J. Brit. Astron. Assoc., 110, 2, 2000, p.93

Letters

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

A cautionary tale

From Mr Michael Maunder

Reading the British Medical Journal is not one of my regular hobbies, but astronomy leapt out of the contents list - 'Uranus attacks! Herschel's legacy'. That eye catching title and what followed make fascinating reading, and reminded me vividly of the message the late Steven James gave in his superb series of lectures to the BAA. The gist of this was the part defective eyesight can play in astronomy, and how the brain interprets what we think we see. 'See' what I mean from the following transcript:

The past few years have seen a resurgence of news stories and films with a 'danger from outer space' theme. These ranged from last year's films Independence Day and Mars Attacks! to this year's spate of movies showing the earth threatened by approaching comets, and culminating in reports that the asteroid Toutatis was entering our solar system and threatening to do for us what the last one did for the dinosaurs. Fortunately it missed by three million miles, a close shave in astronomical terms.

Towards the end of the 18th century, the astronomer William Herschel also provoked consternation by observing that a large comet was heading in our direction. He confirmed this with a series of measurements over subsequent weeks showing it increasing in size, from about two seconds of arc on 17 March 1781, to three seconds on 19 March, to four seconds a few days after that and so on. Clearly something was on its way, but, unlike the asteroid earlier this year, it never arrived.

In Herschel's case this was because he had actually discovered the planet Uranus, and it was not approaching the earth, but moving away. Yet Herschel's measurements clearly show the size increasing, and instrument error could not explain the systematic increase in size, night after night. Historian Norris Hetherington has suggested a simple answer: Herschel's strong prior beliefs had overridden his natural objectivity, and his measurements reflected this. Despite careful calculations Herschel simply succeeded in supporting his prior hypothesis that the blob in his eyepiece was a comet. Herschel's legacy therefore - apart from discovering the planet Uranus - is a reminder that humans are inherently biased in recording or measuring events, even those as objective as the movement of planets. We are easily led astray by our natural inclinations to see what we want. Naturally this failing does not apply just to astronomers: Schulz and colleagues have pointed out that controlled studies that are not double blinded tend to overestimate effect sizes by 17%.2 Although this is perhaps not as earth shattering as reporting that a comet is heading towards the earth, it is a useful reminder that as scientists we just cannot help kidding ourselves.

[Mark Pettigrew, Research fellow, York]

I have noted my arms do get shorter as anno domini progresses and the written page needs larger print. Nothing new there, of course, but my recent trip to Bali to view the Mercury transit brought home to me how eyesight can and does play funny tricks. Orion was at the zenith, and not at all easy to observe being much 'smaller' in common with the familiar full Moon phenomenon. The rising Moon at the horizon appears huge, apparently shrinking when overhead in winter. In the crystal clear tropical skies I could not make out M42 at all, just the bright spot of the Trapezium, something I've rarely spotted before with the naked eye. I knew the fuzzy patch should be visible, but saw no evidence, and began to have doubts concerning my eyesight. However, that appeared to be OK as the Milky Way was brilliantly obvious and I had no problem seeing M41 in Canis Major. Averted vision, all the tricks, yet M42 had vanished. Back to the UK and the M42 fuzz became obvious again.

Without my years of experience of observing Orion, I could easily have been misled into thinking I'd discovered a new comet or something, a phenomenon akin to Pettigrew's note to BMJ. It must be something to do with the object's altitude, compared with the much more normal viewing situation. At neck-cricking altitudes, the patch could appear to shrink like a full Moon into a small patch, just like my bright spot, a reverse effect in explanation of Herschel, perhaps. Sky clarity and similar factors must come into the equation, but preconceptions had to be part of it. Had I lived a long time in the tropics, perhaps never seeing the extended patch, I'd be unaware of the large size seen from these shores.

My second and real concern has to be in the almost universal use of computer enhancement of astronomy observations. It seems that the observer can tweak the observation until the effect they like or think is right comes out of the printer. Herschel would have loved that. The media certainly do. Publishing only bright and cheerful nebulae and the like creates an atmosphere of expectation, which the novice observer will seek to emulate. We all know that the eye only sees green well under dark adaptation. Generate any coloured or contrast effect you like and nobody can check the originals because you simply won't be able to in the future. Maybe that blemish you spend ages removing is the thing historians need in generations to come. The original recording media will have gone to 'Great Floppy in the Sky' (or the the Science Museum if you're lucky) in the way of 51/4 inch floppies etc. Do you have the wherewithal to read those disks on which much of the pioneering work was done, that short time ago?

