PRACTICAL APPLICATION OF CARTOGRAPHY
SECTIONS 6 - 17
Paragraph §1 gives us no idea of Marinus's dates, only that he prepared serious commentaries on the locations of various journeys and places throughout the known world. However, since Ptolemy draws attention to the various errors that have been introduced by copyists of Marinus’s work we must assume that a fair amount of time has elapsed since his work was carried out. Therefore, the translation of 'usuauoV' to read 'latest' rather than 'last' is questionable and we should perhaps allow time for a much as much as two or three generations to have elapsed to make sense of Ptolemy. Less time than that would have allowed first hand accounts to have reached Ptolemy by virtue of generation overlap and reporting. Thus a margin of about one hundred years might seem to be an optimum figure. In which case we should be considering dates for Marinus that would precede and overlap the Claudian occupation of Britain and might account for some of the anomalies in the map references.
In paragraph §2 Ptolemy agrees that the work of Marinus is so comprehensive as to need no additions – 'kan aphrkesen hµin apo uouuwn µonwn uwn upoµnhµauon poieisqai uhn uhV oikouµenhV kauagrajhn, µhden ui periergazoµenoiV.' However, he qualifies this by indicating that it is only right that if he, (Ptolemy) finds any passage that is at odds with a more modern concept or based on untrustworthy sources, it should be set right. Thus Ptolemy clearly takes on the role of editor rather than author, together with the supervisory role that all editors assume. We should not expect, therefore, that Ptolemy will introduce any new material, merely assume that he will conscientiously monitor the work of Marinus – 'oson woµeqa dein, uh uandroV pragµaueia suneisenegkein epi uo eulogwueron kai eucrhsuoueron'. Thus, t would seem, Ptolemy rather neatly brackets himself into the role of an editor, equipping the office with an over–riding censorship with the commentaries of Marinus.
Paragraph §3 contains a critical statement that seems quite unequivocal in its meaning. Ptolemy claims that Marinus considers the earth to extend too far to the east and to the south. 'aj' hV oieuai dein epi pleon proagein kai uo hµkoV uhV egnwVsµenhV ghV proV uas anauolaV, kai up plauoV proV uhn µeshµßrian' Ptolemy does not elaborate on this statement except by a later comparison but it poses the question of how he makes the comparison and of how he measures the distance. (The theory of Posidonius rated one degree as equivalent of five hundred stadia; Eratosthenes rated one degree as equivalent to seven hundred stadia The correct, modern day equivalent would be in the order of six hundred and twenty–six stadia.) If both Ptolemy and Marinus are using the same factor then Ptolemy must be relying on a theoretical argument to sustain his accusation. If there is any possibility of a difference in factors, then this is one potential source of error.
In paragraph §4 Ptolemy defines longitude and latitude. However, while he correctly states that latitude is the space that extends between north and south poles, he attempts to restrict longitude as being between the rising and setting sun. Nevertheless he states that longitude extends a greater distance than latitude. There seems to be a confusion introduced here between the known, habitable world and the correct properties of the Earth as a spherical object revolving on its own axis in a time base governed by the Sun.
This section is devoted to the computation of total latitude in relation to the latitude of the known world of Ptolemy's time. In §1 Ptolemy quotes Marinus as setting the parallel of latitude through Thule at sixty–three degrees North. By Thule, Ptolemy is assumed, by historians, to mean the Shetland Isles although the basis for this assumption is not made clear. Sixty–three degrees North is actually the parallel of the Faroes and whether this leads us to a need for a distinction between a Thule and an Ultima Thule is mere conjecture. In either case Ptolemy, by simple multiplication, then deduces this to represent thirty–one thousand and five hundred stadia. In §2 he quotes Marinus as terminating the southern limit of the known world by a degree of latitude that would mean the total distance as being eighty–seven degrees south of Thule, or, presumably, twenty–four degrees south of the equator. Ptolemy again computes this as being equivalent to forty–three thousand and five hundred stadia.
