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Papers of Nevil Maskelyne : Quadrant and zenith sector observations

Papers of Nevil Maskelyne

<p style='text-align: justify;'>This volume records meridian observations made at the Greenwich Observatory between 1765 and 1786 during the tenure of Nevil Maskelyne, the fifth Astronomer Royal. One of the major reasons that the Greenwich Observatory was founded in 1675 was to make an accurate survey of the skies for the purposes of determining longitude at sea. An effective lunar-distance method necessitated extensive and accurate fixes of the positions of key stars, including the sun, over many years, against which the motion and position of the moon could hopefully be predicted. Careful observations of the moon itself were also critical, as any lunar distance method needed lots of data in order to develop an accurate theory of the motion of the moon, one of the most mathematically complicated problems of the 18th century.</p><p style='text-align: justify;'>During the tenure of Edmond Halley, second Astronomer Royal, and James Bradley, third Astronomer Royal, a system of meridian astronomy was established, whereby two types of large telescopes were fixed rigidly in the north-south or meridian axis. The mural quadrants measured the height or altitude of a stellar object as it crossed the meridian, whilst the transit instrument, in conjunction with a pendulum regulator clock, was used to record the exact time that a stellar object crossed the meridian.</p><p style='text-align: justify;'>By the time of Maskelyne's appointment in 1765, there were two 8 foot mural quadrants, one facing north and one facing south, and an 8 foot transit instrument. All three had been <i>contrived</i> by John Bird, and all were based on the designs of George Graham, the most celebrated English instrument maker of the 18th century. All three survive today Greenwich displayed in their original positions:</p><p style='text-align: justify;'><a target='_blank' class='externalLink' href=''>mural quadrant</a></p><p style='text-align: justify;'><a target='_blank' class='externalLink' href=''>transit instrument</a></p><p style='text-align: justify;'>All astronomical instruments of this type need to have their arcs of circle accurately divided into degrees, minutes and seconds. The Board of Longitude rewarded John Bird with £500 in 1767 for <i>discovering</i> the method by which he divided the Greenwich mural quadrants. The Board subsequently published two books at their own expense describing Bird's technique and containing detailed engravings of all the parts of the instruments:</p><p style='text-align: justify;'> The Method of Dividing Astronomical Instruments </p><p style='text-align: justify;'> The Method of Constructing Mural Quadrants Exemplified by a Description of the Brass Mural Quadrant in the Royal Observatory at Greenwich </p><p style='text-align: justify;'>This 200 page volume contains 20 years of rough observations made with the mural quadrants. Numerous other manuscript volumes in the RGO 4 archive contains similar transit observations. On the <a href='' onclick='store.loadPage(2);return false;'> flyleaf (RGO 4/28:cover (inside))</a>, a faint pencil note in Maskelyne's neatest hand states that 'this book may suffice to contain 30 years observations'. This note was later overwritten by Maskelyne in ink with the legend 'Royal Observatory at Greenwich'. On this same page, Maskelyne lists all the assistants he employed in the period. Each page of manuscript observations is signed off by Maskelyne; closer inspection shows that many observations were initialled, and hence observed, by his assistants. For example, on Monday 13th April 1772 Maskelyne's assistant Reuben Burrow observed the zenith distance (defined as 90∞ minus the altitude of the object) of the Sun's lower and upper limbs as they crossed the meridian at local noon. Burrow notes that whilst the observation of the lower limb was 'very good', that of the upper limb was 'dubious' to the order of three or four seconds of arc, the weather was cloudy, the barometer stood at 29.36 inches, and the temperature outside was 54∞ Fahrenheit. This can be found on the back of <a href='' onclick='store.loadPage(72);return false;'> page 37 (RGO 4/28:37v)</a>. On Wednesday 2nd December 1767 Maskelyne reported that John Bird had visited, and applied fresh grease to the pivots about which the telescope of one of the mural quadrants rotated. This can be found on page <a href='' onclick='store.loadPage();return false;'>RGO 4/28:18</a>. Sometime between Sunday 26th July 1772 and Friday, August 7th 1772 Maskelyne noted that an achromatic object glass was added to the south mural quadrant by a Mr Dollond. This can be found on page <a href='' onclick='store.loadPage();return false;'> 39 (RGO 4/28:39)</a>. This new type of lens, which solved the great problem of chromatic aberration, was <i>contrived</i> by John Dollond, whose instrument making business still survives today as Dollond and Aitchison opticians. Maskelyne noted that the installation of the new lens necessitated a slight alteration to the zenith distance calculations. Hence, pasted to the flyleaf, we see a small note in Maskelyne's hand, reminding him and future readers to add two seconds to each zenith distance calculation. On Tuesday 17th October 1775 Maskelyne noted that the plumbline that defined a true vertical on the north facing quadrant had been rubbing against another part of the instrument: several months worth of observations, he noted, must be omitted, and are thus all crossed out, as can be seen on the three pages starting <a href='' onclick='store.loadPage(98);return false;'> here (RGO 4/28:50v)</a>. Hence a close reading of these 200 pages of dense and technical calculations can be read in such a way as to build up a rich and nuanced understanding of the daily life of the Greenwich Observatory, right down to detailed daily weather reports!</p><p style='text-align: justify;'>The best way to test the accuracy of measuring device is to compare it to another measuring device. For this purpose Maskelyne installed a lighter, and more portable telescope, albeit 12 1/2 feet high, called a zenith sector, which he used to calibrate the mural quadrants, as can be seen in observations throughout this volume. John Bird made modifications to this sector, which he set up on 12th and 13th of July 1768, as we can see <a href='' onclick='store.loadPage(38);return false;'> here (RGO 4/28:20v)</a>. This instrument, made in 1727 by George Graham, was in the 1830s was transferred to the Cape of Good Hope Observatory, itself set up by the Board of Longitude, and can today be found displayed at Greenwich:</p><p style='text-align: justify;'><a target='_blank' class='externalLink' href=''>zenith sector</a></p><p style='text-align: justify;'>Further reading on the Greenwich Observatory instruments can be found in Derek Howse's Greenwich Observatory Volume 3: the Buildings and Instruments, and Nicky Reeves' Ph.D. thesis Constructing an Instrument: Nevil Maskelyne and the Zenith Sector, 1760-1774.</p><p style='text-align: justify;'>Nicky Reeves<br />History and Philosophy of Science<br />University of Cambridge<br /></p>

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