J. Brit. Astron. Assoc., 109, 1, 1999, p.40-42
J. Brit. Astron. Assoc., 109, 1, 1999, p.40-42
(Note: The Association is not responsible for individual opinions expressed in articles, reviews, letters or reports of any kind.)
From Dr Peter Owen
After reading Martin Ratcliffe's letter in which he used JPL's Ephemeris Generator to calculate that Pluto would return to being the ninth planet in order of distance from the Sun on 1999 February 11 at 14:20 UT (± 5 minutes) I decided to do the same, but using the US Naval Observatory's Multiyear Interactive Computer Almanac (MICA). As the graph shows this gave me a different answer: 1999 February 09 at 11:29 UT (to the nearest minute).
The two-day discrepancy is mainly due to a difference in the calculated distance of Pluto, but there is also a smaller difference for Neptune. As an example, for 1999 February 11 at 14:00 UT the MICA distance for Pluto is 0.00057 AU (85000km) greater than the distance shown on Mr Ratcliffe's graph, and that for Neptune is 0.000075 AU (11000km) less. Although MICA does give a little information about the uncertainties of its calculated positions, there is nothing about geocentric distances, so I do not know whether the differences in the distances are small enough to be explained by these uncertainties.
MICA uses the JPL DE200/LE200 planetary ephemeris (which is also used in The Astronomical Almanac) so I do wonder which ephemeris is used by JPL's Ephemeris Generator. Mr Ratcliffe hoped that his result would avoid future confusion, but my calculation shows that things are not quite so clear as he thought.
Peter Owen
11 The Downs, Blundellsands Road West, Liverpool L23 6XS. [peter.owen@physics.org]
From M. Jean Meeus
Martin Ratcliffe[1] comments that according to Seeds[2] the length of the radius vector of Pluto would be equal to that of Neptune on 1999 March 14. Maybe Seeds found that date on page 1–21 of the first edition of my Astronomical Tables.[3] This 'wrong' date was calculated by me twenty years ago on the basis of the data published in Volume XII of the Astronomical Papers.[4] The numerical integration of the motion of Pluto in this work was based on observational data made not later than 1940, but at the time I wrote my Tables I had no other data for Pluto at my disposal. It is evident that we now possess a much larger and more recent quantity of data.
Later, using the results of a numerical integration performed on the basis of the osculating orbital elements of Pluto as given by Seidelmann et al.,[5] the date 1999 February 10 was found for the instant when the radius vector of Neptune and that of Pluto will be equal.
A few months ago, I received from Prof. A. Vitagliano, of the University of Naples, Italy, his program Solex 3.0 based on the DE406 planetary ephemeris of JPL. From this, I find the following values of the distances of Neptune and Pluto to the centre of the Sun on 1999 February 10, 11 and 12 at 0h Dynamical Time. From these values, we deduce that the 'event' will occur on 1999 February 11 at 09.40 Dynamical Time (09.39 Universal Time).
Feb Neptune distance (AU) Pluto distance (AU) 10 30.13193118 30.13150610 11 30.13189686 30.13177483 12 30.13186253 30.13204364
Jean Meeus
Heuvestraat 31, B-3071 Erps-Kwerps, Belgium [jmeeus@compuserve.com]
[1] - Ratcliffe M., J. Brit. Astron. Assoc., 108(5), 294 (1998)
[2] - Seeds M. A., Horizons: Exploring the Universe, Wadsworth Publishing Co., 1998
[3] - Meeus J., Astronomical Tables of the Sun, Moon, and Planets, Willmann–Bell, Inc., first edition (1983)
[4] - Eckert W. J., Brouwer D., & Clemence G. M., 'Coordinates of the Five Outer Planets 1653–2060', Astronomical Papers, Vol. XII (Washington, 1951)
[5] - Seidelmann P. K. et al., Icarus, 44, 20 (1980)
Tracking down targets by their chemical signature From Mr Maurice Gavin
One of the trials of amateur spectroscopy is the reluctance to remove the spectroscope from the telescope once it is working correctly. This is especially so when the instrument is home-made and built to less than perfect engineering standards. There is a tendency for it to remain on the telescope for many weeks, during which time as many spectra as possible are captured for later analysis. Finding and identifying targets is also problematic even when the telescope is computer controlled. The spectral images, as downloaded to the computer screen from the CCD camera, resemble a series of long parallel streaks rather than singular stellar points – literally a search for a needle in a spectroscopic haystack!
