J. Brit. Astron. Assoc., 109, 3, 1999, p.155-156
J. Brit. Astron. Assoc., 109, 3, 1999, p.155-156
(Note: The Association is not responsible for individual opinions expressed in articles, reviews, letters or reports of any kind.)
From Mr Maurice Gavin
The adjacent schematic drawing, which is self explanatory, may be of interest to members considering a permanent pier[1] for a Meade LX200 telescope working in equatorial mode. The pier occupies the minimum of floor space and is ideally suited to small enclosures. Most steel fabricators should be able to manufacture it at considerably less cost than the optional Meade tripod and equatorial wedge. Its weight will be substantial.
The upper section comprises a hollow section steel column of the required height cut at the top to an angle equal to the observer's latitude. Steel plates are welded to the top and bottom of the column with a third loose plate to receive the telescope. Slotted bolt holes give general adjustment. Final adjustment in altitude and azimuth is provided by the four corner bolts working in adjacent pairs. These are raised or lowered against the overhang weight of the telescope. The separation and pitch of these bolts (in my case 20cm and 1mm respectively) sets the level of adjustment in arc-minutes when a spanner is applied to the nuts. 5cm diameter holes in the top and bottom of the steel column allow a filler material (like dry sand) to be added later if column vibration is found to be a problem.
The lower concrete pier is easily formed in concrete blockwork with the centre core filled with concrete around a steel reinforcement rod. Bolts to receive the steel baseplate can be cast into the concrete cap using a plywood template. To allow for the overhang of a 30cm Meade LX200 SCT at mid-northern latitudes the pier centre should be located about 35cm south of the observatory centre line.
Maurice Gavin
Worcester Park Observatory, Surrey. [100772.47@compuserve.com]
[1] - Moore P. (Ed.), Small Astronomical Observatories, Springer-Verlag, 1996, p.57
From Mr David R Keedy
I must congratulate Michael Hendrie on his excellent report about Comet Bennett 1969i. In the report, J. Brit. Astron. Assoc., 109(1), 14, he says he is often asked how the bright comets Arend-Roland, Bennett, West, Hyakutake and Hale-Bopp compared. Indeed, this question is problematic because of the differing situations in which these comets appeared, such as the darker skies when Arend-Roland ruled the roost, the bright sky background and low elevation of West, the clouded-out period affecting Hyakutake, the antisocial predawn appearance of Bennett, and the light pollution affecting Hale-Bopp. The brightness of a comet, length of tail and other factors have also to be considered when making visual comparisons.
We cannot compare them under similar conditions and our recollections may fade. Nevertheless, I make my own personal, non-scientific comparison. I would say West was the brightest, Hale-Bopp the most enduring and Bennett the most awe-inspiring. In fact, on 1970 April 2, at 0300 hours UT, I clearly recall gasping at the appearance and beauty of binocular and naked-eye views of Bennett, with its long, grand, substantial, curving tail. A proper comet! (The biting cold wind on that morning also made me gasp, for different reasons.) Arend-Roland and Hyakutake, on the other hand, were not so impressive to me, good though they were.
I would most certainly welcome the recollections of other observers, either to my address below or via the Journal.
David R. Keedy
South Shields, Tyne & Wear, NE33 4TU
From the Director of the BAA Jupiter Section
In the October Journal (vol. 108, no.5, p.279), I described images from the Galileo space probe of the subjovian region on Io, and asked whether some white patches there might have been drifting clouds, especially some that were seen in the first image from the G1 encounter but in no other image.
However, some new images have recently been released from the E6 encounter. These include an image taken with exactly the same lighting and viewing angle as that first G1 image, and the distribution of white patches appears identical. Therefore, these patches are fixed features on the surface and not clouds. However, their extreme dependence on viewing conditions is most peculiar and this region is still of great interest.
John H. Rogers
Linton, Cambridge CB1 6UF. [jr@mole.bio.cam.ac.uk]
From Mr J. S. H. Battison
Since one is unlikely to be lucky enough to be located exactly on the centre-line of the eclipse, it would be useful to know the actual duration of the total phase for a given distance from the centre-line.
The booklet published by the Stationery Office in 1996 called A Guide to the 1999 Total Eclipse of the Sun by Steve Bell of the Royal Greenwich Observatory (in collaboration with the US Naval Observatory) indicates that for southwest England, the duration of the total phase on the centre-line will be about 126 seconds and that the path width of the eclipse over land will be about 108km. This conflicts slightly with figures issued by Fred Espenak of NASA, which are of the order of 122s and 104km. Steve Bell recently stated that whilst the centre-line positions are the same, more significance should be placed on the NASA times and track widths.
In 1995, I converted the centre-line coordinates (longitude and latitude) issued by both NASA and the USNO, to Ordnance Survey grid references, and fitted an average straight line to the very slightly curved line (accurate to about 0.2km in the region of Cornwall and Devon). For the purposes of this letter, I have assumed that the coordinates of the centre-line track have not changed. For those who are interested, the formula is:
It is a matter of mathematics to derive the equation for the length of a perpendicular from a straight line to a point. Substituting [1] from above, the following formula is obtained:
As an example, the full OSGR of Newton Ferrers in south Devon is E_o = 255km and N_o = 48km. Using [2] above, d = 25.7km. Using the following formula which may be derived from the geometry of eclipses:
J. S. H. Battison
Albury Road, Guildford, Surrey GU1 2BS
From the Director of the Comet Section
The first known observations of noctilucent clouds are generally associated with the eruption of Krakatoa in 1883. Since then observations have gradually become more common, and it is assumed that either the clouds did not exist prior to this, or that they were very rare.
Another possible explanation is that they were simply not recorded. I have been reading Fire in the Sky by Olson & Pasachoff,[1] which includes two renditions in colour of Coggia's comet made by Charles Piazzi Smyth from Edinburgh in July 1874. Both drawings show what appear to be blue-white clouds on the northern horizon, with dark tropospheric clouds below and silhouetted against them. The drawings were for midnight on July 12 and 10.30pm on July 15. The moon was then near new and the Sun between 9 and 12 degrees below the horizon.
The clouds are shown occurring up to a little above the comet, which was then between 11 and 19 degrees above the horizon and near the meridian. These circumstances are consistent with the clouds being noctilucent and suggest that they were perhaps visible more commonly than is supposed.
Jonathan Shanklin
City Road, Cambridge CB1 1DP [jds@ast.cam.ac.uk]
[1] Olson R. J. M. & Pasachoff J. M.,Fire in the Sky: Comets and Meteors ... in British Art and Science, Cambridge University Press, 1998
From M. Jean Meeus
In 1998 David Tholen and Robert Whiteley announced the discovery of a new class of asteroids. As mentioned in the February Journal (109(1), 5), they said that the object 1998 DK36 orbits wholly within the orbit of the Earth.
However, this object was observed on only two successive days, namely on 1998 February 23 and 24. There are no other observations of 1998 DK36, and with only two positions no orbital elements can be calculated (except if a circular orbit is assumed). So it was premature to announce the 'discovery' of that new class of asteroids. Indeed, Gareth V. Williams, of the Minor Planet Center, let me know that '1998 DK36 may have an orbit entirely inside the Earth's, but it is not certain that this is the case.'
Jean Meeus
Heuvestraat 31, B-3071 Erps-Kwerps, Belgium
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