest that the earlier observations were copied in the same way. What may have happened is that they were transcribed into a variety of different places depending on the type of observation. Further research is required to properly understand how RGO1/1-3 were processed.
The clock times in both the 1712 and 1725 Historias are given in terms of the astronomical day, with each day beginning and ending at midday (rather than the civil day which ran from midnight to midnight and started 12 hours earlier). It is important to note too that all Flamsteed's clocks (apart from the unsuccessful Degree Clock) were set to mean solar time rather than sidereal time. Despite the fact that the observations as published in the Historias were given based on the astronomical day, Flamsteed only started using astronomical time in the observing books when the mural arc came into use in 1689 (RGO1/4-8). The times recorded in Flamsteed's first three observing books (RGO1/1-3) were recorded in terms of the civil day using a 12 hour rather than 24 hour notation. Civil time was also the time system used for eclipse observations in Philosophical Transactions where some of Flamsteed's observations were also published.
Despite the fact that Flamsteed was using astronomical time in the observing books used with the mural arcs, there was an anomaly, at least to start with (and until at least 1693), about the way in which he recorded the times of the Sun's transit, Those observations that were made at the end of the astronomical day were recorded with the following day's observations but the time was given using a 12 hour nomenclature for example 11.55.46 on 18 April 1690. When transferred to the apocrapha this was recorded as A.M. 4.14, by which Flamsteed meant 4 minutes 14 seconds before 0h, 0', 0" astronomical time. In the published observations, it was recorded under 17 April as 23.55.46.
Although he recorded the time of each observation, Flamsteed did not record where in the Observatory the observation was made or the clock used. When he came to publish his observations, those from different locations remained lumped together in chronological order, again without any mention of clock or location. Although other factors mean that it is often possible to distinguish observations made in the Octagon Room from those made elsewhere this is not always possible. Nor is it possible to discern from which of the two Octagon Room clocks the recorded time was taken though this would not matter if Flamsteed always took the time from the same one if they were syncronised (which seems unlikely). Flamsteed seems to have 'corrected' (ie deduced the errors) of the clock he was using in the Octagon Room from measurements of the Sun's altitudes (more on this later). Similar observations of the Sun's altitude were also made in the Quadrant house during the commissioning of the Mural Arc and possibly later. Likewise, the published observations and correspondence show that Flamsteed used a projection screen to view a solar eclipse on at least six occasions but there is no explicit record of whether these observations were made in the Octagon Room or in one of the Summer Houses. For more information on this see: Solar eclipses observed at Greenwich during the time of Flamsteed (1675–1719).
Unlike the published observations of his successors from Bradley's time onwards, those of Flamsteed do not record the adjustments made to the time shown by the clock the cumulative error had exceeded an acceptable value. An examination of the observing books show that by the time he was using the Mural Arc (if not before) such alterations were recorded by Flamsteed in his observing book and transcribed into the apographas (RGO1/15-17). On 10 February 1690, in both the observing book and the apocrapha , this was flagged not only with a comment, but also by the drawing of a pointing hand alongside it. On this occasion, Flamsteed put the clock forward by 25 seconds. Further research is required to see if Flamsteed always commented in the observing book and if this was always transcribed. Occasionally, the comments were also published in the Historia, An example of this occurs in the entry for 30 October 1690 where Flamsteed records 'purgato prius horologio' (after having first cleaned the clock). The following day, in the Observing book, Flamsteed recorded in Latin 'I have put the new clock next the old one in the Arc House, and have set the hands at the same time, 9.39.00.a.m.' This was not transferred to the apocapha nor is there any mention of a change of clock in the published observations. However, a note in the observing book made a few days later on 4 November about Abraham Sharp leaving to teach mathematics was transferred across to the apocrapha.
.....
Things are not straight forward. The first of the 1725 volumes contains observation made between 1676 and 1689 when the equatorial sextant was the main observing instrument. The second contains those made between 1689 and 1720 when with the Mural Arc was the main instrument. However, the way in which Flamsteed presents his observations changes between the two volumes. In Volume 2, the observations are published in the order that they appear in the observing books. In Volume 1, they are divided into six categories (which makes comparison with the observing books a lot more time consuming). The categories are:
The distances of the fixed stars are further rearranged in chronological order by constellation. No explanation is given for the change in presentation between the two volumes. Nor does Flamsteed explicitly explain how he rated his clocks or deduced their errors. Nor did he publish tables of their rate or error, though it is possible (though unlikely) that such tables exist and are waiting to be discovered in the archives.