It seems clear from what is said in §3–§10 that the data base of Marinus contained information of locations well below the equator and that his stellar observations, as quoted by Ptolemy, reflect this. Ptolemy finds it unacceptable that the known world extends so far, yet twenty–four degrees South would only bring us to Madagascar, surely within the reach of the Phoenician merchant sailors and a prime source for the spice trade. In closing this section Ptolemy dismisses these observations – 'wV µhde uouuwn uwn jainoµenwn idion einai ui uwn nouiwuerwn oikhsewn uou ishµerinou.' as being of no significance since the known world does not extend so far!
In this section there is a clash between Marinus, the practical navigator and Ptolemy, the theoretical scientist. Marinus is concerned with true distance and time taken; Ptolemy with exact, measurable distance. Marinus is quite happy to compute the total journey as including all necessary deviations, whatever their cause, and adjusting the total by deducting a percentage based on practical experience. Thus in a long itinerary, Marinus must have given two references to the distance between two points; the actual distance travelled, (and time taken) as opposed to the theoretical distance, obtained by allowing for such deviations and making an a corresponding deduction. How this was expressed in the commentaries of Marinus is not known but it was clearly an anathema to Ptolemy. As he quite correctly points out, it is impossible to construct a facsimile map based on such evidence.
In §1–2 Ptolemy quotes two such itineraries, from Leptis Magna to Agisymba and from Ptolemais in the Trogloditics to the promontory of Prasum. Ptolemy cites Marinus as giving a destination south of the equator of twenty four thousand six hundred and eighty stadia for the first and twenty seven thousand eight hundred stadia for the second. However it is apparent that Ptolemy himself has made these calculations using as a basis the number of days journeying quoted by Marinus and deduces for them a latitude of between forty–eight and fifty–five degrees South.
In §3–4 Ptolemy reports that Marinus subsequently corrects these distances to around twelve thousand stadia by reason of the deviations encountered in the journeying time. It would seem that the data from Marinus that Ptolemy was using might have been in two parts; the one being a journey log, kept by days, and the other a table of distance. Ptolemy quite clearly became incensed by the data in the former only to find it remedied in the latter. He ends by saying he accepts the latter figure but feels that he must demonstrate the sloppy methods of Marinus – 'ex wn ou uo µeiwsai µonon jainoiu' an anagkaion, alla kai uo µecri uosouuou.'
In §5–6 Ptolemy then quotes other examples of these sloppy methods. Marinus is quoted as saying that Septimus Flaccus journeyed three months towards the equator, from Libya to Ethiopia and Julius Maternus journeyed four months towards the equator, from Leptis Magna to Agisymba, where rhinoceros are to be found. As Ptolemy points out, these journeys are apparently between adjacent territories and the distance implied by the time taken is absurd. Marinus apparently does not qualify his statement by quoting actual distances nor advance any reasons for delays and deviations. There could, of course, have been many reasons for the extended journeying time but, as Ptolemy points out, it makes no sense to include such unqualified data – 'kai eui µhdaµh diaurißaV axiologouV eµpoihsai'.
In §7 Ptolemy concludes this section by commenting that it is easy to speak of strange southern lands or to make loose statements about distance or time travelled but while it makes for colourful narrative, it is useless for precise definition.
In this section Ptolemy continues to disparage Marinus and his methods of recording journeys in time taken rather than the actual distance between two points; this time using journeys by sea as examples.
At this point in Ptolemy's narrative it becomes increasingly clear that what we are seeing amounts to a watershed in navigational interpretation. Between the practical, if loosely documented, aids with which the merchants and merchant–sailors had used for so long and the need for more exact methods of interpretation required by those whose needs were not driven by profit so much as by territorial expansion. It is almost a microcosm of the change from an Hellenistic, de facto, make–do, to a Roman, de jure, pragmatism, that is evident in so much of the differences ofeqh between the two cultures.
