German town takes the lead against skyglow J. Brit. Astron. Assoc., 107, 4, 1997, pp. 224-226
(Note: The Association is not responsible for individual opinions expressed in articles, reviews, letters or reports of any kind.)
From Mr H. Robert Mills
Visitors to Stonehenge may feel the need for reliable information concerning its astronomical significance. The monument is situated on Salisbury Plain at the particular latitude at which the azimuth of the Sun at its midsummer rising, and the azimuth of the Moon at its major standstill rising, differ by 90°. These two directions are remarkably in evidence in the rectangle formed by the four station stones 91, 92, 93 and 94 which mark the direction (91-92) representing the azimuth (50°/230°) of the rising and setting of the midsummer Sun and the side 92-93 representing the azimuth (140°/320°) of the Moon rise and setting at the time of the Moon's major standstill. It is noteworthy that a position of Stonehenge situated about 30 miles to the north or to the south of its present site, would not satisfy this important Sun-Moon rectangle.
The model, 48cm diameter, was formed by joining 12 photographic transparencies, each of which comprised 30° of azimuth from the centre. The camera for taking these photographs was mounted on a stable horizontal surface, 1.5m high, at the 'centre' of the stones, and was rotated about a vertical axis, in 12 steps, to cover the whole horizon. The actual width of each photograph covered a field of view of about 35° which comfortably ensured that the 12 transparencies would adequately cover the full horizon. The photographs when mounted showed that Stonehenge was well chosen for its uniformity and completeness. The model could also be used to demonstrate the relationship of significant astronomical azimuths of former times with those of today.
H. R. Mills
83 Firs Road, Firsdown, Salisbury, SP5 1SW
(from a report prepared by the RAS Stonehenge Group for presentation to 'English Heritage')
German town takes the lead against skyglow From the Coordinator, BAA Campaign for Dark Skies
Readers of the Journal might like to know that a European city has not only declared itself against light pollution of the night sky, but has formulated and passed in council an action plan to be rid of it. In mid-1996, the BAA's Campaign for Dark Skies received a letter from its correspondent in southern Germany, Rudolf Stolte, to the effect that Augsburg, a city with a population of about a third of a million people, was considering an attack on skyglow as part of its commitment to the international Agenda 21 environment watch. CfDS sent material and lent its support to the city council's environment committee, and in a letter to me in May 1997, Dr Bruggey, head of Augsburg's environment department, thanked CfDS for its 'moral support' and detailed a wide range of 'sky-friendly' measures adopted. These include adaptation or replacement of all types of lamps to cut out light spill above the horizontal by the year 2005, controls on floodlights and advertising displays, and a ban on sky beams.
BAA members who have not yet asked about the extent of their local authorities' desire to protect the environment above, or who have received negative replies, might like to cite the example of Augsburg, a city which agrees with CfDS' call for 'the right amount of light, and only where needed'.
Bob Mizon
38 The Vineries, Colehill, Wimborne, Dorset BH21 2PX
From Mr Terry Moseley
A rare and interesting series of Jovian satellite phenomena occurs on the evening and night of 21-22 September 1997. From 18.38 to 22.37, and from 22.46 to 00.12 UT, there will be at least two satellite events visible on the disk, while from 18.49 to 21.28, there will be three such events. The climax of the series is the period from 19.24 to 19.33, when there will be four phenomena visible simultaneously - transits of Europa (II) and Ganymede (III), and shadow transits of Europa and Callisto (IV). Four such events simultaneously is very rare, and even more so when both Ganymede and Callisto are involved.
During the period 19.24 to 19.33, the shadow of II will appear as a black spot; III will appear as a dark spot, whose relative darkness will depend on whether it is seen against a dark belt or a bright zone; II will appear as a bright spot at the preceding edge of the disk, moving gradually off it; and the shadow of IV will be a dark spot moving slowly onto the following edge of the disk. The times given are for mid-event, so at 19.24 the shadow of IV should appear as a dark notch in the limb; the start of this event may be visible at about 19.20 or earlier. At 19.33, Europa will appear half off the disk. The most interesting view should be at about 19.30. A fairly high power in steady seeing, on a good telescope of at least 150mm aperture will show these events clearly. Remember to add one hour for BST.