Has anyone out there got their own 'worrying' observations like my vanishing M42? Or do my comments explain the spate of false alarms overloading the nova and comet patrols, which, in turn, have a common origin in media hype?

Michael Maunder
Woking, Surrey GU22 8QY

'The Star of Bethlehem: an astronomer's view'

From Mr John Farquharson

Further to Dr Mitton's review, a couple of 'old chestnuts' have been perpetuated by Dr Mark Kidger in this enjoyable book.

Firstly, Dionysius Exiguus, the Scythian monk who lived and worked in Rome during the sixth century AD and was responsible for computing the Christian chronology, did not omit in error the year zero between 1 BC and AD 1 - the very concept of 'zero' was unknown in Dionysius' time. (Zero did not exist in Roman notation.) Zero was a development of the Arabic (more correctly the Indo-Arabic) system of notation which probably originated in India around the third century BC. By the first century AD the figures 1 to 9 had been devised. Zero, however, did not appear until the ninth century AD and the earliest record of its use was in AD 876 at Gwalior, India. AD 1 is not the first year after Christ's birth year: it is the first year of His life. The meaning of 'anno Domini' is quite explicit: 'in the year of the Lord'. It is generally accepted that the true date of the Nativity should be placed prior to 4 BC (the death of Herod), probably between the years 8 and 6 BC.4

The second 'chestnut' is that King Herod the Great died in 4 BC, between the lunar eclipse visible from Jericho (March 12/13) and the Feast of the Passover (April 10/11).5 This matter was fully argued in a previous Letter,6 but briefly, this misconception is probably due to the reference by Josephus to acts of sedition (against Herod's successor, his son Archelaus) by the people during the Passover. It therefore appears that Herod had died before the Passover of 4 BC April 10/11, but I quote a footnote: 'his passover, when the sedition here mentioned was moved against Archelaus, was not one, but thirteen months after the eclipse of the moon, already mentioned.' Although Herod undoubtedly died sometime during 4 BC it is by no means certain that his death occurred between the lunar eclipse and the Passover of that year.

All in all it makes a fascinating study, and I am sure that the story of the Christmas Star will continue to run...

John Farquharson
Cathcart, Glasgow G44 5TY.

Was 'Vulcan' a comet?

From the Director of the Comet Section

Richard Baum and William Sheehan in their book In Search of Planet Vulcan (Plenum Press, 1997) recount the story of the 1878 total solar eclipse when Professor James Watson and Lewis Swift saw, or thought they saw, one or more ruddy objects showing a disk close to the totally eclipsed sun. At the time they were searching for the planet Vulcan, and it soon transpired that the observations were not particularly consistent, and could not be reconciled with an intra-Mercurial planet. With the publication of Einstein's theory of general relativity the need for Vulcan to reconcile the dynamics of the solar system disappeared and the observations were consigned to the scrapheap. Baum & Sheehan note that the Solar Maximum Mission and SOLWIND satellites recorded several small sungrazing comets, and that these provide an appealing explanation of the observations, though we will never know for certain.

Could this explanation be correct? Since the book was published the SOHO satellite has been very successful in discovering many small members of the Kreutz group of sungrazing comets. Contrary to expectation quite a few of these do not show tails and appear as small disk-like objects, which at a first glance appear almost stellar. Although SOHO once recorded two sungrazers at the same time it is unlikely that four would be visible, so it seems probable that not all of Watson and Swift's objects were comets. Watson was however most certain about the one closest to the Sun and the position and magnitude of this object are compatible with that of a small Kreutz sungrazer some five hours from perihelion. At this point in its orbit the comet would be moving at around 30" an hour, which could explain some of the apparent discrepancy in positions which were measured about five minutes apart. Overall it seems probable that this 'Vulcan' was in fact a comet.

Careful search at future eclipses may reveal a similar object, and although SOHO could catch it first the images are not always viewed in real time, leaving the field open to the assiduous observer.

Jonathan Shanklin
Cambridge CB1 1DP. [jds@ast.cam.ac.uk]