In §1–5 Ptolemy quotes Marinus as relating two voyages, by a certain Diogenes and Theophilus respectively; Diogenes voyaging from Aromata to Rhapta and Theophilus voyaging from Rhapta to Azania. Diogenes encountered adverse winds from the north and was forced to make a detour around the Trogloditics and took twenty–five days; Theophilus encountered adverse winds from the south and took twenty days. Ptolemy comments that the actual sailing days are not recorded, only the length of the voyage. By which he presumably means the actual days, in which progress was made towards the destination, were not recorded. Ptolemy displays a certain lack of comprehension on the practical difficulties of achieving this and by voicing incredulity at the adversity of winds. He then makes the point that while Theophilus had the longer journey, he made the better time. He quotes Theophilus as saying that sailing a night and a day is the equivalent of one thousand stadia and Diogenes as saying that the distance of his journey was only five thousand stadia. That the two statements are irreconcilable is attributable to the treachery of wind direction around the equatorial regions. Ptolemy is clearly unhappy with this situation and comments that such inconclusive reasoning will result in the sort of situation that would infer the Ethiopians and the rhinoceri live in the frigid zones of the earth. Common sense, he argues, dictates a law of similarity which places all like flora and fauna belong to certain latitudes and climates.
In §6–10 Ptolemy decries the method of re–calculating distance, by making allowances for deviations or adverse winds, as being quite untrustworthy. He argues that a knowledge of mathematics and familiarity with zonal characteristics would eliminate this necessity. For, he points out, ethnically speaking, from the equator southwards all are 'Ethiopians' whereas from the equator northwards most are white with some being 'slightly black' nearer the equator and indeed, that in the area of Meroe there are some who are 'actually black' and also some elephants and other unusual animals. Nevertheless, basic knowledge such as this, passed on would be infinitely preferable to quoting the number of days only.
In this short section Ptolemy recapitulates on the previous section in two short paragraphs. In §1 he agrees that the people encountered would have been Ethiopians, by which we understand him to mean people of a black countenance but he insists that Agisymba and the Prasum promontory are no further south than Meroe is north of the equator, sixteen degrees and twenty–five minutes South or, by his reckoning, eight thousand two hundred stadia. (It is by comments such as this that we can gain an insight into the margin of error he introduced by using the calculation of Posidonius for his geodesic time base instead of that of Eratosthenes. The Posidonius factor would give an equivalent number of stadia of about eleven thousand five hundred. If Marinus was using this factor the conflict of distances is easily explained. As a corollary to this, the present day equivalent would be ten thousand two hundred and sixty–five stadia)
In §2 he comments on the journeys of Flaccus and Maternus and accepts that desert travel is conditioned by factors other than natural adversities such as the need to proceed from one oasis to another. He also comments that such well travelled routes as these would, by this very fact alone, have an accurate assessment of distance. In such cases he allows that travellers estimates will be correct.
In this section Ptolemy debates the size of the known, habitable world from west to east and from north to south and states the coefficients he uses. He then challenges Marinus in this matter, claiming that he has seriously overestimated these distances. He then tracks a theoretical journey from the Fortunate Islands to the capital city of the Chinese, which he takes to be the opposite boundaries of the known world, and passes comment on the veracity and accuracy of the merchants who travelled these routes.
In §1 Ptolemy states that the latitudinal bounders are as discussed in the previous section, from the parallel of Agisymba, sixteen degrees and twenty five minutes South, to the parallel of Thule, sixty–three degrees North, a total of seventy–nine degrees and twenty–five minutes, the equivalent of which, by Ptolemy's coefficient, is just under forty thousand stadia. He then states that, by the reckoning of Marinus, the longitudinal boundaries are fifteen hours apart (two hundred and twenty–five degrees), while he himself feels they are no more than twelve hours apart (one hundred and eighty degrees).
