J. Brit. Astron. Assoc., 107, 3, 1997, pp. 164–165


(Note: The Association is not responsible for individual opinions expressed in articles, reviews, letters or reports of any kind.)

Alan P. Lenham

From the BAA Curator of Instruments

As an addendum to Richard McKim's obituary of Alan Lenham (Journal, 107(1), 1997, p.50), I would like to advise members that the 8-inch (20cm) reflector referred to was bequeathed to the Association. This instrument is available for loan, although it requires some refurbishment, including a coat of paint – unless grey and iridescent green applied au camouflage is considered tasteful. I can also confirm the 'fish and chip shop' anecdote, but although this seems amusing it must have been frustrating to have planetary images continually degraded by the air rising from hot chip fat, the shop being due south and at the bottom of the garden. However, John Gould and myself ignored possible restrictions on the advancement of science when we visited the establishment on the day we collected the instrument. The photograph of the Yerkes Observatory staff included in the obituary seems strangely at odds with the terraced house in Swindon which, when we visited, was empty except for a poignant scattering of railway, aircraft and cricket ephemera. I knew Alan Lenham's name for many years, and – like Richard McKim – much regret that I never met him.

R. A. Marriott
24 Thirlestane Road, Far Cotton, Northampton NN4 8HD

More on the twin paradox

From Dr John McCue

Like Mr Watson,[1] I hesitate to enter a minefield, but I also refer to the Twin Paradox question raised by Mr V. Mayes. [2]

Roger Penrose has a neat explanation,[3] and while I do not claim to be an expert on special relativity, I will try to convey Penrose's meaning. He discusses the problem in terms of the Minkowskian geometry implicit in the four-dimensional world of spacetime. This is impossible to represent on paper, but the idea can be conveyed by using two dimensions for space and one for time. Twin A on Earth will then have a vertical 'world line' X Z: he stays in the same place as his time progresses on the vertical axis (see Figure). Twin B makes her journey, to the Moon, say, and returns to the Earth some time later, the trip described by her world line X Y Z.

The reader will be familiar with Pythagoras' Theorem for the distance between two points in a three dimensional, space only, framework:... r^2 = x^2 + y^2 + z^2.

The equivalent formula in three dimensional (two space, one time) Minkowskian geometry is similar but with a crucial difference:... S^2 = t^2 – (x/c)^2 – (z/c)^2 where c is the speed of light. With the fourth dimension added this becomes:... S^2 = t^2 – (x/c)^2 – (y/c)^2 – (z/c)^2.

Here S, the 'distance-between-two-points' measure, represents the time elapsed as experienced by the moving observer along that particular world line. Because of the crucial minus signs, Minkowskian geometry is different from the usual Euclidean geometry and in fact for S, ... XZ > XY + YZ not, as one might think, the other way round.

Clement Durell[4] describes at length how the interval, S, between two events is always the same when measured by different inertial (constant velocity) observers, no matter how they may be moving relative to each other. In terms of the Twin Paradox, the journeying twin B has experienced a shorter passage of time than her earthbound twin A, finding him slightly older when they meet again. Acceleration is clearly important inasmuch as twin B must experience an acceleration (if only expressed as a change of direction) at the Moon, Y. Durell quotes Minkowski himself: 'From now onwards space and time sink to the position of mere shadows, and only a sort of union of both can claim an independent or absolute existence.'

John McCue
40 Bradbury Rd, Stockton on Tees, TS20 1LE

[1] Watson J., J. Brit. Astron. Assoc., 107(1), 47 (1997)
[2] Mayes V., J. Brit. Astron. Assoc., 106(5), 294 (1996)
[3] Penrose R., The Emperor's New Mind, Oxford University Press, 1989, 256
[4] Durell C. V., Readable Relativity, G. Bell, 1926, 71

(– Sorry, but this really is the last word for now on this subject. Ed.)

