Telescopes: α Aquilae and α Cygni Telescopes (1816)

Pond's Alpha Cygni Telescope with details of the mounting and component parts. The Alpha Cygni Telescope was mounted in front of Halley's 8-foot Iron Mural Quadrant (not shown) which remained in place on the wall behind it. Engraving from Volume 2 of Pond's Greenwich Observations

The α Aquilae and α Cygni Telescopes were conceived by the Astronomer Royal, John Pond, in an attempt to settle a differerence in view that had developed between him and John Brinkley (the Royal Astronomer of Ireland and a former Greenwich Assistant under Maskelyne), over whether or not Brinkley had measured a parallax for α Aquilae (Altair), α Cygni (Deneb) and a small selection of other bright stars. Brinkley’s Observations were made with an 8-foot reversible vertical circle at the Dunsink Observatory near Dublin. The paper that triggered what turned out to be a decade long argument was published by Brinkley in 1815. On checking his own Mural Circle observations at Greenwich, Pond found that although there was a discordance between his summer and winter observations, it was much less than that which Brinkley had measured.

To try and settle the question by an independent means, Pond commissioned two identical 10-foot achromatic telescopes of four-inch from Dollond. Each was to be mounted rigidly on a meridian wall, one angled at about 6.5º from the zenith for observing α Cygni as it crossed the meridian; the other angled at about 43º for similar observations of α Aquilae. The α Aquilae Telescope was mounted on the rear (west side) of the pier that carried the Troughton Mural Circle; the α Cygni Telescope on the west side of the Quadrant Wall. As there were already telescopes in these locations (a zenith telescope on the Mural Circle Pier and Halley’s Iron 8-foot Mural Quadrant on the Quadrant Wall), rather than dismount them, the mountings for the new telescopes were made so that they could be mounted over the top of the existing instruments (which in the case of the zenith telescope would still be usable).

A small degree of adjustment was possible at the setting up stage through the provision of elongated screw holes in the telescope clamps. Each telescope was fitted with a double micrometer, the wires being illuminated by light that entered the side of the tube via a hole and was reflected downwards by a mirror. Initially, the two telescopes had different micrometers, one revolution of the α Aquilae Telescope’s micrometer representing 33.3” and that of the α Cygni Telescope representing 17.05”. Why they were different is not known.

The first published observation made with the α Aquilae Telescope is dated 14 November 1816; that for the α Cygni Telescope is dated 18 January 1817. (Click here to view the observations made between1816 and 1818). The observing programme with the α Aquilae Telescope ceased on 21 January 1817 to allow it to be dismounted for the fitting of a new micrometer similar to that of the α Cygni Telescope (on the new micrometer, one revolution represented 17.17”). It was brought back into use on 25 July 1817.  The bill submitted for the two telescopes by Dollond was for £147. 9s. (RGO6/22/95). It is not clear if this included the cost of the replacement micrometer. The cost of the stonemason was almost certainly extra.

 

Pond’s methodology

Previous attempts at measuring parallax had been dependent on a plumb-line to fix the exact point of the zenith. Pond’s technique with the α Aquilae and α Cygni Telescopes was to dispense with the need for one and thereby eliminate a potential major source of error.  Having permanently fixed the positions of each of the telescopes, he used the one on the Quadrant Wall to compare the declination of  α Cygni with that of β Aurigae and that on the Circle Pier to compare the declination of α Aquilae with that of l (55) Pegasi. The stars for comparison, were chosen because they had very similar declinations, which meant they would pass though the field of view. They also had widely differing right ascensions, which meant that they would pass several hours earlier or later (about 9 hours for α Cygni and β Aurigae and 3 hours for α Aquilae and l (55) Pegasi).

The results obtained by Pond convinced him that there was no discernable parallax that could be measured. Brinkley disagreed. Their argument was largely a technical one, based on such matters as the differences in right ascension, the accuracy of refraction tables, the adopted constant of aberration and possible instrumental instabilities and errors.

In his 1822 paper (On the Parallax of α Lyrae), Pond wrote:

‘The history of annual parallax appears to me to be this: in proportion as instruments have been imperfect in their construction, they have misled observers into the belief of the existence of sensible parallax. This happened in Italy to astronomers of the very first reputation. The Dublin instrument is superior to any similar construction on the continent; and accordingly it shows a much less parallax than the Italian astronomers imagined they had detected. Conceiving that I have established, beyond a doubt, that the Greenwich instrument approaches still nearer to perfection, I can come to no other conclusion than that this is the reason why it discovers no parallax at all.’

