The 1970's was a decade of considerable change and growth in Australian comet observing. At the beginning of the decade, there were few Australian amateurs who were actively studying comets. Yet by the end of the 1970's, Australia not only had one of the world's most successful comet hunters, but also an active, nationally organized group of amateur observers, providing a wealth of detailed, high-quality observations of southern comets. What follows is a look at the comets of this era through the eyes of one who participated in this growth.
The first comet of this decade was actually discovered in October 1969, when a 10th magnitude object was discovered by three Japanese observers. Moving rapidly southwards and brightening, it soon became evident that southern hemisphere observers would be in for a real treat. Preliminary calculations indicated a perihelion distance of about 0.5AU in late December, and a closest approach to the Earth in late January of about 0.4AU. Although it would generally remain within about 45 degrees of the sun, the predicted maximum of about 3.0 promised that comet Tago-Sato-Kosaka would be an interesting sight.
Being in the days B.T. (before telescope) my own observations had to wait until December 10 when the comet was a difficult object low in the evening twilight, at about magnitude 4.5. By December 15, the comet had brightened to magnitude 3.7, and a tail 1 degree long was visible to the naked eye. By the beginning of 1970, comet Tago-Sato-Kosaka was an impressive sight fairly high in the evening sky. It was around magnitude 3 and had a naked eye tail 7 degrees long. As January wore on, the comet began to move rapidly northwards and fade. By late January the comet was low to the north-west horizon at magnitude 4.5, still sporting a 3 degree tail to the naked eye.
The wait for the next comet was not very long at all. Indeed in a circular updating information on comet Tago-Sato-Kosaka, mention was made of an 8th magnitude discovery by J. Bennett of South Africa. I observed this comet twice in early January with binoculars as a small, circular, and somewhat uninteresting patch of light, and then forgot about it until early March, by which time it was a magnificent object in the pre-dawn skies.
By March 12, the comet was shining at about magnitude 1 and displayed 2 naked eye tails each about 6 degrees long. On March 15, the dust tail was 12 degrees long to the naked eye, while the comet was magnitude 0.5. Unfortunately, after this date the moon became a hindrance. At this time I had managed to build myself an astrocamera, based around a 50mm f/2.5 slide projector lens, that used cut pieces of 120 roll film. My first photograph with this instrument was of comet Bennett on March 14.
Two scenes concerning this comet are forever imprinted on my memory. The first is of watching the comet rising tail first over the rooftops across the road on March 28. The slightly curved tail was reminiscent of a searchlight.
The other memory is of waking up at 2.30am on March 23, to observe the comet, only to find the sky hidden by a dense fog. After a drive of 40km to pick up a friend, there followed another drive of almost 50km until we found a place where the fog thinned sufficiently for us to observe and photograph the comet, all the while trying to shield our eyes from the headlights of passing cars. Also of clambering in and out of the car boot to reload the camera with sheets of film. Finally the fun of trying to follow the comet with the naked eye until sunrise, only to lose it less than 5 minutes before the sun rose.
An interesting feature of these two bright comets was that their paths crossed in the constellation of Grus; the two comets being about 6 weeks apart. A number of observers displayed photographs of the two comets superimposed. Later still a third bright comet would pass over this same region.
Without a doubt, comet Bennett was one of the great comets of the 20th century, and certainly was the best comet for southern hemisphere observers in the 1970's.
At the time that comet Bennett was putting on its display, there came a report of another comet that could possibly become very bright. However comet Daido-Fujikawa fizzled out and was quickly forgotten.
After comet Bennett passed into the northern sky and was lost to southern observers, there was a break of a few weeks that gave observers a chance to watch a transit of Mercury before yet another naked eye comet was discovered. A member of the Krutz group of sungrazers, comet White-Ortiz-Bolelli burst out from behind the sun as a magnitude 2 object with a long tail. However like many sungrazers, this glory was short-lived, with a rapid falling off in brightness. Two weeks after discovery, the comet was a difficult naked-eye object in the evening twilight at about magnitude 5.5, with a naked eye tail about 8 degrees long.
My best view of the comet was on May 28. Watching the comet set over the Indian Ocean, it appeared as a very ill-defined head of magnitude 4.5, followed by an 18 degree long tail that was very narrow, but split into two some 10 degrees from the coma.
