Francis Stephenson#
SUMMARY OF PREVIOUS AND CURRENT RESEARCH#
In my research career, spanning some 40 years, I have largely
concentrated on the application of early astronomical records
(especially from the pre-telescopic period) to the investigation of
problems in modern astronomy and geophysics. Despite the low
precision of early observations, the lengthy time-span which they
cover affords several advantages over more recent data. Examples
include the study of very rare events (e.g. galactic supernovae) or
long-term trends which cannot be adequately discerned over the short
period covered by accurate modern observations (e.g. in Earth's
rotation). It seems fair to state that I am widely regarded the
leading world authority on this subject, which has become known as
"Applied Historical Astronomy". I have contributed extensively to its
growth from a somewhat arcane field of study to an important
scientific discipline.
My research interests centre round the following areas of Applied
Historical Astronomy: (i) Earth's rotation in the past; (ii)
historical supernovae; (iii) the past orbit of Halley's Comet; (iv)
solar variability. Additionally, I have done much research on (v)
oriental star maps and also (vi) the accuracy and reliability of
pre-telescopic observations.
I have published extensively on the above topics in books, research
papers, conference proceedings and also popular articles - see list of
publications below.
Most of the numerous early observations which I have analysed
originate from only four cultures: Babylon, China (including Japan and
Korea), Europe and the Arab dominions. An important feature of my
research is the systematic consultation of historical documents in
their original language, either personally (as in the case of Chinese
and Latin texts) or with the help of close colleagues (for Babylonian
and Arab material). In my view, such a procedure is essential if an
objective assessment is to be made of early astronomical records.
I have a sound command of Latin and a good working knowledge of
Classical Chinese. In particular, I am in regular touch with
colleagues in China, both South Korea and North Korea, and also Japan.
I have hosted several Royal Society K.C. Wong research fellows from
China. I have made two visits to China under the auspices of the Royal
Society, four visits to South Korea, and two to Japan. I have
acquired some knowledge of Late Babylonian cuneiform - such as found
in astronomical texts - but here I am able to rely extensively on the
expertise of colleagues: especially Professor H. Hunger of the
University of Vienna and Dr C.B.F. Walker of the British Museum. I
have encouraged Dr S.S. Said, formerly of King Saud University,
Riyadh, Saudi Arabia and other colleagues possessing an expertise in
Classical Arabic, to translate and investigate a wide variety of
Arabic texts dealing with eclipses, comets, etc.
A summary of both my past and current research investigations is as
follows.
Earth's rotation in the past
Fluctuations in the rate of rotation of the Earth have been detected
on time-scales from a fraction of a year to millennia. These are
produced by a variety of mechanisms, both tidal and non-tidal. My
principal researches in this subject have been made in collaboration
with Dr L.V. Morrison and major papers were published in 1984 and 1995
(in addition to many other publications). In the first of these papers
(Phil. Trans. Roy. Soc. A, 313 (1984), 47-70) we charted
the decade fluctuations (usually attributed to core-mantle coupling)
in hitherto unrivalled detail since A.D. 1620, mainly using
occultation observations, and evaluated the necessary torques (between
10$^{17}$ and 10$^{18}$ Nm). Our results have inspired other authors
to investigate correlations with the westward drift of the Earth's
magnetic field. In the same paper we also used ancient and medieval
observations (since 700 B.C.) of both solar and lunar eclipses to
demonstrate the existence of marked non-tidal variations in the length
of the day on a time-scale of millennia, a previously unexpected
result.
In our more recent paper - published in (Phil. Trans. Roy. Soc.
A, 313 351 (1995), 165-202) - Dr Morrison and I considerably
refined our previous results on the millennial time-scale,
incorporating many hitherto unused ancient and medieval eclipse data.