Some stars with emission lines (bright points within the spectrum) or cool M-type stars with a 'serrated' spectrum are most obliging. But what of the outer planets Uranus and Neptune – had they spectra worth recording? In the autumn of 1998 both planets were poorly placed at low southern declination after twilight, with only a small window of opportunity clear of local obstructions in the SSW. The evening chosen was not perfect – a recent weather front had cleared the skies to better than average transparency for suburbia, but low scudding cloud with small breaks filled this zone of the sky. The telescope was slewed to Uranus but no stars could be seen in the finder for cloud had temporarily intervened. Nevertheless an exposure of a few seconds confirmed an unusual 'star' was in the field, unlike any previously recorded. The spectrum had a series of dark bands within it. Next on to Neptune, even lower in the sky a few degrees further west above distant rooftops. Again no stars were visible in the finder but a similar spectrum was captured. During the latter exposure of several minutes, passing holes in cloud acted as an intermittent shutter! A later check confirmed the telescope was on target and that spectra of both Uranus and Neptune had been obtained. They compare well to a reference.[1]
Traditionally Uranus is attributed a visually greenish hue and Neptune bluish, explained by absorption at the red end of the spectrum by methane in the planetary atmospheres. The adjacent low resolution spectrograms taken a few evenings later and covering the visual spectrum from blue (left) to red (right), appear almost identical both in the density and location of absorption bands due mainly to methane.
Maurice Gavin
79 Ardrossan Gardens, Worcester Park, Surrey KT4 7AX. [100772.47@compuserve.com]
[1] - Rudaux L. & de Vaucouleurs G., Larousse Encyclopedia of Astronomy, Batchworth Press, 1959, p. 226
From Mr George Y. Haig
In my note on alternative spectrohelioscope designs in the December Journal (JBAA 108(6), 330) I omitted to mention that since the entrance and exit slits in Figure 2 are shown as being at unequal distances from the oscillating mirror, the solar image will be compressed along one diameter, and will appear elliptical rather than circular. This is most easily avoided by relocating components along the optical path between static reflectors to make the slit distances equal.
No relocation is needed if the alternative method of scanning by vibrating the slit carrier is employed.
G. Y. Haig
35 Dalmahoy Crescent, Bridge of Weir, Renfrewshire PA11 3JB, Scotland
From Mr Wm. J. Williamson, Eng.Tech., A.M.I.E.I.E.
I am researching the history of a supposed anti-gravity device which was the subject of a number of articles in Practical Mechanics magazine between 1942 and 1958. I am particularly interested in any information on W. D. Verschoyle, who wrote the first article, and Swinfen Bramley–Moore, who wrote a booklet, The Apple in the Orchard. Does anyone have a copy of this booklet?
I have also lost my copy of the first article which was published in the magazine in February 1942 and reprinted in August 1958. If anyone can supply a copy, or has any information on the subject would they please write to me at the address below.
W. J. Williamson
Leeskol, North-a-Voe, Yell, Shetland ZE2 9DA
From Mr Roy Panther
The historical note by Richard Baum in the October Journal (108(5), 277) will have interested many observers. The well-known Victorian amateur W. F. Denning in his book Telescopic work for Starlight Evenings writes of his observations: '1889 May 22, 9 a.m. Observed vast numbers of luminous particles floating about contiguous to the Sun's margin. They were clearly carried along by the wind; but this being very slight, their motions were very slow, and now and then many of these became nearly stationary. Their directions were far from uniform, though the general tendency was obviously in a common line of flight. I watched them for some time passing in a plentiful shower. These objects are always noticed in summer-time, and I would believe they would much more frequently attract remark but for the fact that they require a longer focus than the Sun and cannot be recognised when on the disk, to which the observer is usually giving the whole of his attention'.
Denning did not specifically say what they were. He concludes that there is little doubt they are either the pappus of different kinds of seed, or convolutions of gossamer, which have been lifted to great heights in the air, and are rendered bright by reflection from the bordering Sun.
Some years ago I had the same experience when observing near the Sun with a 90mm refractor ×80. The objects consisted of small numbers of apparently self-luminous bodies traversing the field of view of the telescope in the same general direction. Using a pair of 6×30 binoculars near the telescope and just hiding the Sun by a nearby chimney, I could see the same phenomenon.
By focusing down from near-infinity all was revealed. The bodies turned out to be minute flies, being wafted about on the slightest breeze, at a distance of between five to twenty metres. Their brightness was due to the wings of these very small insects reflecting light from the almost in-line Sun. In the telescope these objects, although small points of light, were out of focus, the range being too short for accommodation.
Roy Panther
51 Old Road, Walgrave, Northampton NN6 9QW.
(Editor's note: Readers of this Journal should not need reminding that observing near the Sun directly with any kind of optical instrument is extremely dangerous, and should not be undertaken unless you know exactly what you are doing.).