Following hard on the heels of White-Ortiz-Bolelli, was yet another naked eye comet, Abe. Although best viewed from the northern hemisphere, I managed to observe comet Abe with the naked eye in September, low to the north at magnitude 5.5 with a 0.5 degree tail. Shortly after the comet was lost to southern observers. However during December, the comet reappeared from behind the sun in the morning sky and was again visible to southern observers, many of whom had forgotten about it. There was more that one observer who, for a short time at least, thought they discovered a comet! Finally the proud owner of a telescope, I was able to observe the comet during February when it was a faint 10th magnitude object with a central condensation and a short tail.
The first of these was yet another Japanese discovery, comet Suzuki-Sato-Seki. Discovered on October 19, the comet was about magnitude 7 and sported a 0.5 degree tail. However the comet faded rapidly and was never a naked eye object. The other comet was the periodic comet, Encke. This was briefly visible as a magnitude 5 object with a 0.5 degree tail. However due to strong twilight it was not visible to the naked eye.
Having had 6 comets bright enough to be visible in binoculars within 12 months, observers could have been excused for expecting a break. No so! Hardly had comet Abe faded from view when another Japanese discovery, comet Toba, put in an appearance. Although never a spectacular naked eye event, comet Toba was non-the-less a very interesting object that remained visible in binoculars for over 2 months as it approached the Earth following perihelion. Its path across the sky was especially favourable to southern hemisphere observers as the comet rapidly moved southwards, passing within a few degrees of the south celestial pole.
My first observation of this comet was in the pre-dawn skies of April 29, 1971, when it appeared as a small, nebulous patch of light about magnitude 8.5. During May, the comet brightened and grew steadily larger as it approached the Earth. A short, faint tail was also observed. During early June the comet reached a peak brightness of 7.6 and the coma grew very large, reaching 18 arc minutes in diameter on June 7. This large size made the visibility of the comet very dependent on the viewing conditions and the equipment used. After passing the Earth, comet Toba faded rapidly, and was a large, faint, and diffuse object of magnitude 9.2, still with a short, faint tail, at the end of June.
Following comet Toba, there was a wait of almost 12 months until the next moderately bright comet. (Why is it that with comets it always seems to be either a flood or a drought???) Comet Bradfield 1972f, was the first discovered by Bill Bradfield of South Australia. Never a prominent object, it peaked at about magnitude 8.5 in late April 1972, and had fallen below magnitude 9.5 by the middle of May. The comet drought then continued throughout the rest of 1972 and into 1973. It was hoped that comet Kojima, 1972i, would become visible in a small telescope around Christmas, however the comet never exceeded 11th magnitude.
The long drought was finally broken in April 1973, when it was announced that a comet discovered the previous month was expected to become very bright. This was the famous (or infamous) comet Kohoutek, a comet which many have described as the greatest astronomical non event of the century! A most unwarranted description. Discovered on March 7, 1973, at a distance of around 4AU, some early predictions indicated that around the time of perihelion (December 28) the comet would be exceptionally bright. These predictions caught the attention of the media, who then cave considerable coverage to stories of a brilliant "supercomet" that would be visible around Christmastime. Unfortunately, the media did not give the same coverage to more realistic predictions, even when it was clear that the comet would not live up to early expectations. As a result, many people were very disappointed and a great opportunity for positive publicity for astronomy was lost. However, a scientific disappointment the comet definitly was not, as the comet's early detection enabled astronomers to plan detailed observations from both the ground and from Earth orbit. As a result, a tremendous amount of information was gathered that added significantly to our understanding of these most intriguing objects.
From an amateur viewpoint, comet Kohotek presented a fine sight, especially since it had been over 2 years since the last naked eye comet. My observations commenced on October 26, when it was a small, well condensed object of about magnitude 8.0. By October 30 a short tail was visible.
As the comet approached perihelion it brightened steadily and moved further south. On December 5, the comet was an easy naked eye object with a 2 degree naked eye tail, despite the glow of twilight.
My last pre-perihelion observation was on December 23, when it was a difficult object of magnitude 2.8, buried deep in the morning twilight. Other observers were successful in following the comet right through perihelion passage.
Following perihelion, the comet appeared in the evening sky as a 3rd magnitude object with 2.5 degrees of tail visible to the naked eye, despite the twilight.
Before leaving 1973, mention should be made of an unexpected event that occurred during the middle of the year. Observers following the periodic comet Tuttle -Giacobini-Kresak reported that between May 20 and June 3, and again on July 7, the comet experienced outbursts which increased the brightness by a remarkable 10 magnitudes, and it briefly became a naked eye object. Unfortunately, due to poor communications, (ie no WWW!!!) few southern hemisphere observers heard about these events until after they were long past, and the comet quickly returned to its normal 14th magnitude.
