An Ancient Rule for Making Portable Altitude Sundials from an ‘Unedited’ Medieval Text of the Tenth Century

Abstract

The use of altitude dials that worked on same principle as cylinder for travellers may be proved for Christian Europe between tenth and eleventh centuries by 'Canterbury Pendant': a little portable sundial made of silver and gold found in 1938, during work of ground levelling of cemetery inside cloister of cathedral of Canterbury. This sundial shows only third, sixth and ninth hour of day' by a series of points along vertical columns assigned to months of year.The existence of other altitude sundials of same kind seems to be confirmed by words used by Byrhtferth of Ramsey (c. 970 - c. 1020) in a passage of his Manual: the sun ascends point by point on sundial, and again Observe, O clerk, how sun ascends point by point on sundial.2 To same monk was attributed, in 1563, Glossae to De temporum ratione of Venerable Bede (627-735);3 in these we read: 'The point [one of fractions of one hour] is so called from word 'pungendo'; because of this they make some points on sundials. The horologium is a series of hours shown point after point.4 Hermann Lame (Hermannus Contractus, 1013-54) described instrument for first time in a short but complete treatise.5Then there is a text, which until now has escaped attention of scholars, that describes in a clear way construction of an altitude dial of this kind. It is inserted in a little treatise doubtfully attributed to Bede: Libellus de mensura horologii. This brief composition (for convenience hereafter simply Libellus) was published for first time in 1563 in first tome of big work on Bede edited by Iohannes Hervagius (Johann Herwagen) from Basel6 and reprinted in second half of nineteenth century in vol. xc of well known series on patristic Latin texts edited by J. P. Migne.7 The Libellus, in fact, is a collection of different texts (not all on gnomonics) gathered under seven rubrics in this order:1. The first does not seem to have a title and it starts with: Si quem delectat horologium componere.82. The second, Ad merithem inveniendum, describes method of finding meridian line with 'correspondent altitudes' of sun, also known as 'Hindu circles'.3. The third, Horologium quod contra unumquemque mensem habet ad umbram humani corporis pede singularum horarum diei, is drawing of 'shadow scheme' falsely attributed to Bede. It is represented as a portable instrument.94. Then follows Concordia xii mensium, where preceding shadow scheme is reported in a textual way.105. The fifth, De signis et horis xii mensium, is a diagram of length of days, in equal hours, for every month and corresponding zodiacal sign.6. Then, De tribus diebus periculosis lists three dangerous days of year, when one risks death.7. The last one, De Aegyptiacis diebus, is a known text about so called 'Egyptian days', which lists all 22 days of year that are disadvantageous for human activities.12The drawing of item no. 3 and next text (no. 4) are found often together in a single folio,13 while text of no. 2 is added to many medieval gnomonical works independently from kind of sundial described.The contiguity of items 2, 3 and 4 has always drawn, automatically, attention of researchers on 'shadow schemes', which were so popular in medieval times.'4 For this reason first text of Libellus has not been studied carefully and sometimes entire compilation has even been mistreated as a disappointing gnomonic work.We do not know source consulted by Hervagius for his work, but whatever it was it seems now irreparably lost. For all sections relating to computus, Charles Jones suggested that Hervagius used a thirteenth-century manuscript, or manuscripts, from southern Germany, but we do not know anything about manuscript that Hervagius used for his publication of Libellus. …