His justification for this reduction is contained in §2–6 and in §2 he reveals the coefficients he is using; an exact five hundred stadia for one degree of longitude on the latitude of the equator, which he states categorically is the one acceptable by all and based on the calculation of Posidonius. This would make an average of four hundred stadia for one degree of longitude along the latitude of Rhodes which, he states, bisects the habitable world. He divides the journey into three stages; from the Fortunate Islands to Hieropolis on the Euphrates, from there to the Stone Tower and from the Stone Tower to Sera, the capital city of the Seres. All the stages are computed from the number of months and days required for their completion but while Ptolemy is happy to accept that computation for the first stage he is not so happy in accepting those for the other two stages. His reason is that Marinus does not agree that the reports of travellers on these two stages are at all reliable since their aim is merchandising, their deviations inconstant and no allowance has been made for lack of continuity of travel. Additionally the final stage was notorious for its weather conditions in all seasons of the year. Ptolemy therefore deducts a large proportion from the last two stages in order to achieve his final figure.
At this point it is worth while considering the arithmetic involved in these calculations. First of all it is highly unlikely that Marinus would have stated the whole distance, if at all, as a total number of hours or their equivalent degrees. Fifteens hours would have been a deduced figure on the part of Ptolemy who would have translated the computed distances from the commentaries of Marinus. However he would have used the geodesic time base of Posidonius, (one degree equals five hundred stadia) instead of that of Eratosthenes, (one degree equals seven hundred stadia), thus introducing into the result a ratio factor of seven over five. Had the other factor have been used the equivalent hours would have been about ten and three quarters or about one hundred and sixty degrees. Further, since this is an uncorrected figure of Marinus, by Ptolemy's own reckoning it should be reduced by his own factor for deviations. This of course must mean a reduction by a similar ratio factor based on fifteen over twelve, which would give us about eight and two thirds hours or about one hundred and thirty degrees. It is worth noting that the present distance from the Canary Islands to central China extends to about the same number of degrees, without unduly labouring the matter. However this does seem support an argument that Marinus, and, presumably, all those who went before him, could have been using the correct time base of Eratosthenes!
In §7–8 Ptolemy quotes examples of how merchants are prone to make errors in estimating distance and that Marinus was loath to accept their word. A certain Maen of Macedonia wrote a commentary on the same journey by taking second hand reports from others without verifying distances while a certain Philemon estimated the east/west journey on the island of Hibernia to be twenty days, but only by hearsay. He quotes Marinus as saying that such merchants are inclined to boast about the length and difficulties of their journeys, be unconcerned about any notable feature and be only concerned with the business of merchandising their wares.
In this section Ptolemy shows how his figure for twelve hours, one hundred and eighty degrees, is computed. By tracing the route from the Fortunate Islands to Sera, section by section, and making allowances for change of parallels, adverse terrain and other enforced deviations, he finally arrives at a total figure of one hundred and seventy–seven degrees and fifteen minutes. This is slightly under his original estimation and is arrived at by estimating the number of stadia for each section, from the number of days travel needed, adding all the stadia together and then converting it to a total number of degrees by dividing the total number of stadia by the geodesic time base of Posidonius, which, of courses, would have given an inflated figure of total degrees.
The total stadia he computed would, however, have been correct at eighty–eight thousand six hundred and twenty–five. There is therefore, within this passage, the evidence of a doubling up of error that has confounded this work for generations. Firstly, Ptolemy took all the distances quoted by Marinus in days and estimated from them the actual number of stadia from point to point. Secondly, he then converted these distances to an equivalent number of degrees of longitude using the incorrect factor of Posidonius, thus introducing an error exponential of seven over five and thereby effectively cancelling out the allowances he had already made for deviations. Finally, his conclusions were accepted without question by succeeding generations who, had they made the simple division of his total stadia by the correct factor of seven hundred stadia as equal to one degree of longitude, would have arrived at a reasonably accurate figure of total longitudinal degrees for the known world. They would also have been alerted to the fact that almost two thirds of the earth's surface remained unexplored.