Barely had comet Kohotek disappeared into the northern skies than Bill Bradfield reported his second discovery. Already at 9th magnitude when discovered, the comet brightened rapidly as it approached perihelion. however observations were hampered as the comet remained within 30 degrees of the sun. Southern observers were further disadvantaged by the comet's northwards movement.
By the end of February, comet Bradfield was a 6th magnitude object with a short tail, despite a bright moon. When the moon vacated the scene the comet was an easy naked eye object with a 3 degree tail. The best view was obtained on March 15, when the comet appeared as a small, well condensed coma of 4th magnitude, with a narrow ion tail streaming up from the horizon for about 3.5 degrees.
About 1 degree back from the coma, a broad dust tail curved away.
After mid March the comet moved rapidly towards the north western horizon, and by early April the comet was lost to southern hemisphere observers.
Following this activity there was another lull and it was not until late November 1974, that the next comet, Bennett 1974h, appeared. Heading rapidly south, Bennett's third discovery was expected to become visible to the naked eye for a brief period. However it was subsequently found to have been unusually bright at discovery, and the comet faded rapidly, becoming large and diffuse and difficult to observe. Possibly the comet totally vapourized.
Before moving into 1975, there are two periodic comets that deserve mention, Honda-Mrkos-Padusakova and Boethin. Althtough both comets reached 10th magnitude, very few amateur observations were reported. This was a curious phenomenon of the 1970's. In more recent times, amateurs have followed periodic comets to below 12th magnitude consistantly, yet in the 1970's periodic comets seemed to be ignored by amateurs unless they became unusually bright. Although I did not realise it at the time, comet Honda-Mrkos-Padusakova would become a very special comet for me in the future.
In March 1975, Bill Bradfield struck again, with a 9th magnitude object becoming his third discovery. However it was never a brilliant sight, remaining small and diffuse, and fainter than magnitude 8.5.
The next bright comet was discovered in July 1975, by three observers, Kobayashi, Berga, and Milon. Already at 7th magnitude and sporting a 1 degree tail when discovered, the comet brightened rapidly as it approached perihelion. Unfortunately for southern observers, the comet was also rapidly moving northwards. During August, the comet peaked at around magnitude 4 with a faint ion tail, photographs of which showed it to be up to 13 degrees in length. Following perihelion the comet again became accessible to southern observers in late September as a 6th magnitude object with a short, faint tail. Although fading rapidly, the comet's small well condensed coma rendered the comet readily visible in binoculars until the end of October, by which time it had faded to 9th magnitude.
Of greater interest to southern observers were the next two comets, both discovered on the same night, October 5. The first of these was comet Mori-Sato-Fujikawa, never became brighter than 8th magnitude. However it became a south circumpolar object for over 6 months and remained brighter than 10th magnitude until mid March. Unfortunately, most observers (myself included) forgot about this comet with the arrival of a rather showy individual to be discussed shortly. It remained a rather small, diffuse object with no tail being visible in binoculars. The second comet discovered that night was Suzuki-Sato-Mori. Although discovered in Ursa Major, the comet moved swiftly southwards as it passed the Earth, becoming accessible to southern hemisphere observers following conjunction with the sun on October 30. My first observation of the comet was on November 2, when it appeared as a large, diffuse, magnitude 4.4 object, with no tail visible.
Moving rapidly away from the sun and southwards, the comet was an easy naked eye object 2 nights later even though it had faded by 0.5 magnitude. This rapid fading continued as the comet receded from both the Earth and the sun, and this coupled with the comet's large, diffuse appearance, made it a very difficult object by mid month.
The flood of comets continued when Bill Bradfield discovered his fourth comet on November 12. Unfortunately poor communications once again prevented many observers from seeing this comet, as when news of the discovery finally filtered through (by mail from the northern hemisphere!) the comet was too close to the sun for observation. After perihelion the comet was only visible from the northern hemisphere. Originally expected to become a very bright object for northern observers, comet Bradfield faded quickly after perihelion, although it did briefly reach 3rd magnitude.
In the same circular announcing the discovery of comet Bradfield was the notification of yet another new comet. This one also promised to be very bright. This comet, the famous comet West, 1975n, vies with comet Bennett 1969i as the best comet of the 1970's.
A more detailed description is here.Comet West 1973n
While this spectacular show was happening, Bill Bradfield discovered two more comets. The first (1976a) was an 8th magnitude object rapidly moving northwards. The second comet (1976d) was a diffuse 9th magnitude object. Already past perihelion, it faded quickly. Once again, poor communications resulted in many observers being unable to follow these comets.