The complete sequence of events is as follows:
Terry Moseley
6 Collinbridge Drive, Glengormley, Newtownabbey, Co. Antrim, BT36 7SX
From Dr Alan K. Welch
In the report of the 1996 October 30 Ordinary Meeting in the April Journal a diagram illustrates the passage of Comet Hale-Bopp through the inner solar system. Two thoughts spring to mind on studying this diagram.
The first was how near the comet came to the Earth's orbit. On calculating the orbits of the Earth and of Hale-Bopp in a 3D system based on the ecliptic plane, a minimum distance between the orbits of 0.11 AU was obtained. If Hale-Bopp had been four months earlier what an awesome sight it would have made! It would have been nearly 10 times bigger and 100 times brighter. The force on the comet caused by such a close passage to the Earth would only be about 0.025% of the Sun's force hence little change of the orbit would occur.
The second thought was, at what location did Hale-Bopp pass through the ecliptic plane and were any other close encounters possible? Carrying out preliminary estimates indicated that Hale-Bopp crossed the ecliptic plane at about Jupiter's orbit. Repeating the calculation for minimal orbital separation resulted in a maximum distance of just over 0.11 AU early in 1996. This fact may be of some concern to our great-(80 times)-grandchildren. If Jupiter should occupy a spot close to the position of minimal separation then the force exerted on the comet by Jupiter would be over twice that exerted by the Sun, causing massive disruption to the orbit. It is difficult to follow Hale-Bopp through future orbits in order to check for possible close planetary encounters as a change of 1 in the sixth significant figure of the eccentricity will change the return time by about one year. Now that Comet Hale-Bopp has passed, what are the latest orbital parameters, and to what accuracy are these quoted?
Alan K. Welch
63 Churchill Meadow, Ledbury, Herefordshire HR8 2DQ
From Dr R. F. Griffin
It is nice to see spectroscopy (JBAA, 107, 141, 1997) making its way into the Journal. In some respects it may actually be easier than it appears: the determination of wavelengths scarcely warrants the use of a computer, at any rate in the case of the solar spectrum used as an illustration. On the other hand, the identification of the spectral lines, when their wavelengths are ascertained, does merit a degree of circumspection. By way of an example I would like to report my own, rather naive, measurement of the solar spectrum published in the Journal article.
I laid a steel ruler (steel ones tend to be more accurately graduated than wooden ones) on the picture, parallel to the spectral dispersion and, as a concession to modernity, with the edge having metric graduations beside the best-exposed portion of the spectrum, rather below the centreline of the reproduction. I placed the 10cm mark (to use as the zero) as nearly as possible on the rather diffuse feature labelled G, which is dominated by numerous lines arising from the molecule CH (not methane, CH4). I then noted the positions of all the other features that seemed to be visible in the spectrum, interpolating the millimetre scale to tenths as well as possible. The measurements are in the first column of the table. Next I plotted the positions of the few lines that seemed safely identified against the wavelengths of those lines as shown in the second column; I used a scale of 1cm on the graph horizontally= 2mm on the spectrum and 1cm= 100Å vertically. The points fell agreeably close to a straight line, which I proceeded to draw on the graph with the aforementioned steel ruler; the dispersion indicated by the slope of the line was 42.7Å/mm. Then I read back from the graph the wavelengths corresponding to each of the measured lines: they are in the third column of the table. Finally I tried to identify the features that are not in column 2.
It was at that point that some caution was required. Reliable identification of lines in different sources of light, be they terrestrial or astrophysical, really needs to be based on more than just wavelength coincidences. For example, absorption lines of helium can only appear in sources at considerably higher temperatures than the photosphere of the Sun - they are in fact a diagnostic criterion of B-type stars. Ionized helium (He II) requires still hotter sources (O-type stars). So it is not safe to identify as helium an absorption line seen in the solar spectrum. In fact, the identification of lines in the spectrum discussed here is none too straightforward, because at the resolution concerned (perhaps about 10Å) most features are blends, i.e. they consist of two or more lines that appear merged together because they are too close to one another to be seen separately. In the pictured spectrum, the only really definite line that has a single principal origin is the hydrogen line H-beta.
