Published in the November 1998 issue of Universe
A common thread throughout all of these astronomy publications that are read by amateurs and the general public is the thrill and anticipation when a special event is observed in the sky.
Perhaps your interest is solar eclipses, when the Moon covers the Sun, the temperature drops, the birds fall silent, and precious time is invariably spent cursing at the cloud cover or fixing cameras that don't feel co-operative. I have yet to witness a total solar eclipse, and unless fortune favours me in the NSW Lotteries, you won't see me in Europe for next year's August eclipse. Perhaps the eclipse of December 2002 may see me in South Australia (as part of a Buckley-led, Mencinsky-narrated, grand tour of Australian impact craters?).
What about bright comets? We've been fortunate in recent years to witness two Great Comets. Hyakutake sped past us with little warning, whilst we enjoyed a long buildup for Hale-Bopp. It's going to take something very special to beat that March 1996 morning view of Hyakutake from the Blue Mountains. When I not just glimpsed my first Great Comet, but received an initiation in staring at a magnitude zero comet with a head of perhaps 2 degrees in width and a naked-eye tail of more than 30 degrees in length.
There are other special events that occur in the sky, and the morning of Wednesday 18th November sees the curtain rise for what may be the astronomical talking point of 1998 - the long-awaited peak of the Leonid meteor shower.
The Leonid Meteor Shower
The relationship between comets and meteors first became apparent in the 1860s, with one of the pieces of evidence being the discovery of the linkage between periodic Comet Tempel-Tuttle and the annual Leonid meteor shower. Comet Tempel-Tuttle returned to perihelion in February 1998, and has brought with it the possibility of an intense meteor storm in November 1998 and/or November 1999.
It is regarded by many that November 12-13, 1833, marked not only the discovery of the Leonid meteor shower but also the actual birth or initiation of meteor astronomy. During the evening, it was noted by some astronomers that there were an unusual number of meteors in the sky. It wasn't until the early hours of the morning of the 13th that eastern North America witnessed the Leonid meteor shower at its best as the sky was flooded by meteors.
According to the writer Agnes Clerke, "...a tempest of falling stars broke over the earth...The sky was scored in every direction with shining tracks and illuminated with majestic fireballs. At Boston, the frequency of meteors was estimated to be about half that of flakes of snow in an average snowstorm. Their numbers...were quite beyond counting; but as it waned, a reckoning was attempted, from which it was computed, on the basis of that much-diminished rate, that 240,000 must have been visible during the nine hours they continued to fall."
The meteor storm was certainly remembered by those who saw it. Indeed, one report I read states that most inhabitants of north-east America would have witnessed the phenomena, having been awakened either by the commotion in the streets, or by the glare of fireballs shining into their bedroom windows (an experience that perhaps would not repeatable in today's light-polluted urban environments)! Not understanding what was happening, it would have been a terrifying experience to the uneducated population. In 1878, historian R. Devens listed it as one of the 100 most memorable events in American history.
In 1833, the concept of meteor showers was not one readily considered by the astronomical community. Yet, following the incredible meteor display, a study prompted by the desire to know why the meteor storm occurred, showed that the intense display could have been anticipated. At that time, the true nature of meteors had not even been determined - most thought it was an atmospheric phenomenon. Indeed, following the 1833 display, one American newspaper published a story on how the Sun caused gases to be released from dead plants, and these, hydrogen being the most abundant, became ignited by electricity or phosphorous particles in the air. Another newspaper also stated "The strong southern wind of yesterday may have brought a body of electrified air, which, by the coldness of the morning, was caused to discharge its contents towards the earth".
However, a major key to unlocking the mystery was the fact that many observers reported that the meteors appeared to radiate from a point in the constellation of Leo and, as the constellation moved westward during the night, so did the radiant. It was left to Yale mathematician Denison Olmsted, however, to provide an accurate explanation of the event. He noted that the shower was of short duration, not being seen in Europe or west of Ohio. Olmsted also noted that the meteors seemed to radiate from a particular point in the sky. Finally, considering that a large number of meteors had been observed in Europe, the Urals, Arabia, Mauritius, and by sailors in the North Atlantic on November 12, 1832, he theorised that the meteors had originated from a cloud of particles in space, with the radiant point simply being an effect of perspective.
