J. Brit. Astron. Assoc., 107, 1, 1997, pp. 45–48

Letters

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


Cold climate survival

From Mr David Frydman

Michael Maunder's article 'Cold climate photography' in the December Journal (Vol. 106 no. 6) made interesting reading. My experiences over many years in Finland may be relevant. If in doubt, please follow Michael's good advice.

'Long Johns' are normal wear in Finland for men and women for prolonged exposure below –5°C. At prolonged –20°C, one or two pairs of 'Long Johns' may be worn with warm trousers and outer ski wear as needed. Protect the pelvis. Avoid rough wool against the body.

Use zips rather than buttons. Mittens, maybe over inner gloves; neck and wrists sealed but comfortable. Balaclava or Russian hat. Inner socks inside new Norwegian oiled woollen socks or similar. Fur lined boots with special reflective insoles, or at –30°C, Finnish/Russian long felt boots. If using a long scarf, wrap it twice around the front and tie at the back. It can be lifted to cover the face. Special down (feather) outer anoraks with hoods are expensive but good. There are high-tech cold weather outer trousers and tops. Good insurance would be to choose clothes with a cold country native or with the mountain rescue service.

Don't wash your face or shave on Eclipse day – preserve natural oils. Can you last four hours without toilet facilities? Take a non-mercury thermometer and relative humidity meter. Don't fall asleep during or after the eclipse in case of hypothermia. Watch others carefully.

If going as a bus tourist, for brief exposure (5 to 10 minutes at –25°C in dry still air) the demands are much less rigorous – but still take care. Outdoor workers in Finland have very restricted hours at –25°C. However, children in central Finland regularly play outside for long periods at –30°C and go to school at –42°C. They move a lot. Breathing is through the nose, slowly and not deeply. At minus 42°C outdoor play time is not compulsory!

1970s Nikon, Minolta or Canon manual SLR cameras in good condition usually worked at –30°C. Also Russian cameras and binoculars. I have had one lens crack at –27°C. The camera strap and case became distorted and useless at –34°C. A top quality film broke hopelessly – but a competitor worked fine – so don't rely on one film type. Once when the wind chill reached –50°C I became scared and fled with my camera into the Metro. Arrange warm shelter for use when needed. Ice is not slippery when very cold unless under great pressure. I have stayed out for six hours at –15°C, but would not do so at –25°C even if the air was very dry and still. My Finnish friends have been out photographing the aurora all night at –35°C wearing six pairs of 'trousers'. They are used to extreme weather.

I would not advise anyone not used to the conditions to stay out for four hours at –30°C, especially if humid or windy. You may not survive. One hour might be possible. If you do go to Mongolia, and it is really below –20°C, please be very careful indeed.

David Frydman
6 Berkeley Court, Ravenscroft Avenue, London NW11 8BG


Corrections to the BAA Handbook, 1996 and 1997

From the Editor of the Handbook

(Hyperion, Table 7, Handbook 1996, p.67, and 1997, p.68)

I regret there are serious errors in the columns of distance and position angle (R and P) of Hyperion in the Handbook for 1996 and 1997. The errors were occasioned by a program corrupted by an unknown cause. I am grateful to Mr Carl Mills, of the Salford Astronomical Society, whose observation of Hyperion revealed the errors, and to his colleague, Mr Cliff Meredith, who brought the matter to my attention, insisting that the tables were wrong, in spite of my initial convinced assurance that he must be mistaken. (The correct columns are given in the printed version).

Neville Goodman
Yew Tree House, Hillesley, Wotton-under-Edge, Gloucestershire GL12 7RD


CCD video cameras

From Mr Andrew Elliott

In his otherwise excellent paper on video recording the Moon and planets with a CCD video camera (J. Brit. Astron. Assoc., 106(6), 309), Thomas Dobbins stated that five years ago, impressively specified cameras were only available as research-grade instruments costing $3,000–$10,000.

In fact, a monochrome CCD camera module became available from Philips in 1988 at a cost of £400, although this price dropped by half in the next couple of years. These cameras had slightly better specifications than those quoted by Mr Dobbins, having a usable faceplate sensitivity of 0.02 lux and a horizontal resolution of 450 TV lines.

Following the successful use of these cameras for lunar occultation timing in the UK, amateur astronomers in mainland Europe and America made bulk purchases of them and have been using them successfully for occultations and other astronomical observations ever since. A number of amateurs have also been experimenting with the cameras in conjunction with image intensifiers, which vastly increase their sensitivity and allow video recording of asteroid occultations and meteor showers. The latter is a particularly exciting area of current research.

Interested readers may wish to know that further improved CCD cameras, some in miniature, are currently available from retail security outlets in the UK, as well as from some astronomical equipment suppliers, for as little as £200+VAT.