Such an argument is, of course, based on the premise that Marinus did not also use the incorrect factor of Posidonius. There are other arguments, already mentioned, against this and also for the premise that such practical navigators as the Phoenicians could not have been unaware of the true value in distance of one degree of longitude. The relevant passage of Ptolemy's text can be read as supportive of this argument, – 'Oiwn esuin o µegisuoV kukloV µoirwn u–x, penuakosiouV epi uhV epijaineiaV uhV ghV apolaµßanein suadiouV, oui uaiV oµologouµenaiV awaµeurhsesi suµjwnon esui.' – ‘The portion of one degree of the great circle is deemed to be a portion of one over three hundred and sixty, equalling five hundred stadia on the face of the earth, a figure that has been proven to be correct.' Proven by whom? Posidonius? But not by Marinus apparently. Nor indeed that this factor was used by him, accepted by him, or even that he was aware of it. It is a bland, theoretical assumption, on the part of Ptolemy, that Posidonius has arrived at the true factor and that everyone else must therefore have acknowledged this. In it also lies the almost untenable assumption, on our part, that there was some direct link between Marinus and Ptolemy, other than the commentaries. That they were of roughly the same era, inhabited the same scientific circles and agreed on the same scientific laws and principles. This cannot be assumed as fact with any real degree of certainty, for there is nothing in Ptolemy's text that would allow us to do so. Marinus could have been of a much earlier generation, have belonged to totally different community of thought and practised an entirely different work ethic. Ptolemy was very much the archetype of the theoretical scientist when set against Marinus, who had all the hallmarks of the practical artisan.
If this argument holds, it is by way of being an indictment on a posterity that read as fact, an unsupported statement that could so easily have been successfully challenged.
Section Thirteen & Fourteen
In these section Ptolemy argues the same case as in the previous section but this time by sea journeys. He takes the journey from India to Cattigara in two stages. Firstly from the promontory of Cory (between India and Ceylon) to the Golden Cheronese and secondly, from the Golden Cheronese to Cattigara, a seaport of southern coast of China, on a longitude slightly east of the capital itself. Breaking it down into sections, he finds that, once again, the allowances that he has to make for deviations is about one third of the total distance travelled. In the final paragraph of Section fourteen, he is thus able, by projection, to compute a total circumference for the Earth, according to the way he assumed that Marinus calculated journeys, of two hundred and seventy thousand stadia. This, he insists, must be subject to a one third deduction because of the fact that Marinus does not allow for deviations. Thus he arrives at a figure of one hundred and eighty thousand stadia which, of course, agrees with Posidonius.
This is a case of forcing facts to fit a preconceived solution. Two hundred and seventy thousand stadia is within 7% of the figure Eratosthenes calculated (two hundred and fifty–two thousand). Had Ptolemy accepted the distances calculated by Marinus and not gone to great lengths to try and prove him wrong, a great deal of misconception would have been avoided. Ptolemy's sentence, 'wsue sunagesqai kai uou dia uhV PodiaV µhkouV, suadiouV epuakisµuriouV kai disciliouV eggisua' – 'so that to total all of the longitudinal distances along the latitude through Rhodes, is. as near as possible, two hundred and seventy thousand stadia.’ triumphant a vindication as he must have intended it to be, is an awful warning to us all.
In this section, after first reiterating his decision to reduce the distances of Marinus, Ptolemy turns his attention to specific instances where he disagrees with Marinus as regards certain locations.
In §2 he challenges the assertion of Marinus that Tarragon (Tarraco) is opposite Caesarea (Iol Caesarea on the Algerian coast) and also that Tarragon is on the same longitude as the Pyrenees. Well Marinus was quite correct; almost exactly so by modern computation. Why then such a claim by Ptolemy? The answer must lie in the co–ordinates given by Marinus and in the use of individual rectilinear graticules to map specific regions.
