Two complete English translations of the Tract have been made to date. The first was undertaken by John J O'Farrelly in 1893. A hand written copy and a typed copy are in the archives of the Royal Irish Academy, Dublin. O'Farrelly had considerable technical advice from Maxwell Close, at that time Treasurer of the Academy. Close's knowledge of the history of astronomy was thorough and his assistance would have been substantial. The typed copy in the Academy Library carries with it Close's handwritten commentary on the blank facing pages. The commentary was completed in 1901. Neither the translation nor the accompanying commentary has been published.
The translation of Maura Power was completed in 1914 under the direction of Bergin. It was published later that year by the Irish Tract Society. Her translation was made without reference to the earlier work of O'Farrelly and Close. As with the earlier translation it is based principally on the Irish of Stowe B, with reference to both the Marsh copy and the second RIA copy where correlation was possible.
The following English edition of the Tract is substantially that of Power. In various places however, Power's translation ranges from obscure to contradictory, largely through an apparent lack of familiarity with the subject matter. In these parts, reference to O'Farrelly's translation invariably restores the intended sense of the passage. Using the work of Close and O'Farrelly, I have amended Power's translation in minimalist fashion. The required 'substantial' changes are all acknowledged in footnotes.
In addition, numerous 'minor' changes have been made to technical terminology that was ill fitting in Power's work. For example, the term Pol Airtic, is invariably translated as 'Arctic Pole'. This is quite misleading to a contemporary reader, since in the geocentric framework of Ptolemaic astronomy, the pole refers to that of the celestial sphere. In Chapter 35, we are told that the Pol Airtic can be viewed from the equator at an elevation of zero degrees, i.e., on the horizon. The author clearly refers to the North Celestial Pole. Reference simply to the Arctic Pole is hence, an apparent impossibility. To avoid this confusion, I have used the term Arctic (Celestial) Pole.
Another frequent point of confusion in Power's text arises from the fact that the Irish term cercall can variously mean 'orbit', 'circle' or 'sphere'. Power generally opts for the term 'orbit' when it refers to celestial matters. This has often been inappropriate. Chapter 16 deals at length with imaginary grid lines that define the celestial chart; 'meridian' and so forth. I have used the word 'circle' in such cases. I have retained the term 'orbit' only when it applies to the path of a planet, moon or sun, in accord with a geocentric model. A further variation occurs in Chapter 29 of the Tract where the term is also used as an alternative to speir, meaning a celestial sphere in Ptolemaic terminology. A rather isolated use of the term cercall is employed in a description of Cleomede's analogy of the potter's wheel. In this case it refers to the movement of small balls along grooves on the wheel. Power's use of the word
A slightly irritating element of O'Farrelly's and occasionally Power's translation has been the retention of the personal pronouns when referring to celestial objects such as planets and the sun or moon, the sun being 'she' and planets such as Mars and Jupiter being 'he'. While it sits comfortably in the Irish, its retention in an English translation can be misleading. I have therefore removed these accidental tags of Irish grammar.
A similar problem results from the variations in Irish terminology for the planets, stars and constellations. This has often let to confusion in Power's English translation. For example, in a passage describing the period of Saturn's orbit, the planet is referred to as the retlann (star). The reference to the planets as stars was not new in Irish (or Latin for that matter), during the Middle Ages, but the use of the English word 'star' when referring to a planet would be misleading to contemporary readers.
'Minor' alterations such as these have not been footnoted.
In a few places, there has been obvious corruption of the text. In Chapter 7 for example, it appears that a whole line of text had slipped from an early transcription. Close supplied the gist of this missing line and I have included it in italics, suitably footnoted. In Chapter 22 another obvious corruption of the text seems to have resulted from faulty restoration of contracted verbs. By changing the verbs to conditional mood, O'Farrelly was able to redeem the intended meaning. I have included these amendments in this translation.
For some reason, numerical terms have been very prone to scribal error223. Some, such as that for the angular displacement of the sun and moon at the end of Chapter 25 date back to early times in the life of the Tract. Another in Chapter 21 seems to have slipped past the print proofs of the ITS publishers. The angle in the Irish text is correct, but the English one in error. I have made these corrections, duly footnoted.
Concerning the value of pi, Power expressed surprise that the Irish author had taken it as being 3 rather than 3 1/7. Assuming it to be a scribal error, Power corrected it to conform to the traditional value. The calculations within that Chapter however, take pi to be simply 3. I have left it as such, for it is quite clear that this was the value used by the author.
I have retained the almost all of the diagrams as they appear in the ITS publication. In the case of Figure 6, I have substituted the amended version of Maxwell Close. Both the Stowe B and Marsh version are distorted to the point of obscuring its meaning. The diagram of Close conforms to the Irish text and the equivalent diagram in the Latin edition of Stabius. One or two other
In spite of these amendments, the following English translation remains substantially a re-edition of Power's work, hopefully with its confusions and contradictions rectified.
I have included most, but not all, of Close's commentary to the Tract by way of footnotes. He was on occasion prone to elaborate in minute detail on elements that were rather tangential to the main body of the Tract. A few I have omitted, some I have curtailed. The occasional footnote from Maura Power has also been included. I have added additional footnotes where appropriate. The footnotes of Power and Close are all acknowledged.