The feature D, due to sodium, actually consists of two lines about 6Å apart; the feature known as the b lines (labelled Eb in the picture) is really three magnesium lines at 5167, 5173 and 5183Å, with an almost equally strong iron line at 5171Å thrown in; and the spectrum is littered throughout with weaker lines of the same elements and many others. Some of the lines that I measured in the spectrum between the b lines and D are very faint, though probably real in the sense that they do correspond to places where there are local excesses of absorption in the solar spectrum. To identify some of them it was necessary to have recourse to an atlas which shows the spectrum in greater detail. It was thereby possible to assign all of the measured features to lines that could be regarded as their respective principal origins, whose wavelengths and identifications are listed in the last two columns of the table below. In most cases, however, other strong lines within a few ångströms of the listed identifications must have contributed to the visibility of the features concerned. Incidentally, the 'skylight filter' used in photographing the spectrum is probably the optical element that was responsible for cutting off the H- and K-line region.
R. F. Griffin
The Observatories, Madingley Road, Cambridge CB3 0HA
From Mr Peter Macdonald
The clock tower at the Houses of Parliament has become the symbol of Britain, and the mellow tones of the bells are known and loved throughout the world, yet the clock was built against a background of controversy - indeed so bitter were the arguments and so long the delays involving construction, the wonder must be that it was ever completed.
The idea for the clock came about in the 1830s when the ancient Palace of Westminster was destroyed by fire and plans for the new Houses of Parliament incorporated a clock on the northern tower. The Astronomer Royal, George Airy, was appointed referee for the design and construction of the clock and a contract was let to Edward Dent who had recently completed a clock at the Royal Exchange. The Astronomer Royal's specification required that the first blow of each hour should be struck within a second of the time, and there was considerable feeling among the clock-making fraternity that such accuracy could not be maintained in a large turret clock where four sets of very heavy hands are subject to all weathers.
That such accurate running was achieved is due largely to the inventive genius of Edmund Beckett Denison, later to become Lord Grimthorpe, whose double three-legged gravity escapement was considered the greatest advance in horological design for many centuries and has since been adopted for most turret clocks. An interesting and comprehensive account of the clock's history and of the many difficulties and disagreements encountered by the various personalities involved has been written by John Darwin,[1] former Resident Engineer at the Palace of Westminster. There has been some speculation over the years regarding the naming of the bell, the most popular explanation being that it was named after Sir Benjamin Hall, the Commissioner of Works. The story relates that the Commons were debating the issue and Sir Benjamin, a man of considerable proportions, was replying on behalf of the government when a back bencher exclaimed, 'Why not call it Big Ben?' and the name stuck. Unfortunately there is no record of this in Hansard.
The bell was cast but never reached the belfry since it cracked while being struck experimentally in New Palace Yard. A new bell was made from the metal of the old and on 1859 May 30 the clock was started with hands on just two of the dials. The remaining hands were soon fitted and in July the clock began striking the hours. The chiming mechanism was brought into use in early September and at the beginning of October the hour bell cracked. Thus the clock was operating in its originally intended form for less than a month. For the next three years the hours were struck on the largest of the quarter bells, after which the hour bell was brought back into use, having been put through a quarter of a turn so that the damaged surface is away from the hammer. It is this crack which gives Big Ben its characteristic tone.
The music of the chimes is taken from Handel's Messiah. It is an extension of a phrase in the aria 'I know that my Redeemer liveth'. Introduced towards the end of the eighteenth century for the clock at St Mary's Church in Cambridge,[2] and known originally as the Cambridge Quarters, the passages have become familiar as the Westminster Chimes. In 1924, daily radio broadcasts of the bells were begun, and with a few interruptions, they have continued ever since. During that time the bells have heralded many events of national importance, announcing both the nation's joy and the nation's grief. When the clock underwent major overhauls in 1934 and 1956, radio duties were taken up by Great Tom of St Paul's. In 1976, metal fatigue almost tore the mechanism apart, and, once again the St Paul's clock proved an admirable deputy on the air. So great was the damage that consideration was given to replacement rather than repair. Fortunately repair was possible and the clock was returned to full working order in time for the Queen's visit to the Palace on the occasion of her Silver Jubilee in 1977.
Recently I was privileged to visit the clock-room and bells. To stand in the belfry at the Palace of Westminster at 12 o'clock must be every horologist's dream. It is well worth climbing the three hundred steps to make that dream come true.
Peter Macdonald
46 Vista Way, Harrow, Middx. HA3 0SL
[1] Darwin J., The Triumphs of Big Ben, Robert Hale, 1986
[2] 'The Song of the Hours', Horological Journal, 999, 378 (1941)
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