THE ASTRONOMICAL GEOGRAPHY OF THE KNOWN WORLD
We have already examined the broader theories of astronomical geography
whereby the relation of the globe to the remainder of the universe was
explained. In this chapter we shall speak only of those aspects of
astronomical geography which were intimately connected with man’s
knowledge of the various parts of the known world, or _oikoumene_, as
distinguished from the sphere as a whole.
PHENOMENA RESULTING FROM DIFFERENCES IN LATITUDE
Within the _oikoumene_ the phenomena resulting from varying elevations
of the ecliptic in different latitudes were fairly well understood. The
facts that there are two summers between the tropics (particularly in
India) and that the sun there passes vertically overhead twice a year
had been commented on by Pliny and Solinus, whose observations in this
connection found their way into Isidore’s _Etymologiae_[1095] and thus
to the works of the plagiarists of Isidore in our period. The _De
imagine mundi_,[1096] the _Image du monde_,[1097] Gervase of
Tilbury,[1098] and John of Holywood[1099] all tell us that the same
phenomenon was said to occur in Arabia which lies between the tropics.
Similarly the long days and nights of far northern latitudes were
described on the authority of Solinus and Isidore. In the _De imagine
mundi_,[1100] from which Gervase copies, it is said that in the island
of “Chili” (Thule) there are six months of daylight and summer and six
of night and winter. Giraldus Cambrensis also quotes Solinus[1101] and
Isidore[1102] to the same effect and adds a brief description of how the
sun continuously circles around the horizon during the long Arctic day
and how its light disappears completely when the luminary departs
southward towards the Tropic of Capricorn.[1103]
“CLIMATA”
The ancient geographers had divided the earth’s surface into _climata_,
or climates, which, as we have already seen,[1104] were not atmospheric
regions but mathematical strips running east and west and bounded by
parallels of latitude. Pliny,[1105] for instance, had conceived of seven
climates, the first in the latitude of India, where the length of the
longest day is fourteen hours, and the seventh in that of the
Borysthenes (Dnieper) and of Venetia, Umbria, Milan, and Aquitania,
where the longest day is fifteen and three-quarters hours. Martianus
Capella[1106] added an eighth climate in the north between the parallel
of the Borysthenes and that of the Rhipaean Mountains. Furthermore, he
applied names to the strips. It must be added, however, that neither
Pliny nor Capella were precise in the data they gave, and neither
indicated in degrees the latitude of the parallels which bound their
climates.
More definite is the information we find in the two works of Ptolemy.
The _Almagest_[1107] and _Geography_[1108] give accounts of the
characteristic astronomical phenomena that occur along a series of
parallels, thirty-eight in number according to the former, twenty-one
according to the latter.[1109] The positions of these were determined by
the length of the longest day at each one. Though there is no explicit
mention of the older division by climates in the text of either of
Ptolemy’s books, such a division not only appears upon the map of the
world made by Agathodaemon on the basis of material supplied by Ptolemy
but also upon certain of the special regional maps which were probably
the work of Ptolemy himself.[1110]
At all events, the conception of the seven or eight climates did not
disappear but at a very early period, whether by Ptolemy or not, was
correlated with the Ptolemaic parallels.[1111] That is to say, certain
of Ptolemy’s parallels were used to designate the imaginary lines
marking the centers and bounds of the climates. This practice was
adopted by the Arabs and from them transferred to the knowledge of the
Christian West in various astronomical treatises. Among the Latin
manuscripts of the _Toledo Tables_,[1112] for instance, there are series
of astronomical tables for each of the seven climates, according to
which the climates occupy the space between latitude 16° N., with a
longest day of thirteen hours, and 48° N., with a longest day of sixteen
hours. The length of the longest day and the latitude are given for each
parallel that bounds the climates. Except that Ptolemy notes minutes as
well as degrees and in the _Toledo Tables_ the minutes have in most
cases been omitted, the figures correspond essentially with those of the
_Almagest_ and _Geography_. Thus: Ptolemy’s eleventh parallel according
to the _Almagest_ (or tenth according to the _Geography_) has a longest
day of fourteen and a half hours and is at latitude 36°. In the _Tables_
the southern edge of the fourth climate likewise has a longest day of
fourteen and a half hours and is at latitude 36°.
Again, in John of Seville’s translation of Al-Farghānī’s
_Astronomy_[1113] and in the _De sphaera_[1114] of John of Holywood, who
had borrowed from Al-Farghānī in this matter, we find a similar
correlation. In both cases the figures of latitude correspond
essentially, though with slight divergences in detail, to those of
Ptolemy. The boundaries of each climate, however, have here been
displaced by one parallel to the south of the parallels used in the
_Toledo Tables_ and those which we may presume were the Ptolemaic
boundaries of the climates.[1115]
The table, Figure 11 (in the Notes to Ch. X), gives some idea of the
relative degree of accuracy of these figures as they were employed in
the West during the Middle Ages. But just as in the case of other
figures for latitude and longitude, as we shall shortly have occasion to
see, this material was not utilized for geographical purposes during our
age.
