Introduction

The eclipse observations made by medieval Arab astronomers are among the most accurate and reliable data from the whole of the pre-telescopic period. Careful records of both solar and lunar eclipses are contained in a number of compendia – some known as zijes (astronomical handbooks containing various tables along with explanatory text). These include measurements of the times of occurrence and other details such as magnitude estimates. Although the main emphasis in this chapter will be on timed data, solar magnitude estimates and horizon observations of eclipses will also be considered.

Many of the observations discussed below were investigated by Newcomb (1878) and Newton (1970). However, these authors relied on published translations which sometimes contained significant errors, while their own interpretations are occasionally suspect. Furthermore, in neither case was a direct solution made for ΔT.

Sources of data

Most of the accessible eclipse observations by medieval Arab astronomers are contained in a single treatise – the zij compiled by the great Cairo astronomer Ibn Yunus, who died in AD 1009 (his date of birth is unknown). A few eclipses are also recorded in works by al-Battani (who lived between AD 850 and 929) and al-Biruni (AD 973–1048).

Ibn Yunus cites reports of some thirty solar and lunar eclipses from between AD 829 and 1004. His treatise, dedicated to Caliph al-Hakim, is entitled al-Zij al-Kabir al-Hakimi.

Introduction

In order to determine the value of ΔT from an eclipse observation, it is necessary to be able to calculate accurately the positions of the Sun and Moon at any selected epoch. By definition, the Sun has negligible acceleration on TT. However, the longitude of the Moon contains an appreciable quadratic term – part of which (owing to the reciprocal action of the tides) can only be determined empirically. Since many of the eclipses recorded in history are remote from the present-day, the effect of the lunar accelerative term on their calculated visibility is substantial. Furthermore, knowledge of the tidal component of this acceleration (usually denoted by ṅ), leads directly to a determination of the effect of the tides on the Earth's spin – see section 2.4. Consequently, it is important to investigate both the numerical value of ṅ and its constancy during the historical period. Each of these questions will be considered in the immediately following sections (2.2 and 2.3).

Evaluation of the lunar tidal acceleration on TT

As discussed in chapter 1, the gravitational component of the lunar acceleration (coefficient of T equal to 6″.05) is well established. Up to about 1970, all estimates of the non-gravitational lunar acceleration were based on analyses in a UT framework. Results for the secular acceleration of the Moon (c) and Sun (c′) on UT can be easily converted to ṅ using the formula:

Introduction

Despite their relatively low precision, pre-telescopic observations cover a sufficient time-span for long-term trends in the length of day (LOD) to become apparent. These trends cannot be discerned from modern measurements. Here we have the main reason why archaic observations are so important in the study of the Earth's past rotation.

The analysis of ancient and medieval eclipse records is just one of several techniques which have been utilised in recent years to investigate variations in the LOD over the historical past. Before discussing in detail the application of eclipse observations to this problem, it is necessary to briefly consider other available methods and to explain why eclipses are to be preferred to other types of data.

Observational requirements for determining Δ T in the pre-telescopic period

Any early astronomical observation which is of value in studying changes in the LOD in the past must satisfy a wide variety of criteria. These may be listed as follows:

1. (i) The observation must involve at least one of the brighter and more rapidly moving objects in the solar system (i.e. the Moon, Sun or one of the inner planets Mercury, Venus and Mars).

2. (ii) The exact Julian or Gregorian date of the observation must either be specified directly or be able to be determined unambiguously.

3. […]

Introduction

Compared with the careful observations of similar age which are recorded on the Late Babylonian astronomical texts, many of the eclipse records in ancient Greek and Roman history come as something of an anticlimax. Although numerous descriptions of both solar and lunar obscurations are preserved in these sources, commencing as early as the seventh century BC, most accounts are too vague to be suitable for investigating the Earth's past rotation. The majority of writings which mention eclipses are literary rather than technical, and include historical works, biographies and even poems. Late nineteenth and early twentieth century authors such as Nevill (e.g. 1906a), Ginzel (1899), Cowell (e.g. 1906b), and Fotheringham (e.g. 1920b) paid much attention to these observations. However, this was largely because little other material was available at the time.