Following this hectic burst of activity, comet observers had a chance to catch their breath until the next naked eye comet which was visible in the middle of the year. During August 1976, the periodic comet d'Arrest made its closest ever approach to the Earth. As a result the comet became much brighter than usual, reaching a peak of magnitude 5.5 and remaining visible to the naked eye for about 5 weeks during late August and September. A more detailed description is here.Comet 6P d'Arrest
After 8 bright comets in just over 12 months, there followed a very lean period lasting another 12 months, until the discovery of comet Kohler in late 1977 broke the drought. In between however, there was one interesting periodic comet. This was comet Grigg-Skjellerup. The 1977 return was the first after the comet had been perturbed into a new orbit. It was also a return which saw the comet make a comparatively close approach to the Earth, with a minimum distance of only 0.18AU. As a result, the comet briefly reached magnitude 10 during mid April as it moved rapidly northwards across the sky.
Comet Kohlor was around 10th magnitude at discovery, the comet brightened steadily as it approached perihelion. Unfortunately it never came closer to the Earth than 1AU. Even so, it was faintly visible to the naked eye for a short period at magnitude 6.5. The coma was quite large and diffuse, and several observers reported a faint ion tail. After peaking in brightness in late October, the comet faded slowly as it drifted south-eastwards, falling below magnitude 10 by late January 1978.
A few weeks after comet Kohler had faded from view, Bill Bradfield discovered yet another bright comet. Still approaching perihelion, the comet brightened rapidly. Unfortunately it was also rapidly moving northwards and towards the sun. Even so, southern hemisphere observers were definitely favoured. The comet was faintly visible to the naked eye during late February, and by early march was developing a strong tail. The tail was 3 degrees long on March 8, but the waning moon drowned out the best views of the comet.
Once again only a few weeks were needed before the next comet discovery was made, this time by R. Meier of Canada. This comet is probably best remembered as an "if only" comet! At magnitude 10 when discovered nothing seemed unusual. However it was quickly realised that the comet was over 3AU from both the Earth and the sun. Perihelion would not occur for another 6 months when the comet would be about 1AU from the sun. Therefore there was a possibility of a very striking display later in the year, and one such prediction was made. Alas however, this prediction contained an error in the position of the comet. In actual fact calculations showed that the observing conditions could not have been worse. When the comet was at perihelion the comet and the Earth were on opposite sides of the sun! Even so, the comet reached magnitude 4, although for southern hemisphere observers it was never well placed when bright, always appearing close to the sun. If only the comet had reached perihelion 6 months earlier or later!
During September 1978, 2 more comets were discovered. The first, comet Haneda-Campos, subsequently proved to be a new periodic comet with a period of just under 6 years. Typical of many short-period comets, it was never a prominent object, remaining small and featureless, and reaching only magnitude 9.5. Similarly the second comet, comet Macholtz, also remained small and featureless, and below magnitude 10.
While these two comets were still visible, the Australian comet observer D. Seargent discovered a comet. At magnitude 5.5, comet Seargent was easily visible to the unaided eye, and displayed a small, well condensed coma and a 2 degree tail. Already past perihelion, the comet faded rapidly, falling below magnitude 10 by mid November.
A week after Seargent's discovery, Bill Bradfield was at it again with his fourth discovery in 6 weeks! Once again however, the comet was past perihelion and faded rapidly.
Once again a burst of comet activity was followed by a long spell when none were visible. The drought was broken by Bill Bradfield (again!) when he spied a 10th magnitude comet in Hydra. Unfortunately for southern hemisphere observers the comet, although still brightening, was moving northwards rapidly. This northwards motion, combined with a spell of poor weather, allowed only a very few observations.
Just after comet Bradfield faded from view, Meier discovered his second comet. However this comet remained too far north to be satisfactorily observed from the southern hemisphere, and it remained below magnitude 11.
The final comet of the 1970's was discovered on December 24, yet again by Bill Bradfield. Already at magnitude 5, the comet continued to brighten as it passed close to the Earth on its outbound journey.
A more detailed description is here. Comet Bradfield 1979l
Thus the 1970's ended as they began, with a bright naked eye comet. During the 10 years a total of 35 comets became visible in amateur instruments, 25 of which became bright enough to observe in binoculars and 17 with the naked eye. Looking back, it hardly seems possible that there were so many comets visible. Where did we find the time to observe all the other objects in the sky?