Later on, in Book VII, Ptolemy directs that the following graticules be constructed to cover Europe and Africa, with the commencing northern and western co–ordinates:
. Europe Map 1 Ireland and Britain 7°W 63°N
. 2 Spain 1°W 47°N
. 3 France 16°W 44°N
. 4 Germany 27°W 49°N
. 5 Balkans 30°W 48°N
. 6 Italy 28°W 45°N
. 7 Sardinia/Corsica 30°W 40°N
. 8 Eastern Europe 43°W 63°N
. 9 Asia Minor 43°W 48°N
. 10 Greece 44°W 39°N
. Africa Map 1 Morocco/Algeria 5°W 40°N
. 2 Libya 27°W 39°N
. 3 Egypt 47°W 32°N
. 4 Ethiopia 9°W 37°N
. 5 Turkey 45°W 46°N
These graticular representations would have had longitudinal lines at intervals of one degree, each representing five hundred stadia and marked accordingly. The equivalent representations of Marinus, had they existed based on Eratosthenes, would have used one that had longitudinal lines marked at seven hundred stadia. From whatever western point the count of longitude began at 0°W., and it has been suggested that it was the longitude of Ferro in the Canary Islands) and from the northern point (set at Thule, 63° N) there would begin an immediate divergence in the value of each degree towards the east and south respectively, at a ratio of seven over five. That is, one degree on Ptolemy's graticule would only equal five sevenths of a degree on the graticule of Marinus. So that the longitudinal co–ordinates plotted by Marinas would, if used uncorrected by Ptolemy and plotted on his graticule, appear to be slipping cumulatively to the west.
If the longitude of Tarragona was plotted by Ptolemy in this fashion it would indeed appear to miss Iol southwards and the Pyrenees northwards. If §2 he also challenges Marinus on co–ordinates between Libya and Sicily and, in §4, between Egypt and Cyprus and in both of these cases Marinus is correct.
So far it has been a matter of longitudinal divergence but in §3 and §5, relating to the respective positions of Trieste, Ravenna and Pisa it is evident that there is also a latitudinal divergence also. Therefore in our rectilinear graticule as well as the corrected longitudes showing a cumulative move westwards, the corrected latitudes will show, by way of paradox, a tendency to creep towards the north!
Consider the position. Ptolemy has set the parallel of Thule, presumed to be the Shetlands Islands, at sixty–three degrees North. By setting this as the northern boundary of the graticule and plotting the distance of our latitudinal position from the north we would have effectively increased its physical position northwards while apparently reducing its latitudinal degrees. While this had a disastrous effect on the positioning of northern Scotland, its effect is felt throughout all parallels of latitude. Unless one is able to encompass the whole picture on one graticule, instead of a multitude of smaller records, then there is no correspondence between them.
It would seem that Ptolemy, when stating that Marinus has positioned places in the wrong clime, or climate of hours, is reading what appears to be the reasonably correct co–ordinates of Marinus in relation to his own graticular divergences. Thus the true correspondence of the Libyan locations with the Sicilian locations, that of Trieste with Ravenna and the locations in the Egyptian Delta with corresponding points on Cyprus would all seem to be the result of comparing specific degree co–ordinates, from different graticules, each effected by such longitudinal and latitudinal corrections. The relationship he objects to between Ravenna and Pisa is particularly the result of latitudinal slippage.
In §6 Ptolemy mentions Britain, the fact that Chichester, which he knows to be south west of London, is shown, by reason of the co–ordinates given by Marinus, to occupy a latitudinal 'climate' to the north. 'Kai Loundiniou uhV BreuaniaV Noioµagon eipen nouiwueraw µilioiV n–q, Boreioueran auuhn dia uwn kliµauwn apojainei.' – 'And Noiomagus in Britain, which is fifty–nine miles in a south–westerly direction from London, is shown by reason of climate to be more northerly.'