GEOGRAPHICAL COÖRDINATES
At the present time the study of regional geography is largely dependent
on a precise knowledge of the geographical coördinates of places. The
foremost duty of the explorer is to know where he is from day to day and
to find this out by astronomical means, if possible. In classical times
and among the Moslems the importance of such observations was not only
well understood, but several methods of carrying them through were
described by astronomers and geographers, and the latitudes of a great
many stations had been determined astronomically. Longitude, on the
other hand, long remained a stumbling block, and before the twelfth
century, certainly, no systematic attempts to ascertain the longitudes
of any large number of places had ever met with success.
A few relics of classical and Moslem study in this field became familiar
in the West as a result of the intense interest in Arabic astronomy
prevailing in Europe between the tenth and thirteenth centuries.[1116]
Various figures representing the results of Arabic corrections of and
additions to the data given in Ptolemy’s _Geography_ found their way
into Western astrological tables. The most interesting of these occur in
a list of the latitudes and longitudes of some sixty odd cities appended
to the Paris manuscript of the _Marseilles Tables_ of Raymond of
Marseilles[1117] and also to most of the Latin versions of the _Toledo
Tables_.[1118] This list and certain figures scattered through the
astrological tables and canons[1119] reveal the results of the
reductions made by Al-Khwārizmī and by Az-Zarqalī of Ptolemy’s gross
overestimate of the length of the Mediterranean, to which we have
referred in a preceding chapter.[1120] The European student of these
astrological works might have drawn a by no means contemptible map from
the figures to be found in them had he been interested in what these
figures could teach him of geography. Figure 6 is a map compiled from
the coöordinates given in the Paris manuscript of the _Marseilles
Tables_.
At the end of this list of geographical coördinates in many manuscripts
additional figures not derived from Moslem sources are given. These show
the positions of such points in Europe as London, Hereford, Paris,
Toulouse, Barcelona, Marseilles, Novara, Cremona, Florence, and
Naples[1121] (see Fig. 12, in Notes to Ch. X). They were undoubtedly
determined by observations made during our period or shortly after.
[Illustration:
FIG. 6—Sketch map constructed from the list of geographical positions
appended to the Paris manuscript of Raymond of Marseilles’
_Marseilles Tables_. The outline of the coast, arbitrarily indicated
by a shaded band, is shown merely to give some idea of the type of
map that might have been constructed from the data given in the
tables. This may be compared with the Henry of Mayence map (see
above, p. 124) shown in outline in the inset. The original Henry of
Mayence map reveals far greater detail and upon it east (not north,
as in this figure) is at the top.
This list is based on the observations of the eleventh-century Arabic
astronomers Al-Khwārizmī and Az-Zarqalī. Cities and other points
have been plotted according to the coördinates of this list. The
resulting map of the Mediterranean region and the Near East is
remarkable for its comparative accuracy. For a key to the names
represented by the numbers on the diagram and for the figures for
the latitudes and longitudes, see J. K. Wright, _Knowledge of
Latitudes and Longitudes_, 1923, pp. 87–88.
]
METHODS OF FINDING LATITUDE AND LONGITUDE
That such observations were carried out is entirely possible, for there
is absolutely no doubt that methods of finding latitudes and longitudes
were well understood in theory and were sometimes put to practical use.
Rules are given for finding latitude in Az-Zarqalī’s _Canons_, in Plato
of Tivoli’s translation of the _Astronomy_ of Al-Battānī, and in many
other astronomical and astrological treatises.[1122] Two principal
methods were recommended. You may either measure with the astrolabe the
altitude of the sun above the horizon at noon at the spring or autumn
equinox and find the latitude by subtracting this angle from 90° or you
may measure the altitude of the celestial pole above the horizon, which
is the same as the latitude. As to longitude, the fact that there are
differences in local time between points east and west of each other was
recognized and clearly explained by several writers of our age.[1123]
The _Marseilles Tables_ give a rule for finding longitude by the
observation of eclipses. Roger of Hereford indicates that he himself, by
observing an eclipse in 1178, ascertained the positions of Hereford,
Marseilles, and Toledo in relation to Arin, the world center of the
Moslems.[1124] Gerard of Cremona describes a method of finding longitude
by noting the distance of the moon from a given point in the heavens and
thereby dispensing with eclipses,[1125] though it is doubtful whether
this method was used until the sixteenth century. The lack of accurate
instruments for ascertaining time must have rendered it extremely
difficult to calculate longitude under any circumstances. Making
allowances for this, it is surprising to find how accurate the few
coöordinates that have come down to us seem to be, if our interpretation
of them is correct.[1126]
The geographical interest of these figures and of investigations of this
sort was not appreciated by the majority of the men of our age. The
application of astronomical considerations to the problems of navigation
was still in its infancy. The purpose of the investigator of the twelfth
and early thirteenth century in finding geographical coöordinates was
astrological. He wished to make use of them to transpose tables made
originally for the meridian and parallel of one station to the meridian
and parallel of another. Their influence on the cartography of the age
was absolutely _nil_. It is probably safe to make the categorical
statement that the maps and geographical treatises of the century and a
half preceding the year 1250 were drawn and written with almost complete
disregard of any astronomical considerations whatsoever.