We deduced that the mean length of the day has increased by 1.7
milliseconds per century (ms/cy) in the last 2700 years (compared with
the expected figure due to tidal friction of 2.3 ms/cy) and also
demonstrated the existence of non-tidal oscillations (of amplitude
some 5 ms and periodicity 1500 years). We showed that our result for
the mean non-tidal rate of decrease in the length of the day (i.e. 0.6
ms/cy) is in close accord with current estimates of the effect of
post-glacial rebound. In particular, our values for the Earth's
rotational clock error and length of the day from ancient times to the
modern period are tabulated annually in the Astronomical
Almanac.
Much of my research on Earth's past rotation was undertaken while I
was at JPL, Pasadena, during the winters of 1989-90 and 1990-91,
having been awarded a one-year Senior Resident Research Associateship
by the National Research Council, Washington, D.C. My extensive book
entitled "Historical Eclipses and Earth's Rotation" considerably
expanded our previous publications on this subject. This book,
published by Cambridge University Press in 1997, has received several
excellent reviews and is widely regarded as a standard work on the
subject.
More recently (since about 2005) I have extended my research to
include the following issues: (a) analysis of Babylonian timings of
first contacts for both solar and lunar eclipses; (b) the
investigation of observations - many previously unused - of solar
and lunar eclipses made between AD 300 and 800; and (c) the study of
possible archaic references to large solar eclipses (prior to about
700 B.C.). In particular, the last of these extensive investigations
has revealed that before about 700 B.C. no solar eclipse observations
are of sufficient reliability to provide useful information on
variations in the Earth's rate of rotation. Scientifically, this is an
inconvenient but unavoidable conclusion.
Historical supernovae
Early observations of "new stars" - giving details of position, changing
brightness, etc. - are of particular significance in astrophysics as no
outburst of a galactic supernova has been definitely recorded since
the invention of the telescope. Although about 200 supernova remnants
(SNRs) have been discovered at radio, x-ray and other wavelengths, the
ages of most of these can only be crudely estimated. A confident
association between a carefully observed temporary star and a SNR thus
yields an exact age for the latter and this provides a valuable
calibration for theoretical models of the supernova process.
When I commenced my researches in this topic around 1970, only three
historical supernovae (appearing in A.D.1054, 1572 and 1604) had been
definitely recognised and their remnants identified. Working first on
my own and later with Dr D.H. Clark I made many careful historical and
astronomical investigations of temporary stars recorded in the
pre-telescopic era. Emphasis was placed on those objects which were
unusually bright, visible for many months and in low galactic latitude
- all characteristics of supernovae. As a result we were able to
affirm direct associations between two further supernovae (appearing
in A.D. 1006 and 1181) and SNRs of acceptable age. We thus
considerably enlarged the sample of SNRs whose precise age was
established. The results of this research have since found general
acceptance. Our joint book (The Historical Supernovae, Pergamon
Press, Oxford, 1977) soon became a standard text on historical
supernovae, and has so remained.
In 1998, I began a new phase of research into historical supernovae
with Dr D.A. Green (Cambridge University) and our resulting book
entitled Historical Supernovae and their Remnants, was published
by Oxford University Press in 2002. In this work, we discuss
historical supernovae in detail, both historical observations and the
present day radio and X-ray remnants. In particular, we have
investigated hitherto unknown texts - from both East Asia and the
Arab lands - and also developed a variety of techniques for better
understanding of the terminology in the early sources. We conclude
that over the past millennium, only five galactic supernovae have been
definitely recorded - in AD 1006, 1054, 1181, 1572 and 1604. In the
previous millennium, possible supernovae ocurred in AD 185, 386 and
393, but in each case there is an element of doubt in the
interpretation of the records. We emphasise that apart from these
eight objects no other temporary star recorded in history deserves
consideration as a potential supernova.
In a paper on this subject ("A reappraisal of some proposed historical
supernovae"), published in 2005, we devised a series of selection
criteria to use in the investigation of further records of possible
supernovae. In this paper we tested these criteria on several further
proposed supernovae which had been discussed in recent years and found
that in no case were there valid grounds.
Subsequently, in 2009, David Green and I made a detailed study of
some 50 early records of "guest stars" recorded in East Asian history. This
was entitled "A catalogue of `guest stars' recorded in East Asian
history from earliest times to A.D. 1600".