This serves to demonstrate precisely the effect of such latitudinal slippage. Whether a map reference for London is given by Marinus in his commentaries, the location of Chichester is given the latitude of 53° 25˘. London is known by Ptolemy at a latitude of 49° 25˘. It is immediately apparent to him that this places London to the south of Chichester which he knows is incorrect. However, for his own reasons he does not alter the text of Marinus, being content to draw the reader's attention to the anomaly. It would seem, from this and other instances, that Ptolemy has been content to leave the map references of Marinus intact but to draw the readers attention to any with which he does not agree. In §1 of this section, when Ptolemy writes – 'kai uaV kaua µeroV de uwn polewn diaqeseiV pollach diorqwsewV hxiwsaµen.'– 'and we have deemed it worthy in often correcting the given disposition of cities.’ he may well have intended diorqwsewV' to be read in the wider context of 'drawing attention to' rather than 'correcting'. There is no clear evidence to support his actually correcting any of the co–ordinates of Marinus but there are many instances where he has inserted those that were missing or drawn attention to what he considers to be glaring errors. It could well be that he was meticulous in his role of editor of the commentaries of Marinus and he felt constrained to leave the original text unaltered.
In §7–9, Ptolemy discusses similar inconsistencies in respect of Thrace, the Hellespont, Byzantium and Armenia, which all appear to be the result of such latitudinal slippage and in which Marinus is correct. In §10–11, Ptolemy draws attention to the comments of Marinus on the course of the river Nile; once again the confusion would appear to of the same order.
This short section is concerned with provincial boundaries in which it is difficult to follow Ptolemy's reasoning. The relationship of the boundaries of Moesia, Rhetia and Pannonia to Italy are as Marinus claims, and that of Thrace and Moesia to the Pontic Sea, except that Thrace was a separate client kingdom until AD 46. Ptolemy's objection to the claims of Marinus as regards the Sogdiani and the Saci who dwelt to the north of the Himalayas and India, and the parallels along which they co–existed, does not seem to hold true. All of this confusion may indeed be the result of incompatibility of sectionalized maps but it does begin to seem that Ptolemy had no actual recourse to visual map aids at all. The physical relationship of Italy to the adjacent provinces of Rhetia, Noricum and Pannonia and of Dalmatia to Pannonia must surely have been common knowledge. The parallels through Byzantium and the Hellespont, continued through into Asia, do in fact pass to the north of the Himalayas and would have separated the two regional peoples, as Marinus claimed, without going through the Pontus as Ptolemy claims would have been necessary. Unless Marinus was writing before AD.46, the annexation of Thrace would have caused no misunderstanding. This must surely support the argument that Marinus was using the correct geodesic time base.
In this section Ptolemy is concerned generally with the changes that have happened since Marinus wrote his commentaries and for those corrections that Marinus was unable to incorporate, presumably before his death. Ptolemy then mentions specific locations in Asia and the corrections in hours and in climate that he feels are missing or incorrect in the commentaries of Marinus. In essence what Ptolemy argues is that Marinus consistently places locations to the east and to the north of what Ptolemy considers to be the correct position. – 'Di' ou kai uhn kliµauuwn kai uwn wriaiwn µonwV epoihsauo diorqwsin; enia d' hdh kai uoiV nun isuorouµenoiV ouc ecei suµjwnwV.'
Paragraphs,§2–4, are concerned with locations in Arabia and the west coast of India, §5 is concerned about the sea journey from India to China and the overland return journey, while §6–12 details the coastal voyage down the eastern side of Africa. This east/north slippage is consistent with the other objections of Ptolemy and would seem, once again, to be the result of the wrong time base. the main difference is that these locations are outside the Roman Empire and Ptolemy relies on the reports of travellers for his data.