From Mr Christopher Newman
Having recently calculated the theoretical and practical limiting visual magnitude of my 80mm refractor, using the equations on page 101 of the 1999 BAA Handbook (2 + 5logD for 100% efficiency and 1 + 5logD for 65% efficiency), I am somewhat disturbed to discover the calculated magnitudes, 11.5 and 10.5 respectively, are in fact considerably brighter than my actual limiting magnitude. Given that I am submitting data to the Variable Star Section with reference to stars down to magnitude 12.3, close to my actual limit using averted vision while using an 80mm Vixen APO refractor at ×80 magnification, it is not inconceivable that my results will be looked at with a high degree of scepticism. I do recall reading some time ago that the magnification used also has a bearing on the actual limiting magnitude, though I am unable to locate the source. Given that my skies are never free of light pollution, and living close to the sea and one of the largest chemical and industrial complexes in Europe, I must conclude that from a dark site my actual limiting magnitude will be at least 2.5 times (1 full magnitude) fainter than the theoretical limit when using high powers.
Obviously there must be some leeway in the limits set, given the variability of optics, local conditions, elevation above horizon, eyesight and the accuracy of the magnitude data for any given object etc. Though I have no comparison by which to judge my eyesight, and I do need glasses though not at the eyepiece, I feel there may be a case for investigating this further and possibly, as a result, modifying the equations used in the Handbook to account for the magnification factor of the equipment.
Chris Newman
56 Kirkham Rd, Nunthorpe, Middlesbrough, Cleveland TS7 OHQ.
From the Director of the Instruments & Imaging Section
The formula in the Handbook could well be revised as, looking at it simply, a difference of 100 to 65% efficiency should not produce a whole magnitude difference. I have been doing a little research through my pile of Sky and Telescope magazines and have found two relevant articles which might be of interest.
Bradley E. Schaefer, Nov. 89, pp 522–5: Based on a practical observing test of limiting magnitude of stars in M67 by 250 observers, Schaefer found that only three things really mattered: Aperture, magnification and zenith magnitude. However, for the same conditions (6-inch reflector, same power and sky conditions) he found a scatter of up to 1.5 magnitudes which he could not explain. He produced a computer program to fit the results, which also included the observer's age, and this gives a limiting magnitude of 14.3 for a 6-inch reflector under perfect skies with a magnification of ×150. This corresponds to 3.4 + 5logD. He also found that in skies with a zenith limiting magnitude of 4, compared to a perfect sky of 7, the telescopic limiting magnitude dropped by about two magnitudes, although his program predicts a loss of only around one magnitude.
Roger N. Clark, April 1994, pp 106–8, discusses Steven James O'Meara's feat of detecting Comet Halley when at mag 19.6 with a 24-inch reflector on Mauna Kea. He then refers to results published by H. R. Blackwell in 1946, to construct a probabilistic limiting magnitude functon. He gives the percentage of time that a star of a certain magnitude will be seen for various apertures against a perfect sky. Schaefer's result, above, corresponds to his seeing the star for 70% of the time. Using Clark's formula for an 80mm objective gives a limiting magnitude of 12.2 for detection 98% of the time, 12.7 for 90% and 13.2 for 50% observational detection. These limits correspond to 2.7, 3.2, and 3.7 + 5logD, respectively, for a perfect sky. Compare this with Chris Newman's 12.3 in a poor sky, needing a correction of perhaps one magnitude. This takes Chris to 13.3 and is a fair match to the predicted 50% detection level of Roger Clark (13.2).
Bob Neville
19 Bradden Way, Greens Norton, Towcester, Northampton NN12 8BY. [RJNeville@aol.com]
From the Director of the Computing Section
Mr. Newman's letter will surely be of considerable interest to many members and I would welcome serious discussion in the Journal with a view to either improving the formulae given in the Handbook or removing them altogether. I must admit that the more I read on the subject the more I am inclined to take the latter course.
In any discussion we must be careful to distinguish between the percentage of light through the optical system to the eye and the percentage of light actually entering the eye of a particular observer at a particular time. The latter can vary enormously from one observer to another for several reasons. One major cause of variation is the age of the observer. The writer is saddened by the fact that the dark-adapted pupil of his eye probably had a diameter of 8mm when he was aged 20 but now, over 50 years later, it is only about 3mm!
For further information interested readers should refer to the article 'Vision and the Amateur Astronomer' by K. P. Bowen in Sky and Telescope, 1984 April. The subject is also discussed in the Amateur Astronomer's Handbook by J. B. Sidgwick.
Gordon E. Taylor
20 Badgers Walk, Deanland Wood Park, Golden Cross, Hailsham, East Sussex, BN27 3UT
From the Editor of the Handbook
Mr Newman has rightly drawn attention to the fact that the formulae given in the Handbook for the visual limiting magnitude of a telescope do not tally with actual experience. Though limiting magnitude will depend on a number of factors – sky quality, optical quality of the telescope, observer's eyesight and experience, for example – an approximate guide to the expected performance of a telescope may nevertheless be of some value to Handbook users.
The formula 3.7 + 5logD would appear to yield values closer to those determined empirically by experienced observers. I would be interested in readers' comments on the accuracy of this formula, and the value of giving a formula at all in the Handbook.
Jacqueline Mitton
8a Canterbury Close, Cambridge, CB4 3QQ. [jmitton@dial.pipex.com].
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