 

Later history and current location

The α Cygni Telescope was dismounted in 1839 when the Safe Room was created by Airy at the western end of the Meridian Building, the telescope being put into store in the Octagon Room (RGO6/54/75). The α Aquilae Telescope was likewise dismounted in 1848 and put into store in the Octagon Room when work began on converting the Circle Room for the new Transit Circle Room (RGO6/54/78). Both instruments were then hung on the western wall of the Transit Circle Room in 1850 following its completion (RGO39/2)

Both instruments are now in the care of the National Maritime Museum. In 2014, they were in storage.

α Aquilae Telescope (Object ID: AST0996)
α Cygni Telescope (Object ID: AST0995)

 

Contemporary account

An introduction to practical astronomy Volume 2 pp.560-564. Rev. W. Pearson (London, 1829). Click here to view the accompanying Plate (Plate XII)

 

The relevant papers of Pond and Brinkley

Date
Author
Details
1810 Brinkley Extract of a Letter from the Rev. John Brinkley, D.D.F.R.S. Andrew's Professor of Astronomy in the University of Dublin, to the Rev. Nevil Maskelyne, D.D.F.R.S. Astronomer Royal, on the Annual Parallax of α Lyrae. Phil. Trans. R. Soc. Lond. January 1, 1810 100 204 (PDF)
1815 Brinkley Appendix to the Account of Observations made at the Observatory of Trinity College, Dublin which appear to point out an Annual Parallax in certain fixed Stars, &c. &c.. The Transactions of the Royal Irish Academy Volume 12 pp.119–124 (1815). See also pp.33–76 and pp.77–118
1817 Pond On the Parallax of the Fixed Stars. Phil. Trans. R. Soc. Lond. January 1, 1817 107 158–175 (PDF)
1817 Pond On the Parallax of the Fixed Stars. Phil. Trans. R. Soc. Lond. January 1, 1817 107 353–362 (PDF)
1818 Brinkley On the Parallax of Certain Fixed Stars. Phil. Trans. R. Soc. Lond. January 1, 1818 108 275–302 (PDF)
1818 Pond On the Parallax of α Aquilae. Phil. Trans. R. Soc. Lond. January 1, 1818 108 477–480 (PDF)
1818 Pond On the Parallax of the Fixed Stars in Right Ascension. Phil. Trans. R. Soc. Lond. January 1, 1818 108 481–485 (PDF)
1821 Brinkley An Account of Observations Made with the Eight Feet Astronomical Circle, at the Observatory of Trinity College, Dublin, since the Beginning of the Year 1818, for Investigating the Effects of Parallax and Aberration on the Places of Certain Fixed Stars; Also the Comparison of These with Former Observations for Determining the Effects of Lunar Nutation. Phil. Trans. R. Soc. Lond. January 1, 1821 111 327–360 (PDF)
1822 Pond On the Changes Which Have Taken Place in the Declination of Some of the Principal Fixed Stars. Phil. Trans. R. Soc. Lond. January 1, 1823 113 34–38 (PDF)
1822 Pond Appendix to the Preceding Paper on the Changes Which Appear to Have Taken Place in the Declination of Some of the Fixed Stars. Phil. Trans. R. Soc. Lond. January 1, 1823 113 39–52 (PDF)
1822 Pond On the Parallax of α Lyrae. Phil. Trans. R. Soc. Lond. January 1, 1823 113 53–72 (PDF)
1823 Pond On Certain Changes Which Appear to Have Taken Place in the Positions of Some of the Principal Fixed Stars. Phil. Trans. R. Soc. Lond. January 1, 1823 113 529–540 (PDF)
1824 Brinkley Remarks on the Parallax of αLyrae. Phil. Trans. R. Soc. Lond. January 1, 1824 114 471–498 (PDF)
1825 Pond On the Annual Variations of Some of the Principal Fixed Stars. Phil. Trans. R. Soc. Lond. January 1, 1825 115 510–512 (PDF)

 

Further reading

The distances to the stars, Murray, C. A., The Observatory, vol. 108, pp. 199-217 (1988)

The Historical Search for Stellar Parallax, Fernie, J. D.,Journal of the Royal Astronomical Society of Canada,

Vol. 69, pp.153–161
Vol. 69, pp.222–238
Vol. 70, p.40

Measurements of the Distances of the Stars, Dyson, F. W., Journal of the Royal Astronomical Society of Canada, Journal of the Royal Astronomical Society of Canada,ol. 9, pp.407–422