The past orbit of Halley's Comet
The apparition of Halley's Comet in 1985/6 provided ample incentive
for research into its past history. My interest was largely inspired
by a translation of East Asian records of Halley's Comet going back to
earliest times which Dr K.K.C. Yau and I undertook in 1985 at the
request of the editor of {\it{Journal of the British Interplanetary
Society}. This demonstrated to me that despite the careful work of Dr
D.K. Yeomans of JPL (along with Professor T. Kiang of Dunsink
Observatory, Republic of Ireland) the more ancient history of Halley's
Comet (i.e. before 12 B.C.) was most uncertain. As a result, I
requested Professor H. Hunger of Vienna University to make a search of
selected Late Babylonian astronomical diaries in the British Museum.
This led to our discovery (with Dr Yau) of Babylonian observations of
Halley's Comet in both 87 and 164 B.C. (Nature, 314 (1985), 587-592). This discovery of the earliest reliable sightings of
Halley's Comet (there is a brief Chinese record in 240 B.C.) attracted
widespread attention. As a result of these our researches, every
apparition of Comet Halley since 240 B.C. has at least one
identifiable observation.
In these same studies, various orbital solutions based on numerical
integration were tested. Of these, the calculations by D.K. Yeomans
and T. Kiang were shown to agree remarkably well with observation. I
have since shown that owing to the paucity of good cometary records
before 240 B.C., no reliable sightings of Halley's Comet at this
archaic period can be found. However, the fact that the motion of
Halley's Comet can be confidently traced back over more than 2200
years provides convincing evidence that it has made no "close
encounters" with stray asteroids, etc. or suffered anomalous mass-loss
from any other source in that same period.
A matter of current concern to me is the analysis of medieval East
Asian observations of Halley's Comet. When comparision is made with
the results of numerical integration, these reveal that for four
successive apparitions (from A.D. 989 to 1222), perihelion passage of
the comet was delayed by approximately 3 days. The most likely cause
is unusual outgassing of the nucleus, causing a rocket effect.
Solar Variability
In principle, sunspot observations provide the most viable index of
solar variability in the historical past. However, during the whole of
the pre-telescopic period there are only about 150 known sightings
(nearly all from China and Korea). As well as producing the first
comprehensive catalogue of these observations to be published in a
Western language (jointly with Dr D.H. Clark) - and a subsequent
revision of this paper (with Dr K.K.C. Yau) - have also discussed in
depth the utility of these early records, which are particularly
challenging because of their uniqueness.
My principal research paper on solar variability was published in
Phil. Trans. Roy. Soc. A, 330 (1990), 499-512, as part
of a symposium on "The Earth's Climate and Variability of the Sun over
Recent Millennia" held at the Royal Society in February 1989. The main
conclusions of my research were threefold: (a) Telescopic observations
of sunspots provide a reliable index of solar activity since about
A.D. 1715. (b) The apparent scarcity of such observations in the 17th
century (e.g. during the Maunder Minimum) is only partly due to
diminished solar activity; the relative inaccessibility of
astronomical data of any kind at this early period must also be taken
into account. (c) Although the temporal distribution of recorded
sunspot sightings in the pre-telescopic period (largely from China and
Korea) shows considerable variation, most of these features would
appear to result from the incompleteness of the observational record
rather than indicating real trends in solar activity. In general the
most effective usage of early astronomical records involves analysis
of measurements - not simply frequency trends.
Recently, Dr D.M. Willis of Rutherford Appleton Laboratory and I
published a paper on conjugate auroral sightings made in the late 18th
century by both British seamen north of Australia and East Asian
astronomers (in China and Japan). This is the earliest known example
of conjugate auroral sightings. More recent joint work -- much of it
already published -- includes investigations of concurrent sightings
of aurorae in China, Korea and Japan; the earliest drawing of sunspots
(from England in AD 1128); independent sunspot (from England) and
auroral (from Korea) evidence for an intense recurrent geomagnetic
storm in AD 1128"; identification of possible intense historical
geomagnetic storms using combined sunspot and auroral observations
from East Asia.