Olmsted was not just an armchair theorist. He had witnessed the 1833 display and wrote "Imagine a constant succession of fireballs, resembling rockets, radiating in all directions from a point in the heavens".
Further research revealed that a display had been seen on November 12, 1799. The Prussian scientist and explorer Alexander von Humboldt, at the time observing in Venezuela, described the scene as "there was no part of the sky so large as twice the Moon's diameter not filled each instant by meteors". An observer in Florida that night recorded that the meteors were "at any one instant as numerous as the stars".
Humboldt also wrote that he had learnt from South American natives that a similar "rain of stars" had been seen in 1766. In 1837, Heinrich Olbers combined all of the available data at that time and concluded that the Leonids possessed a period of 33 - 34 years.
In 1863 and 1864, Yale professor Hubert Newton traced accounts of the Leonids over a period of more than 1000 years. He found reports of strong displays of the Leonids in the years 585, 902, 931, 934, 1002, 1202, 1366, 1533, 1582, 1602, and 1698. These few dates were sufficient to indicate a periodicity of around 33 years, and in 1866 it was proposed that a dense cloud orbited the Sun with a period of 33.25 years, and Newton predicted that the next return would occur on November 13-14, 1866.
An independent discovery was made on January 6, 1866, by Horace Tuttle, observing from Harvard College Observatory, Massachusetts, in the United States. As a result of the two individual discoveries, the comet was named Tempel-Tuttle. Perihelion occurred on January 12, 1866, around which time it reached its peak brightness of 5th magnitude. It then faded quite rapidly and was not sighted again after February 9, at which time it was observed to be fainter than 10th magnitude.
In 1837, after the comet had already been determined to have an elliptical orbit, Theodore von Oppolzer further refined the orbital calculations for Comet Tempel-Tuttle and derived a period of 33.17 years. Meanwhile, using observations from the 1866 Leonid shower, an accurate orbit for the Leonids was computed, and a number of people independently noted a striking resemblance between Comet Tempel-Tuttle and the Leonids.
This was not the first identification of the source of a meteor shower. That achievement came in 1866 when the Italian astronomer Giovanni Schiaparelli established that the orbit of the Perseid meteor shower matched that of periodic Comet Swift-Tuttle. The linking of the Leonids to Comet Tempel-Tuttle, however, helped astronomers to visualise the process of an orbiting stream of meteoroids and how it formed.
Today, we think of a meteor stream forming as a comet nears the Sun and sheds debris. Over time, this material spreads out along the orbit of the comet. The Leonids are special since, like their parent comet, they orbit the Sun in a direction virtually opposite to that taken by Earth, the other planets, and most comets. Consequently, when Earth collides with the Leonid meteor stream, it does so at a very high speed, which in turns leads to brighter and faster meteors.
1869 saw one last notable display, with observed rates in the order of 200 per hour. Following this, rates returned to the normal rate of 10 to 15 meteors per hour. Astronomers confidently predicted that the next Leonid meteor storm would occur in 1899, and expectations were further heightened when observers in 1898 detected hourly rates in the order of 50 - 100.
However, hardly anything happened in 1899. Observers noted a maximum hourly rate of 40 on November 14, but nothing that compared to what astronomers, and indeed the public, were expecting. American meteor expert Charles Olivier was to later write, "this was the worst blow ever suffered by astronomy in the eyes of the public". Commentators today would place such items as Comet Kohoutek's dismal performance in 1974, and the optical problems surrounding the Hubble Space Telescope following its launch in 1990, in the same category, if not higher. Can you think of any other monumental astronomical bloopers?
What went wrong with the Leonids? Some astronomers had issued a few cautions in advance. In particular, calculations by John Adams and George Stoney revealed that the meteoroid stream passed quite close to Saturn in 1870 and to Jupiter in 1898, close enough that it may have shifted the orbit. Indeed, by 1899, the orbit had been shifted nearly 0.02 AU closer to the Sun than the Earth at the expected meeting point, and thus the stream's distance from Earth was double that of the 1866 display. It is worth noting here that the intensity of a Leonid meteor storm is far from predictable, simply because it is not known just how dense pockets of the meteor stream will be prior to their encounter with Earth.
