MIRA 11
October 1985


Halley's Comet Edition


This famous photograph of Halley's comet was taken in 1910, during the comet's last appearance in our skies.  As you may have heard, Halley will again be visible to us, as it passes round the Sun on its third return since the one predicted by Edmond Halley.




The History of Halley's Comet

I am sure that many of you know the story of how Edmond Halley's 'discovery' of the periodicity of the comet which now beers his name, however it will be recounted here for those who may not.
In 1682 a comet was discovered.  When Edmond Halley (1656 — 1742) calculated its orbit he noticed that it bore a strong resemblance to the orbits of the comets of 1531 and 1607.  It had just been shown that comets followed orbits around the sun in accordance with Newton's theory of gravitation, but it was thought that these orbits were parabolic (i.e. the comets comes in from outside the solar system, passes the Sun and then leaves the solar system, never to be seen again), rather than elliptical, where the comet travels round and round the Sun in a closed orbit.  Observational accuracy at the time was not sufficient to decide between these two alterntives.  However, Halley, with his knowledge of the similarity of the orbits of the three comets, concluded that they were in fact the same object.  As the returns of this comet were about 76 years apart he predicted that the comet travelled in an elliptical orbit of period 76 years, and would return in 1758.
On Christmas day 1758 the comet was rediscovered by German amateur astronomer Johann Georg Pelitzsch.  Thus the first periodic comet had been found, though Halley did not live to see it.
Since 1758 Halley's comet returned in 1835 and then again in 1910, (when the photograph was taken).  I recently received the following letter from someone who saw Halley‘s comet when it was last in our skies.

Dear Sir,
I saw Halley's comet in 1910.  I was 1O years old.  I am now 85 years old.  It was at Rochester in Kent, where I then lived.  I remember being rather afraid, as we were told that if the tail touched the Earth it would be the end of us all.
Yours faithfully,
JM Cox

It is interesting to hear that a fear of comets was present only 76 years ago.  In fact, in 1910 the Earth passed through the tail of Halley's comet, and a number of people made improbable amounts of money selling everything from hard-hats to comet pills to protect
against the harmful effects that may have been caused by the tail of the comet.
Appearances of Halley's comet have been traced beck as far as 87 BC and perhaps the best known return of the comet coincided with the battle of Hastings, and its image is recorded on the Bayeux Tapestry.  (William the Conqueror told his soldiers that the comet was u good omen and thus persuaded them to cross the Channel to fight the English).  It is even thought that Halley's comet may have been the Star of Bethlehem.
The 1985/6 return will not be very spectacular.  In fact, it Will be about the least favourable return possible.  As amateur astronomers, we must try to explain to.the general public (friends and relatives) that Halley's comet will not be a brilliant object, without sounding so pessimistic that they will be put off even looking for it.  We should also be prepared to answer any questions concerning the general nature of comets.  This is particularly important because comets are much misunderstood objects, and are the subject of a great deal of superstition.  The cartoon below comes from an issue of the magazine ‘Punch’ in 1910 and shows the type of misapprehension the public is under.


Although Halley's comet will be a rather disappointing object visually, this return will be very important because there are a number of spaceprobes being sent to the comet to discover as much as possible about the nature and origin of comets.  I expect these missions will raise more questions than they answer, but it will be interesting to see the results.  The probes are listed below.

PROBE       FLYBY DATE         DISTANCE

Giotto        13 March 1986      500km
Vega—1     6 March 1986       10000km
Vego~2     9 March 1986       3000 - 100000km
Planet-A    8 March 1986       100000km




COMET GIACOBINI-ZINNER (1934e)