My studies (partly with Dr Willis) of Korean records of sunspots and
aurorae have focused largely on the data in such works as the {\em
Koryo-sa}, {\em Choson Wangjo Sillok} and {\em Chungbo Munhon Pigo}. I
have lately studied in detail the numerous auroral sightings in the
earliest surviving volume of the {\em Sungjongwon Ilgi} (the first few
years of King Injo). One of my aims was to verify the reliability of
these records by investigating the accuracy of parallel reports of
lunar and planetary observations.
Oriental Star Maps
The positions of many comets and "new stars" (novae and supernovae)
were carefully recorded by East Asian astronomers in relation to the
traditional constellations. These star groupings differ markedly from
the their occidental equivalents. I have undertaken an extensive
investigation into the history of celestial cartography in both China
and Korea. The principal results of this research were published by
the University of Chicago Press in 1994 (in vol. II, part 2 of the
{\it{History of Cartography}). This article was the first
comprehensive discussion of Chinese and Korean star maps to appear in
a Western language.
Partly as the result of visits to both China and South Korea - I have
built up an extensive collection of high quality photographs of early
oriental star maps. My studies of these reveal that the accuracy of
star positions is very variable from chart to chart - ranging from
about 1 degree to more than 5 degrees. Comparison of star charts of
widely differing dates has also revealed that for many individual
constellations containing fainter stars the recognised configurations
have varied considerably down the centuries. This remarks is
particularly true in the 17th century when the Jesuits intoduced
Western mapping techniques to China.
Chronology
The dating of eclipses recorded in ancient Greek and Latin writings
attracted much attention during the 19th and early 20th centuries.
However, after the extensive work of John K. Fotheringham between
about 1910 and 1920, little further progress was made for more than
fifty years. Using the results of my studies of Earth's past rotation
with Dr Morrison, I have been able to compute the local circumstances
of ancient eclipses with much improved precision. This led to
my renewed interest in chronological studies of eclipses. Much of
this work is published in by 1997 book on {\em Historical Eclipses and
Earth's Rotation}. However, detailed investigations of eclipses
recorded by Herodotus and Thucydides have more recently been published
jointly with my former research Student Dr L.J. Fatoohi.
Until recently, studies of the Chinese calendar were very much
neglected by Western scholars. Professor Liu Baolin of Nanjing, who
spent a year at Durham, made a careful study with me of both the
history and operational rules of the traditional Chinese calendar;
this work was published in a series of papers.
Accuracy and reliability of pre-telescopic observations
Many ancient and medieval astronomers timed the various phases of both
solar and lunar eclipses. These observations enable the accuracy with
which time was measured in the past to be investigated in some detail.
Both alone and in collaboration with Dr S.S. Said, Dr Fatoohi and
another former research student Dr J.M. Steele, I have made the first
comprehensive investigation of this subject, working with Babylonian,
Chinese and Arab and European observations. This project, the results
of which have been published in a series of papers, has significantly
improved existing knowledge of time measurement in the historical
past.
Other projects
In a paper published in 1992, Dr W.S. Rada (a frequent visitor to
Durham) and I compiled and translated -- giving exact Julian dates
where possible -- some 30 medieval Arab records of meteor showers.
Scarcely any of these observations had been previously published. The
main motive in making this compilation was to encourage further
research in meteor showers orbits by making hitherto unused historical
data generally available. It is well established that on account of
orbital perturbations individual meteor showers vary considerably in
extent from year to year and there is some evidence for periodicity.
Also a gradual change in the date of a shower can often be detected
down the centuries, only part of which is due to the discrepancy
between the sidereal and tropical years.
The late Professor Xu Zhentao (who spent a year working with me at
Durham University on a Royal Society K.C. Wong Fellow) and I
also studied and published the various astronomical references on the
oracle bones of the Shang Dynasty in China (ca 1300 - 1100 B.C.).
These include allusions to eclipses, comets, stars and planets.
Professor Xu had specialised in the study of the ancient Shang script.
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