On an ancient sundial unearthed at Pompeii

Abstract

An ancient horizontal marble sundial unearthed at Pompeii is described and discussed in terms of the following: the meaning and astronomical significance of the Greek shorthand inscriptions on the object, and its design and functioning as a time-keeping device on the basis of hyperbolic shadow projection by a vertical gnomon. Detailed calculations are carried out for the epoch just prior to the volcanic eruption, of the astronomical coordinates of representative stars in the constellations recorded on the sundial, as well as the solar coordinates required for the evaluation of the shadow markings. These calculations not only confirm the high quality of the workmanship displayed in the construction of the sundial, but enable an accurate estimate to be made of the original length of the missing gnomon: 2 digiti = 1/8 pes in Imperial Roman units.

Keywords:

• horizontal marble sundial
• Pompeii
• Greek astronomical inscriptions
• shadow markings
• gnomon length
• constellations
• astronomical coordinates
• Greco-Roman hour unit

ACKNOWLEDGEMENTS

The author is indebted, firstly, to Dr Alexander Jones, Leon Levy Director of the Institute for the Study of the Ancient World and Professor of the History of the Exact Sciences in Antiquity at New York University, and to the Publications and Interpretive Manager of the Institute, Ms Kianoosh Hashemzadeh, for their kind permission to use Figure 1 for this publication. Moreover, their careful attention and helpful responses to my various queries are greatly appreciated. The image of the horizontal sundial was obtained from a photograph taken by Guido Petruccioli with permission from the Museo Archeologico Nazionale di Napoli. Details of the recent display of the sundial in New York are mentioned in the Preface.

The author also wishes to thank Dr John Egger for assistance with the Greek orthography, in particular with the diacritics.

A debt of gratitude is owed to my benefactors for their legacies of the fine collection of classical texts from the nineteenth century cited in the article: to the late Pentz family of Wellington in the Cape (Martha G. Pentz, John M. Pentz and J. Fred Pentz), and to the late Revd Fr Ernest O. Pugsley. All of these are held in affectionate remembrance by the author.