By November 1900, interest in the Leonids had diminished considerably, which was most unfortunate as those who kept watching in anticipation experienced rates of more than 1000 per hour on November 15-16, causing a small community near Hudson Bay, Canada, to panic. The following year, observers in California reported a storm of almost 2000 per hour while observers in Arizona and Mexico described the meteors as "too thick to count".
The next predicted return of the Leonids in the 1930s also disappointed astronomers, though perhaps not as much as at the turn of the century, as expectations were now somewhat more modest. Increased activity began in 1928, when maximum hourly rates of 50 were observed. The following year, a peak of only 30 per hour was reached, but this display was affected somewhat by moonlight. 1930 saw the Leonids pick up somewhat, with hourly rates of 130 to 190 per hour accompanying "many brilliant meteors with long enduring trains".
The Leonid meteor storm was expected in 1932, but it was known that moonlight would interfere with calculations of the intensity of the display. Even taking this into account, the rates were not comparable to the heady days of the 1830s and 1860s. The Leonids peaked on November 17, 1932 at an hourly rate of around 240.
Furthermore, the Leonid's parent comet, Tempel-Tuttle, was not sighted, nor had it been seen in 1899. There was some speculation that Comet Tempel-Tuttle, unseen since its discovery apparition in 1865-1866, had crumbled away, just leaving a train of debris to add to the meteor stream. With no active comet remaining to replenish the meteor stream, some felt that the Leonids would begin to decline from their extraordinary levels of previous centuries, and that the decline had already begun.
In the years that followed, the level of activity for the Leonids remained greater than normal up until 1939, which when one considers it began in 1928, was twice the periods of enhanced activity during 1831-1836, 1865-1869, and 1898-1903.
The displays in 1962 and 1963 returned to slightly above normal, with hourly rates in the order of 15 to 20, while rates picked up to 30 per hour in 1964. In 1965, observers in Hawaii and South Australia witnessed an excellent enhancement in activity. The Hawaiian observers saw the hourly rates for the Leonids increase from 20 to around 120 within the space of 2 hours on 16th November and as Australia turned towards the radiant, 38 Leonids were reported with an average brightness of magnitude -3! Some of the Leonids that were observed from Woomera in South Australia were reported to be as bright as magnitude -5.
Astronomers were still pessimistic about the expected meteor rates for the Leonid maximum in November 1966. Given the less than expected displays in 1899 and 1932, astronomers believed that the meteor stream's orbit had been perturbed such that a close encounter with Earth was no more. All they were willing to state was that the maximum rates would probably be greater than 100 per hour.
On the night of November 16/17, Milon and a dozen other amateurs were observing under the skies of Kitt Peak in Arizona. They began their meteor observations at 2:30am local time and counted 33 Leonids during the next hour. They resumed their watch at 3:50am and reported 192 Leonids during the next hour. During the initial stages of this meteor watch, they had been keeping records of their magnitude estimates, but this soon gave way as the numbers of meteors overwhelmed them. Around 5:10 am, they were seeing 30 meteors every minute, with rates climbing to several hundred by 5:30am and an estimated peak of 40 per second at 5:54am! This corresponded to a maximum rate of 144,000 per hour, one of the greatest displays in history.
Milon later stated "The meteors were so intense that we were guessing how many could be seen in a one-second sweep of the observer's head". Charles Capen, observing in southern California, wrote "We saw a rain of meteors turning into a hail of meteors and finally a storm of meteors too numerous to count".
Predicting just when the Leonids will peak is very difficult, since no one knows exactly how and where the debris is distributed within the stream. Furthermore, following the 1966 storm, Canadian meteor expert Peter Millman used radar data to determine that the thickness of the Leonid stream was only 35,000 kilometres - the Earth moved through the thickest part of the Leonid stream in just one hour!
As Comet Tempel-Tuttle has repeated its passage through the Solar System, it has spread its debris along its orbital path. This wide scatter of debris throughout the Solar System accounts for the normal Leonid meteor seen each year in November. However, the densest part of the meteor stream remains within a few hundred million kilometres of Comet Tempel-Tuttle, which Earth moves through repeatedly during the course of a few years.