Andrew Elliott
40 Ryhill Way, Lower Earley, Reading RG6 4AZ


A comet, a bus window and a traffic light

From Mr Nick Martin

On the night of 1996 March 23/24, at about 02.10 UT. I was observing comet Hyakutake and noted an appearance of rapidly flickering bands in the coma, resembling a miniature version of the flickering bands seen in some auroral displays. Fascinated by this observation I discussed it with the Comet Section Director, Jonathan Shanklin, who mentioned that other observers had noted a similar phenomenon. While no cause could be suggested, a professional astronomer had put the idea to Mr Shanklin that this appearance could be related to the near-monochromatic illumination of the coma due to the strong emission from C2 ions which gave the very characteristic blue-green appearance to the coma seen in so many photographs.

Some time later I was sitting by a window of a bus stopped at traffic lights when I observed the red light at a very oblique angle through the bus window. It displayed the banded appearance shown above. At this viewing angle, vertical striations in the bus window cause considerable image unsharpness and I interpreted the bands as representing a moiré pattern formed between the striations in the glass and the (at that time unresolvable) vertical component of the moulded diffuser pattern on the traffic light lens. The pattern was made up of small square elements divided by vertical and horizontal lines. Visibility may have been assisted by the restricted wavelength range of the red light. (The vertical bars found in front of some traffic lights were not present)

Detailed photographs of the comet at the time – especially some processed images – show many fine filamentous structures in the coma. These narrow structures were not visible to the eye at the low illumination levels in telescopic images, but a broader pattern of moiré fringes might be. Such fringes could be formed in an analogous way to the fringes I saw across the traffic light, with convection cells in the atmosphere acting like the striations in the window glass. On this basis, the bands would appear when the air was fairly still so one was observing the comet through a relatively stable pattern of convection cells. The flickering of the bands would represent movements of the cells. This requirement for a temporarily stable convection pattern would also explain the transitory nature of the flickering bands and their infrequent observation.

Other observers have noticed possibly related but different phenomena when observing Hyakutake. It would be interesting and informative if they could send any comments on this letter direct to the Comet Section Director, Jonathan Shanklin, who has kindly offered to collate them. There is also a suggestion that these bands may bear some relationship to the shadow bands observed just before totality in solar eclipses. The current approach of Comet Hale–Bopp may offer the opportunity to check my theory – possibly by observing the comet through narrow band pass filters if it develops much filamentous fine structure.

Nick Martin
Bonnyton House, by Ayr, Scotland KA6 7EW


A comet revisited

From Mr David Keedy

Michael Hendrie's well-written report, 'The two bright comets of 1957' (J. Brit. Astron. Assoc., 106(6), 315), brought back wonderful memories for me of my first observed comet, Arend–Roland C/1956 R1, or as I prefer to call it 1956h. The month was April in 1957. I was thirteen years old, with wonderment in my eyes.

Although my written records of the event are fragmentary and brief, I clearly recall observing Comet Arend–Roland with a 30mm ×12 hand-held terrestrial telescope. Despite the small field of view, the strong central condensation and pronounced tail were all too obvious, a most pleasing sight. At its brightest, I estimated the comet to be about zero magnitude, despite never before seeing such an object except in photographs.

I also had the opportunity to sight the comet in a friend's 75mm diameter refractor, which then for me was a large telescope! The icing on the cake was seeing Patrick Moore's Sky at Night television programme in which Arend–Roland was discussed. The main point of my letter is to thank Michael Hendrie for allowing my thoughts to dwell once again on that exciting time, some four decades ago.

David R. Keedy
14 Mortimer Road, South Shields, Tyne and Wear NE33 4TU


Adjustment of the polar axis of an equatorial mounting

From Dr P. M. R. Hemphill

While perusing old BAA Journals I came across one or two methods of adjusting the polar axis of an equatorially mounted telescope which were highly complicated and mathematical to a degree beyond my ken.

In 1960 the late Dr W. H. Steavenson (1894–1975) very kindly wrote me several letters of sound advice on many aspects of telescope construction, including his method of adjusting the polar axis, which he had used on his 30-inch reflector. His method is simple to understand and is non-mathematical. I do not know if it is generally known to our Membership. In case it is not, these are his instructions verbatim, from a personal communication of 1960 May 23, which may be of interest to some of our readers.

'Azimuth (to be done first). Pick on any star due to pass through or near the zenith in the next half-hour. Put on a fairly high power and pull out the eyepiece until the image enlarges to about the size of Jupiter. Bisect the disc by either the northern or southern edge of the eyepiece diaphragm and clamp in declination. Follow in RA. If the disc drifts north, the upper end of the polar axis is too far west; if south, too far east. Adjust mounting in azimuth by stages until no drift takes place, changing to another star as soon as the first has passed the meridian by half an hour or so.

'Altitude. Choose any star in the north-east, about the same height as Polaris. Proceed as before with bisected disc. If star drifts north, upper end of polar axis is too high; if south, too low. Adjust by stages until no drift occurs. With a star in the north-west the indications are the reverse of these.

'A preliminary approximate adjustment should be made when Polaris is either directly above or directly below the Pole, thus cancelling the effect of the separation of one degree between the latter and Polaris. The true Pole lies nearly on a line between Polaris and Eta Ursae Majoris.'

The above method is a little slow, but gives a more accurate adjustment than could be obtained by the use of circles.