The following terms are used in the column headings of the various sets of observations in the Historias (some sets just one, others two):
| 1 | Temp App. / Temp Ap. | Apparent time |
| 2 | Tempora per Horologium Oscillatorium | Time by the pendulum clock |
| 3 | Tempora ex Altitud. Correcta | Times corrected by altitude(s) [of the Sun] |
| 4 | Tempora ab Observationibus Correcta | Times corrected by observations |
| 5 | Tempora Correcta | Corrected times |
| 6 | Tempora (inde) ab Observationibus Correcta | Times corrected (from) observations |
| 7 | Tempora vera Apparentia | True apparent times |
| 8 | Temora vera & correcta | The true and correct time |
Not all the manuscript observations appear in the printed Historias. One example of those omitted is the third set of observations of the Sun's altitude that he took for correcting the clock at the time of the solar eclipse of 1676 (the three sets of observations having been taken on 31 May and the 1 June. It should be noted here that there is also an unexplained discrepancy in these particular sets of altitude observations in the observing book and those subsequently published in the Historia and Philosophical Transactions, which also differ from one another. Other examples of observations of the sun's altitude being omitted from the Historia have also been found. Also omitted, are the observations of Sirius made with with the Sirius Telescope in the late 1670s.
All this presents a problem when trying to find the observations from which Flamsteed obtained the rate and hence the errors of his clocks. There are occasional references to comparisons between the the Sextant House and Octagon Room Clock in the first volume of Flamsteed's observing book (RGO1/1). How they were made is not stated but it was presumably by means of his portable clock or a watch.
We can be pretty sure that to start with at least, Flamsteed determined the errors and rate of the clocks used in the Octagon Room clocks by making equal altitude observations of the Sun. By 1678 however he also had the means to determine their error and rate from observations made with the Sirius Telescope, but no evidience has been uncovered to suggest that he did?
Likewise, the errors and rate of the mural arc clock could easily have been determined from observations of selected stars as they crossed the meridian. But an examination of the observations shows that he probably determined them by applying the equation of time to timings of the Sun as it crossed the meridian.
Hypothesis:
In volume 2 of the Historia, timings made in the Octagon Room were converted into the equivalent time shown by the Quadrant (Arc) House at the same moment, the comparison being made with the help of the horologium ambulatorium
Possible evidence (1).
The timings of the observations of Jupiter's satellites on 20 September 1702 recorded in the Observation Book (RGO1/7) differ from those published in the Historia, in one key respect. Each of the clock times is preceded by another that is 7minutes 27 seconds ahead. It is therefore surmised that the first time is the time by the Octagon Room Clock and the second (the one that was eventually published), the equivalent time on the Quadrant (Arc) House clock.
Possible evidence (2).
A similar state of affairs occurs with the recording of the observations of Jupiter's satellites made on 3 February 1691. In this case, the Octagon Room Clock would appear to be 5 minutes 20 seconds slow compared to the Quadrant (Arc) House Clock.
Given that relatively few observations were made in the Octagon Room after 1689, this raises the question: were all the post 1689 Octagon Room timings converted to the equivalent time on the Quadrant (Arc) Room Clock?
Volume 1 of the Historia contains numerous off meridian measurements of the Sun's altitude, all of which appear to relate to the Octagon Room Clocks. There is a dearth of off equivalent measurements in Volume 2 and most of these took place between 8 April and 21 September 1690 in conjunction with the calibration of the Mural Arc. This could be explained by the hypothesis above.
Further Research?
Investigate the recording of all the eclipses of the Sun, the Moon and Jupiter's satellites to see how many of these are recorded in a similar way.
Little in-depth research has been done into Flamsteed's methods of recording and structuring his observations since Francis Baily's investigations of the 1830s and those more limited ones by Derek Howse in the 1960s and early 1970s. Since that time, all the known correspondence of Flamsteed has been published and many resources digitised. A serious, systematic in-depth investigation of how Flamsteed operated is now long overdue, but beyond the scope of this website. It would be a major undertaking for any research group.
In her book Under Newton's shadow: astronomical practices in the seventeenth century (1985), Lesley Murdin incorrectly wrote (p.129):
'To find the error of his clocks, Flamsteed attached two 5-foot telescopes to a wall so that he could take the times of well observed stars crossing the Meridian'
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