Deaf astronomers wanted

From Ms Jane Dewey

On 14 March at the start of SET 97, two special planetarium shows for the deaf were given in the Caird Planetarium at the Old Royal Observatory in Greenwich as part of a day of signed astronomy events. As the result of the success of these events, the idea of an informal deaf astronomy society with a base at the National Maritime Museum has been suggested. This group would organise meetings, discussions, trips etc. If you are interested in finding out more then the Museum would be delighted to hear from you. Hearing astronomers who can sign are also welcome. You can contact the museum by letter or fax at the address below.

Jane Dewey
Education and Interpretation Dept., National Maritime Museum, Greenwich, SE10 9NF. Fax: 0181 312 6632

The daylight comet of 1500

From Mr Barry Hetherington

One of the rarest of astronomical sights must be that of a comet in broad daylight; probably only rivalled by 'daylight' supernova observations. In my recently published book[1] I recorded only four such instances in pre-telescopic times: in AD302 - seen from Japan; in 1222 - 'Halley's', as seen from Korea; in 1402 - visible in daylight for 8 days; and in 1471 - comet Regiomontanus, as seen from China.

Another record has just come to light from the year 1500. In this year Pedro Alvares Cabral left Portugal with a fleet of thirteen ships on a journey around Africa to the 'Indies': '...He sailed from Brazil on 2nd May taking a southerly course at first and then altering to the east towards the Cape of Good Hope. It is recounted that ten days after leaving, a comet was sighted in the east which became so bright over the next ten days that it was visible day and night.'[2]

I also give details of a comet visible at this time from the northern hemisphere. Johann Werner observed a comet between June 1 and 24. From China it was first seen on May 8 above Capricorn/Aries; it reached Pegasus, swept Cepheus, and approached Draco. It went out of sight on July 10. If these observations refer to the same comet then those from China and the south Atlantic overlap. However, it is difficult to explain why the Chinese observers did not comment on its great brilliance. It is possible that there were two comets visible at this time, but if so, then the northern one should also have been visible to Cabral, and he would surely have mentioned this.

I have also received a letter from Graeme Waddington of Oxford stating that the entry relating to Abraham Robertson in my 'Centenaries for 1997'[3] is incorrect. The source of the error is volume 48 of the Dictionary of National Biography, which states 'In 1784 Robertson succeeded Dr Austin as lecturer for Dr Smith, who was then acting as a physician at Cheltenham. On the death of the latter in 1797, Robertson took his place as Savilian professor of astronomy.'[4]

Graeme Waddington corrects this by stating that Thomas Hornsby held the Savilian chair of astronomy from 1763 until 1810. Robertson was appointed to the Savilian chair of geometry in 1797. In 1810, following the death of Hornsby, he was appointed to both the Savilian chair of astronomy and the post of Radcliffe Observer which he held until his death in 1826.

Barry Hetherington
22 George Street, Darlington, Co. Durham. DL1 5DW

[1] Hetherington B., A Chronicle of Pre-Telescopic Astronomy, John Wiley & Sons, Chichester, 1996
[2] Knox–Johnson R., The Cape of Good Hope, A Maritime History, Hodder & Stoughton, London, 1989
[3] Hetherington B., 'Centenaries for 1997', J. Brit. Astron. Assoc., 106(6), 331 (1996)
[4] Lee S. (ed.), Dictionary of National Biography, vol. 48, Smith Elder & Co., London, 1896, p.398

A legacy from J. Hedley Robinson

Fron Dr A. J. Hollis

In the February 1997 issue of Engineering First (the journal of the Engineering Council) there is a small footnote entitled "Stargazers' bargain." It describes the construction of an observatory at Teignmouth Community College, Devon, organised by a band of local engineers and carried out at nominal cost by contractors. This observatory - opened by Patrick Moore - houses a 10.25-inch Newtonian donated by Hedley Robinson's son.

Members will recall that Hedley was Mercury and Venus Section Director between 1965 and 1979 and that he lived in Teignmouth. His observatory regularly featured in books and made use of golf balls to provide the support for the rotating roof.

It is gratifying to know that his equipment will continue to be used by local people and hopefully will be put to good use in the community. I hope that the telescope or observatory will be named in his honour.

Andrew J. Hollis
Ansteys Lea, Clay Lane, Marton, Cheshire CW7 2QE

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