As noted in chapter 3, the mainstay of investigations made around the beginning of the present century was undoubtedly untimed observations of large solar eclipses. Attempts to date the various records and identify the places of observation proved an almost irresistible challenge to Fotheringham and his contemporaries, and much effort was expended in these pursuits. Considerable interest was also shown in using ancient eclipses to date historical events. (For a recent summary, see Stephenson, 1993.)

There seems little doubt that many records in Greek and Roman history relate to eclipses which were either total or fell not much short of this phase.

Introduction

In the previous chapter, a variety of Babylonian timings of lunar eclipse contacts were analysed. Several of these records also noted that the Moon rose (ki E-a) or set (SU) whilst eclipsed. Additionally, some damaged texts do not contain useful measurements of time but nevertheless affirm that the Moon was eclipsed at its rising or setting. Such fairly straightforward observations (which enable limits on the value of ΔT to be deduced), would require no instrumental aid. In the following pages these various observations will be investigated together with a few rather more careful reports which give an estimate of the fraction of the Moon covered at moonrise or moonset.

Since the eclipsed Moon is in direct opposition to the Sun, it invariably rises close to sunset or sets near sunrise. It thus usually reaches the horizon when the sky is quite bright – often when the Sun is above the opposite horizon. However, as noted in chapter 4, despite these seemingly unfavourable conditions the Babylonians systematically measured the time of moonrise (relative to sunset) and moonset (in relation to sunrise) around full Moon with considerable care. Many examples of this practice are found in one of the earliest surviving astronomical diaries – dating from 568 BC, the 37th year of Nebuchadrezzar II (see SH I, pp. 47 ff.) – and it may extend back much further in time.

Introduction

More solar eclipses are recorded in the history of China than in the annals of any other civilisation. Not only were these events regarded as important astrological omens by the Chinese from an early period, but they also played a major role in the maintenance of the calendar.

Eclipses of the Sun have been systematically observed in China from at least the eighth century BC. The many hundreds of reports which are preserved since then are part of a huge corpus of accounts of celestial phenomena of various kinds (including eclipses of the Moon, lunar and planetary conjunctions, comets, novae and supernovae, meteors, sunspots and the aurora borealis). Most of these events were noted by official astronomers, who were employed by the ruler to keep a regular watch of the sky for ominous happenings. Nearly all of the original reports have long since been lost. Existing records – as found in the standard dynastic histories and other historical compendia – are usually no more than summaries of what may well have been detailed descriptions. These secondary sources are readily accessible in major libraries throughout the world, having been printed and reprinted many times. Block printing was discovered quite early in China (eighth century AD) and as a result older manuscripts have been phased out and are now relatively rare.

During the early centuries of its history, Chinese writing gradually evolved from simple pictograms to an advanced form of ideographic script.

Introduction

More celestial observations are preserved from Babylon than from any other contemporary civilisation. Yet until about a century ago, when large numbers of clay tablets devoted to astronomy began to be unearthed at the site of Babylon, little was known about the achievements of the skywatchers of this once great city. What could be established was mainly based on ancient Greek texts and the Old Testament. Both the Prophet Isaiah (e.g. 47:13) and the ancient Greek writer Strabo (Geography, XVI, 1.6) stress the Babylonian preoccupation with astrology. As noted in chapter 3, the ancient Greek historian Diodorus Siculus (Library of History, II, 9) implies that the lofty ziggurat – built during the reign of Nebuchadrezzar II (604–563 BC) was used as an observatory. Figure 4.1 shows a schematic view of Babylon in the days of Nebuchadrezzar II – as visualised by Herbert Anger (Unger, 1931, figure 7).