In 1981, Donald Yeomans of the Jet Propulsion Laboratory in the United States published a study entitled "Comet Tempel-Tuttle and the Leonid Meteors". Yeomans analysed historical data of the Leonid displays during the years 902 to 1969 in an attempt to map the distribution of debris in the meteor stream. His study indicates that our prospects are excellent. Intense meteor displays appear to be possible for a few years either side of comet Tempel-Tuttle's perihelion if the comet passes less than 0.025 AU inside or 0.010 AU outside Earth's orbit. During the recent apparition of Tempel-Tuttle earlier this year, when the comet reached 8th magnitude primarily for Northern Hemisphere observers, its orbit passed just 0.0080 AU inside Earth's orbit - though the comet wasn't at that point at that time!
According to Yeomans, if the peak of the Leonids arrives at the instant when Earth passes through the comet's orbital plane, then the 1998 peak will occur at 19:43 UT on 17th November (05:43am EAST on 18th November - don't forget to add an hour for daylight saving). Of course, at this time, the Sun would have already been above the horizon for New South Wales observers, so we would only see the buildup to a peak before the ever-brightening dawn overtook even the best of the Leonid fireballs. Western Australia observers would be in a better position, though as discussed below, the best rates are predicted for northern Asia.
However, other studies differ. Canadian astronomers have modelled the Leonid displays as recorded over the past two centuries. Their data accounts for the poor showings in 1899 and 1933, as well as the storm of 1966, and their results show that the Leonid meteoroids seem to be concentrated at the point which Earth reaches some 2 hours 40 minutes before crossing the actual orbital plane. For 1998, this corresponds to a peak rate occurring at 17:02 UT on 17th November (03:02am EAST on 18th November, when astronomical twilight is just about to begin - much, much better!
It should also be noted that the Leonid meteor showers of 1996 and 1997 showed peaks about 2 hours before the crossing of the orbital plane.
In other words, observers can expect a very good showing, but not a storm of historic proportions - still, wouldn't it be nice to be surprised.
One study has assumed a peak zenithal hourly rate of 10,000 meteors per hour, though this would only be for a very short time (less than an hour) and centred over China and Mongolia. The study notes that this rate could be out by a factor of 10 - either way!
What do we mean by a zenithal hourly rate? It is the number of meteors that would be seen in an hour if stars of magnitude 6.5 could be seen, and the radiant of the meteor shower was straight above - at the zenith. In practice, how often do you observe under such conditions? Indeed, the Leonids is a northern sky radiant, so our numbers will be downgraded considerably.
Based on the above rate, and assuming that the peak occurs at the time of the orbital plane crossing, then New South Wales observers might expect a peak rate of about 300 per hour, while in Western Australia, the number could be a factor of 10 greater. Of course, if the shower peaks 2 hours earlier, as it has in 1996 and in 1997, then New South Wales could be in for a treat.
This is an observing opportunity not to be missed. The predictions for 1999 favour western Asia and Europe, and American observers in 2000 will have to contend with a morning Moon. If you're thinking that there's always next time in another 33 years, think again. In 2029, Jupiter's gravity will tug the Leonid meteor stream away from Earth, and it will not be until 2098 or 2131 (neatly sandwiched between two very bright returns of comets Swift-Tuttle and Halley) that observers will be able to expect a Leonid shower of storm proportions.
Whilst meteor observing is best done under dark skies, there should be enough bright meteors to warrant a watch from most suburban skies. Position yourself away from streetlights and trees to give yourself an open sky. However, if you can, drive to a darker sky, or better still, arrange a holiday, watch the Leonids, and don't forget to attend the Society's New Moon observing weekend at Ilford a few days later, when we can hopefully compare grand tales.
Remember - the morning of Wednesday, 18th November. Set the alarm clock. Wake up early. Be prepared to spend some time under the morning sky (take the opportunity to spot Mars which will be just outside the border of Leo).
The Leonids do not come with a money-back guarantee. You could be clouded out. You could sleep through the alarm clock. The predicted rates could simply fail to materialise.
Back in the 1960s, Sky & Telescope magazine wrote, "Any prediction as to what the Leonids will do in a given year cannot be much more than an intelligent guess". In all likelihood, however, this should be the best meteor shower you will ever observe, and it will cap off an incredible comet-oriented decade of astronomy.