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date: 22 March 2018

astronomical instruments

Although the introduction of an astronomical instrument (the gnomon, an upright stick for measuring shadow-lengths) is credited to Anaximander in the 6th cent. bce, reliable information on the form of such instruments comes only from the later Hellenistic era, with extant examples (mostly sundials; see clocks) and detailed descriptions in the works of Vitruvius, Heron, Ptolemy (4) and his commentators. As early as 432 bceMeton observed the solstice at Athens with a heliotropion, but this may have been no more than an upright pillar fixed on a level platform to mark the shortest shadow. We know nothing about the instruments used by successors of Meton, such as Callippus in the 4th cent., to determine the times of solstice and equinox, nor of those used by Timocharis and other early Hellenistic astronomers to measure stellar declinations. Perhaps the earliest instrument, apart from sundials, of which we have a detailed description is the device constructed by Archimedes (Sand-reckoner 11–15) for measuring the sun's apparent diameter; this was a rod along which different coloured pegs could be moved. However the ‘equatorial armillary’ at Alexandria (1) described by Ptolemy (Almagest 3. 1) may be even earlier: it certainly predates Hipparchus (3). This was a bronze, ungraduated ring fixed permanently in the plane of the equator, which displayed the time of equinox by the crossing of the sun's shadow.

Hipparchus is the first Greek who certainly used instruments employing the Babylonian division of the circle into 360 degrees. He is also known to have employed the ‘four-cubit dioptra’ mentioned in Almagest 5. 14 and described by Pappus and Proclus. This was a device for measuring the apparent diameter of sun and moon. Hipparchus also had some instrument for determining the angular distance between heavenly bodies, but this need not have been as elaborate as the ‘armillary astrolabe’ constructed for that purpose by Ptolemy (Almagest 5. 1), which is a representation of the principal celestial great circles by connected, pivoting, graduated bronze rings. Some type of graduated sighting instrument akin to that described by Heron in Dioptra would have sufficed (although Heron's instrument was intended principally for terrestrial surveying). It is probable that Hipparchus devised the plane astrolabe, the purpose of which was to tell the time at night from the stars' positions: although the earliest surviving description of this is from the 6th cent. ce (Philoponus), the underlying mathematical theory of stereographic projection is expounded in Ptolemy's Planispherium.

Other instruments described in the Almagest are the meridian ring (a graduated circle) and the plinth (a graduated stone quadrant), for measuring the sun's declination (1. 12), and a ‘parallactic instrument’ (similar to the medieval ‘triquetrum’) for determining the moon's zenith distance (5. 12). While none of these was elaborate, great skill and precision in making and fitting the parts and graduating the arcs must have been demanded of the craftsman, in order to attain the accuracy which Ptolemy expected from observations (agreement to within 10 minutes of arc with calculated values). Extant artefacts from the Hellenistic period demonstrate that a high level of craftsmanship was attained, notably the ‘Antikythera instrument’ (which is a device for representing, rather than observing, the motions of sun, moon, and planets).



D. J. Price and A. G. Drachmann, in C. Singer and others (eds.), A History of Technology, 3 (1957), 582–619.Find this resource:

Archimedes' instrument

A. Lejeune, Annales de la Société Scientifique de Bruxelles, 1947, 27–47.Find this resource:

Armillary astrolabe

A. Rome, Annales de la Société Scientifique de Bruxelles, 1927, 77–102.Find this resource:

Plane astrolabe

O. Neugebauer, Isis, 1949, 240–256.Find this resource:

Antikythera instrument

D. Price, Transactions of the American Philosophical Society 64/7 (1974).Find this resource:

M. T. Wright, Antiquarian Horology 18 (2003), 270–79.Find this resource:

M. T. Wright, Antiquarian Horology 29 (2005–2006), 51–63 and 319–329.Find this resource:

T. Freeth and others, Nature 444 (2006), 587–591.Find this resource:

T. Freeth and others, Nature 454 (2008), 614–617.Find this resource:

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