Among writers of the ancient Greek and Roman world whose works are still extant, only the great Alexandrian astronomer Claudius Ptolemy (c. AD 150) hints at the true scale on which celestial observation was practised at Babylon. In his Mathematike Syntaxis (Mathematical Systematic Treatise) – which later became known as the Almagest – Ptolemy specifically mentions sets of Babylonian eclipse observations to which he had access. Examples of his comments are as follows:

Introduction

Because so many observations of lunar eclipses are preserved on the Late Babylonian astronomical texts, it is necessary to devote two chapters of this book to their discussion. This is a reflection of the great importance of the lunar eclipse records from Babylon in the study of long-term changes in the length of the day. The present chapter is restricted to timed contacts, while in chapter 7 a variety of untimed observations of eclipses of the Moon will be considered.

Most Babylonian lunar eclipse timings are expressed relative to sunrise or sunset, although very occasionally they are referred to moonrise or moonset instead. I have divided these measurements into four categories: (i) those for which only a single contact timing is preserved; (ii) timings of two separate contacts (mainly partial eclipses); (iii) three or four contact timings (total eclipses only); and (iv) timings of eclipse maxima (partial eclipses only). Observations in these categories will be discussed in turn in sections 6.2 through 6.5. In addition, a number of eclipses were also timed in relation to the culmination of certain reference stars (see section 6.6).

Times of the very earliest eclipses (whether observed or predicted) were nearly always quoted with low precision. Between 731 BC (the earliest surviving record, representing a prediction) and just before 600 BC times were almost exclusively expressed to the nearest 10 deg.

Introduction

In the previous chapter, East Asian records of total and near-total solar eclipses were analysed. Emphasis was placed on observations which either affirmed or specifically denied that the Sun was completely obscured. Other reported details – such as measurements of the times of the various contacts or rising or setting of the luminary whilst eclipsed – were not considered. The theme of the present chapter is the investigation of further observations of both solar and lunar eclipses from the same part of the world which in principle are of value in the determination of ΔT. These may be grouped in three main categories: (i) timed contacts; (ii) estimates of the proportion of the Sun covered at maximum phase; and (iii) instances where the Sun or Moon was said to rise or set whilst visibly obscured.

In the pre-telescopic period, there is a significant number of Chinese observations in each of the above categories. However, accounts of eclipses from Korea tend to be extremely brief and no useful records are preserved in the history of this country – whether in the Koryo-sa (‘History of Koryo’) or the Choson Wangjo Sillok (‘Veritable Records of the Choson Dynasty’). Although many early Japanese observations of both solar and lunar eclipses give estimates of magnitude or local time, these are contained in a miscellany of sources. Not only is the place of origin often obscure (see also chapter 8), it is frequently difficult to distinguish observation from prediction.

The Utrecht proof of the renormalizability of gauge-invariant massive vectormeson theories which appeared in 1971, as observed by influential contemporary physicists, ‘would change our way of thinking on gauge field theory in a most profound way (Lee, 1972) and ‘caused a great stir, made unification into a central research theme’ (Pais, 1986). More precisely, with the exhilaration of such a great change in perspective, confidence had quickly built up within the collective consciousness of the particle physics community that a system of quantum fields whose dynamics is fixed by the gauge principle was a self-consistent and powerful conceptual framework for describing fundamental interactions in a unified way. An immediate outcome of the new perspective was the rise of the so-called standard model, consisting of the electroweak theory and quantum chromodynamics (QCD) (section 11.1). With the encouragement of the empirical successes of the model and the power of its underlying concepts, efforts were made to extend the model to a grand unification and to gravity, assuming the universality of the gauge principle (section 11.2).

The emergence of the gauge field programme (GFP) suggests a dialectical comprehension of the developments of 20th century field theories: taking GFP as a synthesis of the geometrical programme and the quantum field programme. Some justifications for such an outlook are given in section 11.3. But the dialectical developments of field theories have not ended with a Hegelian closure (a final theory or a closed theoretical framework). Some discussions on the stagnation of GFP and on a new direction in the field theoretical research appear in section 11.4.