192
astronomical observations (and the composition of the first comprehensive star catalogue) Hipparchos may have used an armillary sphere (Lloyd, 1984: 344–345),
which, similarly, would not be more precise than the error of the magnetic compass.
3
This margin of error becomes more evident if we consider the discrepancies
found between ancient star measurements. For example, Plutarch’s claim of his star
coordinates deriving from measurements he made using an armillary sphere has
been challenged by a number of modern studies as untrue (Duke, 2002: 36; Graßhoff,
1990; Rawlins, 1982: 359–373). Similarly, it has been noted that the discrepancies
in the measurements of the position of stars between Hipparchos’ Commentary to
Aratus and Ptolemy’s Almagest are too large and statistically correlated (systematic)
to be accidental. Instead, it has been argued that perhaps Hipparchos created a catalogue of star positions by taking measurements in equatorial coordinates and that
these were subsequently converted to ecliptical coordinates using analog computation (Duke, 2002). A discussion on the importance of precision in ancient Greek
astronomical calculations is superfluous here, but it is important to note that pursuing a higher degree of precision than the ancient Greeks would have been capable
of, is unnecessary and could introduce a meaningless and false sense of extreme
accuracy.
For all but one of the sites included in this study, no magnetic anomalies and no
systematic instrument error were detected. The only exception is the temple of Isis
at Dion (Greece), where a metal bridge has been constructed to give access to the
site, as a result of the rising water table. Since a magnetic compass is almost useless
in this environment, the orientation of this temple was deduced based on Google
Earth, using the compass readings only as a general guideline.
4
Magnetic readings were corrected to true azimuths by applying the relevant magnetic correction computed for the date and place of each survey.
5
The readings were
taken along the surviving walls of the structures and as close to the foundations as
possible. In order to minimise erroneous orientation measurements, multiple readings were taken for each structure (e.g., on either side of a wall and along more than
one wall). As a means of verifying the accuracy of each measurement, a minimum
of three readings (where there was agreement between readings) and maximum of
five (until there was agreement between more than two readings) were recorded for
each structure.
3 See for example the slightly later, first century BCE Taichu calendar in China, which seems to
have been created using an armillary sphere, but its measurements are one degree off from complete accuracy (Xiaochun & Kistemaker, 1997: 64). For a discussion on difficulties in obtaining
accurate measurements of stars using an armillary sphere consult Duke, 2002: 37–38.
4 In those cases where metal poles are used to rope off the temples (as for example at the Erechtheion
and the Parthenon in Athens), the survey permits granted entry to the structures, thus allowing sufficient distance between these objects and the points from where the orientation measurements
were taken. The multiple readings taken from several points in these structures, and their cross
referencing with Google Earth, confirmed the accuracy of the measurements.
5 Magnetic corrections were calculated using the online Magnetic Field Calculator of the National
Centres for Environmental Information (https://www.ngdc.noaa.gov).
E. Boutsikas
astronomical observations (and the composition of the first comprehensive star catalogue) Hipparchos may have used an armillary sphere (Lloyd, 1984: 344–345),
which, similarly, would not be more precise than the error of the magnetic compass.
3
This margin of error becomes more evident if we consider the discrepancies
found between ancient star measurements. For example, Plutarch’s claim of his star
coordinates deriving from measurements he made using an armillary sphere has
been challenged by a number of modern studies as untrue (Duke, 2002: 36; Graßhoff,
1990; Rawlins, 1982: 359–373). Similarly, it has been noted that the discrepancies
in the measurements of the position of stars between Hipparchos’ Commentary to
Aratus and Ptolemy’s Almagest are too large and statistically correlated (systematic)
to be accidental. Instead, it has been argued that perhaps Hipparchos created a catalogue of star positions by taking measurements in equatorial coordinates and that
these were subsequently converted to ecliptical coordinates using analog computation (Duke, 2002). A discussion on the importance of precision in ancient Greek
astronomical calculations is superfluous here, but it is important to note that pursuing a higher degree of precision than the ancient Greeks would have been capable
of, is unnecessary and could introduce a meaningless and false sense of extreme
accuracy.
For all but one of the sites included in this study, no magnetic anomalies and no
systematic instrument error were detected. The only exception is the temple of Isis
at Dion (Greece), where a metal bridge has been constructed to give access to the
site, as a result of the rising water table. Since a magnetic compass is almost useless
in this environment, the orientation of this temple was deduced based on Google
Earth, using the compass readings only as a general guideline.
4
Magnetic readings were corrected to true azimuths by applying the relevant magnetic correction computed for the date and place of each survey.
5
The readings were
taken along the surviving walls of the structures and as close to the foundations as
possible. In order to minimise erroneous orientation measurements, multiple readings were taken for each structure (e.g., on either side of a wall and along more than
one wall). As a means of verifying the accuracy of each measurement, a minimum
of three readings (where there was agreement between readings) and maximum of
five (until there was agreement between more than two readings) were recorded for
each structure.
3 See for example the slightly later, first century BCE Taichu calendar in China, which seems to
have been created using an armillary sphere, but its measurements are one degree off from complete accuracy (Xiaochun & Kistemaker, 1997: 64). For a discussion on difficulties in obtaining
accurate measurements of stars using an armillary sphere consult Duke, 2002: 37–38.
4 In those cases where metal poles are used to rope off the temples (as for example at the Erechtheion
and the Parthenon in Athens), the survey permits granted entry to the structures, thus allowing sufficient distance between these objects and the points from where the orientation measurements
were taken. The multiple readings taken from several points in these structures, and their cross
referencing with Google Earth, confirmed the accuracy of the measurements.
5 Magnetic corrections were calculated using the online Magnetic Field Calculator of the National
Centres for Environmental Information (https://www.ngdc.noaa.gov).
E. Boutsikas