By Rob Moseley

While everyone eagerly awaits the return of Cornet Halley we have been able to watch an interesting preview of the show during the Summer months with the apparition of Comet Giacobini-Zinner.
This cornet was first discovered by M Giacobini in 1900 and rediscovered by E Zinner in 1913.  Because the period of this comet's orbit is about 6.5 years, good apparitions occur every 15 years.  The last well observed return was in 1972.  G-Z was recovered last April at at a distance of 4.2 AU from the Sun.
The comet has been very well placed in the sky, moving along the Milky Way through Cygnus, Cassiopeia and Perseus - keeping very close to the Galactic Equator.
Up to the time of writing (Sept 1st) it has been a small, but steadily brightening object, quite easily found even among the congested star-fields of this part of the sky.
Towards the end of July I noted signs of a tail structure developing (see Fig 1) - though this is necessarily exaggerated in the drawing.  The comeL was rated as 9th magnitude at this time.
By August 20th two ghostly tails could be seen (see Fig, 2), although the coma was still quite small, with a star-like central condensation.  By this date it had brightened to mag.8.
This impression was confirmed by an observation I made on Aug 26th using the 18" reflector at Conder Brow Observatory, Lancaster.  Though dogged by clouds all night a successful 2 minute exposure on Royal X Pan was made at 03.31 UT.  The resulting astromtric measures taken from the plate were good enough to forward to America - where they may be used to help in last minute course corrections for the ICE probe (the detached US Lunar orbiter), which will pass through the tail of G-Z on Sept 11th, six days after the comet reaches perihelion (its closest point to the Sun) on Sept 5th.
Giacobini-Zinner will continue to be visible in small telescopes right through the Autumn months, moving along the winter Milky Way through Auriga, Orion and Monoceros - though it is doomed to be well and truly upstaged by another "hairy star"!

The drawings of P/Comet Giacobini-Zinner (figs 1 & 2)


1985 July 23   00.15UT                                     1985 Aug 20   01.55UT
10" Spec  x120                                                10" Spec  x120
The comet passes a 9th mag star                       Very close approach to a 11th mag star
in Cassiopeia




WATCH OUT!
THERE'S A COMET ABOUT . . .

By Richard Barrett

We are now on the eve of one of the most important periods ever for our Society, and for the popularisation of astronomy in general with Comet Halley growing steadily brighter both in the sky and in the popular imagination as the weeks pass by.
Although this time round the cornet is very badly placed for us in the UK, let's keep our fingers crossed for some fine weather during November and December when it will be comfortably visible in binoculars and small telescopes.  It won't be a spectacular naked eye object even in early December, when it will be best placed in the SW after sunset.  But, having said that, miracles are possible.  The infamous fade-out of Comet Kohoutek in 1973 could be counterbalanced by a glorious outburst of comet Halley in 1985.  Remember, we still know very little about what makes comets tick.  A lot has been inferred about the structure of cometary nuclei but nothing observed.  Hopefully the space probes will be able to provide some real facts.
As media attention grows we will all be expected to become comet "exports" by family and friends, so now is the time to start boning up on a few basic facts in order to satisfy newly aroused curiosity in the sky.
"How far away is it?"  "Will it flash across the sky?"   "When can I see it;'"   "What's the difference between a planet and a comet?". . .
These are a few of the questions I've already been asked.  Some are intelligent, some are crass, some are funny. . . some are downright brilliant!  So be prepared.
The Coventry & Warwickshire Astronomical Society will be opening the College Observatory to the public from 7pm to Midnight during the weekday evenings of National Astronomy Week (the best views will be from 10pm onward.)  Even if the weather is cloudy people will be able to view an exhibition as well as visiting the observatory and telescope.  This will give us a chance to gather new members.  In the meantime we can all start the publicity effort by informing as many people as possible by word of mouth. There will be wider publicity nearer the time via local press and radio.
However Halley performs as amateur astronomers we must take this once-in-a-lifetime opportunity to get out there and observe this most famous of comets for ourselves.  All the drawings, photos and written notes we can muster can then be incorporated into another commemorative issue of MIRA in the New Year.  Good Observing!