P. M. R. Hemphill
Springhill, Kidmore Lane, Sonning Common, Reading RG4 9SH


More on space and time

From Mr Gerald North

Referring to Dr F. W. Thompson's letter in the December Journal, the answer is that the time dilation effect is not direction dependent. If it were then one might expect the time dilation undergone during an outward journey to be reversed on the return journey – so cancelling out the 'twin paradox'. This is not the case. As Dr Thompson and I both described, time dilation arises because of the physical changes experienced as a result of acceleration forces.

In the situation outlined by Dr Thompson, B stays put while A and C accelerate away from B in opposite directions. A, B, and C were initially of identical ages. After undergoing mirror-image journeys A and C arrive simultaneously back at B. Thus B experiences no acceleration forces while A and C each experience equal (even if opposite) forces at each of the stages of their respective journeys. The magnitude of the time dilation effects will thus be the same for both A and C. As far as B is concerned A and C arrive back at the same age as each other, although they are then younger than B (A and C think they have been away only for a short period, while B thinks they have been away for a long time).

The foregoing is the argument from B's point of view. One can also take the situation as viewed by A or by C. This is more tricky. If we choose A as our viewpoint then we have to remember that we (on A) are experiencing acceleration forces. Despite the distracting complication that C is rapidly departing from us, we must remember that C has also experienced forces of the same magnitude and that C will experience the same distortions of mass, length and time that we, on A, will be subject to. If there could be some magical way of simultaneously communicating with C (there is not – but perhaps we might imagine onboard cameras whose signals are passed between A, B and C by some sort of fantasy 'hyperspace link' in the best Star Trek fashion), we would find that we would not notice the distortions of mass, length and time onboard C, because they are the same as for ourselves. However our view of B would be distorted. One of the changes we would notice is that time onboard B would seem to be running 'in fast forward'.

As far as we, onboard A, are concerned we 'agree' with C and it is B which is the odd one out. At journey's end we remain in parity with C and we both will have 'fast forwarded' into B's future. Notice how we have reached the same conclusion from a different standpoint. That is what I meant about being careful when comparing inertial frames of reference.

Gerald North
9 Camperdown Street, Sidley, Bexhill-on-Sea, East Sussex TN39 5BE

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From Mr John Watson

I hesitate to enter the minefield of the discussion of the Twin (or Clock) 'paradox', but the contentions of Gerald North and Dr F. W. Thompson in the December Journal, that the twins may be distinguished because only one of them has been accelerated, should not be allowed to pass unchallenged. The hypothesis of the occurrence of differential aging is based on the Special Theory of Relativity, which is concerned only with bodies in inertial movement; in this respect the introduction of accelerating forces into the argument might seem in itself misplaced. As E. R. Harrison has stated, 'acceleration by itself is not the cause of the asymmetric aging of the twins. If B takes a longer journey, but with exactly the same acceleration [as before], the difference between the ages of the twins will be even greater... The greater the distance travelled, the greater the difference in aging.'

If the travelling twin B carries a clock, one wonders how the clock 'remembers' that because it was once accelerated, it has been given special status and must now conform to the regulations by running slower than A's clock. However, as thought experiments are permitted here, one can devise an arrangement in which Twin B is accelerated before any measurement is commenced, thereafter attaining a known constant velocity v. Clocks carried by A and B are then synchronised at the moment that B makes a close pass of A, whilst continuing the journey in inertial motion to some distant body located at a known distance from Earth; the time registered on B's clock is transmitted back to Earth in digital form at the moment B makes a close pass of the destination. Twin B has experienced no acceleration during the time of the experiment, yet according to the special theory of relativity, if T is the calculated arrival time, a time discrepancy of T´ = T×sq.rt (1–Beta squared) should be recorded as twin B's world-line diverges from that of twin A.

We now face the dilemma raised by Mr V. Mayes. Special Relativity allows for no states of absolute motion or rest, but only relative motion. Which twin is in motion, or are both in motion? There seems to be no determinate answer, and the hypothesis appears to lapse into absurdity and to be untenable. If this be so, there is no differential aging. Any past accelerations which might have been experienced by either twin are irrelevant. The experiment devised by Dr Thompson appears to reach a similar impasse.

The alleged differential aging of the Twins is a topic much-beloved by writers of popular books on cosmology. As it is now over 90 years since Einstein propounded his Special Theory, could not some authority, in a convincing way, lay this spectre finally to rest?

John Watson
Iddons, Henley's Down, Catsfield, Battle, E. Sussex TN7 9BN


Earthshine during a solar eclipse

From Mr Graham Young

Does anyone know if it is possible to observe and photograph Earthshine on the Moon during a total solar eclipse? Has it ever been tried?

I imagine that during New Moon the full Earth must be a brilliant ball seen from the lunar surface, far brighter than our own full Moon due to the larger apparent size and albedo, and even brighter than the gibbous Earth our lunar observer would see while we see our 'old Moon in the new Moon's arms'. If anyone can help I anticipate trying in August 1999 from Hungary!

Graham Young
37 Polepark Road, Dundee DD1 5QT


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