Planetary Notes

By Rob Moseley

NORTH TROPICAL DISTURBANCE ON JUPITER

On July 13th 1985 WE Fox, the veteran Jupiter observer and Director of the BAA Jupiter section, issued an alert through the Astronomer magazine concerning an unusual "pink glow" he had detected in the N latitudes of the planet

This event has lifted British Jupiter observers out of the doldrums of the past few years caused by the low altitude of the planet as it passes through the Southern constellations of the zodiac.

Given reasonable seeing conditions the reddish coloration centred on the NTrZ can be seen even in a small  telescope.

The following notes represent an abstract from my own observing log, and the drawing a typical view of Jupiter during this apparition (note the Great Red Spot moving onto the disc).

Key to nomenclature:

NPR . . . North Polar Region
NTZ . . . North Temperate Zone
NTB . . . North Temperate Belt
NTrZ. . . North Tropical Zone
NEB . . . North Equatorial Belt
W2. . . . System II longitude


                    DATE . . . 1985  July 19    UT 00.20
                    LONGITUDE of CM SYSTEM I . . 49.9°
                    LONGITUDE of CM SYSTEM II  . 348.8°
                    INSTRUMENT . . 10" Spec  x240 (Tolles)
                    SEEING . . IV   Transp.  Fair
                    OBSERVER . R Moseley


(In the course of the limited number of observations I was able to make during the 1984 apparition due to the extreme S declination of the planet noted the NTrZ as "very pale" on several occasions.)

1985 July 3    00.00 - 00.30UT   W2 102° (mean)    Seeing IV
I noted that all the latitudes N of the NTB appeared uniformly dull, but no particular colour was seen.

1985 July 1O   01.30 - 03.20UT   W2 24.3° - 276.9°    Seeing III/IV
Again, "the S half of the disc generally paler than the N". . . which was almost featureless.  No unusual colour.

1985 July 19   00.00 - 00.45UT   W2 348.8° (mean)   Seeing IV
NTZ / NTrZ  "the whole region was dusky grey, especially when compared to the equivalent S hemisphere latitudes."
NTB   "diffuse and pale, but warm in tone"
NEB   ". . . narrow, but on intense reddish brown"
(N.B. At this time I was still unaware of Fox's alert.)

1985 Aug 19   23.00 - 23.45UT   W2 82.2° (mean)     Seeing III
My first cursory glance into the eyepiece revealed a most unusual aspect to the N latitudes of the disc.
NPR   "very obscure, merging with the temperate zones in a dull creamy yellow tone."
NTZ   "dull, becoming warmer in tone to S."
NTB   "thin but definite, placed well S and sometimes giving the impression of being a N component of the NEB."
NTrZ  "this suffused by a tawny red tint, visible across the face of the disc, and involving the NTB."
NEB   "appreciably darker than the SEB, with an intense darkness on the N edge around the CM with suspicions of projections onto the NTrZ."

1985 Aug 29   21.10 - 23.39 UT   W2 60.4° (mean)    Seeing III/IV
The NTrZ was again seen to be suffused with a tawny hue, which intensified towards the dark edge of the NEB.  The NTB was very obscure, hardly visible.  The NPR and NTZ both appeared a dull, uniform coppery grey.  The reddish coloration centred on the NTrZ appeared to lessen towards W2 135° - but this may have been due to colour fatigue in my eye.

In all cases the instrument used was a 10 inch Newtonian employing eyepieces of Orthoscopic and Tolles design, yielding powers of 120 and 240 respectively.

Summary
The poor summer weather and indifferent seeing conditions have combined to make my personal efforts rather sketchy.  But they do indicate that prior to July 10th no unusual colouring was present in N latitudes between W2 100° - 200°, although the whole N hemisphere appeared somewhat dull and uniform.
By mid—August the reddish coloration of the zenogrophical latitudes centred on the NTrZ (ie appx. +22°) had extended to at least W2 100° from Fox's quoted longs. of 300° - 350° in his original alert of July 13th.
More recent studies (up to Sept. 23th) confirm that the "Fox Effect" has not spread beyond about 100° eastward.