Willibrord’s Astronomical Horologium

One of the most famous manuscripts of the eighth century is Paris, Bibliothèque nationale de France, lat. 10837. It is famous because it contains the calendar of St Willibrord (d. 739) – an Englishman from Northumbria whose early story is told by Bede in his Ecclesiastical History (731). Willibrord travelled to Rath Melsigi in Ireland to study and then, in 690, travelled to the Frankish kingdoms where he became a missionary to the Frisians, bishop of Utrecht, and abbot of Echternach (nr. Luxembourg). Some of this is told in a biographical note of 728 in the calendar itself, which allows us to connect it to Willibrord’s circle.

The manuscript contains several other items alongside the calendar. Principally, these are a copy of the Hieronymian Martyrology and the oldest surviving Dionysian Easter table with its distinctive AD-dating. Surrounding the Easter table are a variety of notes about calendars and related matters.

It is here, on 42r, that we find Willibrord’s horologium. And I wanted to offer a brief account of it because it regularly circulates on the internet because it looks good. But few people really know what it does.

Here it is:

Willi Horologium

And here is a crude translation:

 Willibrord Horologium

The horologium is elegant. The user is to orientate themselves facing south, which is why south is at the top of the diagram. One does this by positioning themselves with the rising sun in the east somewhere to their left. But where? Sunrise varies depending on the time of year. The horologium therefore helps the user by representing the different possibilities for the solstices and equinoxes. But, of course, these also vary depending on how far north or south you live. It is therefore notable that the diagram represents an 18-hour solstice – longer than the solstice used in Roman or Greek science, but fitting for someone living in Northern Britain. This means that intellectuals in Northern Britain in the seventh century were not just absorbing the principles of Roman and Greek sciences, they were experimenting with them and adapting them. Why? Well, in this particular case, the clue might be in the times marked: sunrise, 3rd hour, 6th hour, 9th hour, and sunset – namely the canonical hours. The horologium was intended to be used to assist with liturgical observance.

The horologium is not just liturgical. The different heights of the Sun represented for the solstices and equinoxes suggests that this represents the heavens as charted using an armillary sphere, not a basic sun dial or star chart. This is unnecessary from a purely practical point of view. But then so are the glosses. North, east, south and west are given etymologies from Isidore of Seville, plus a note on religious symbolism, and the name of a corresponding Greek Hora (a goddess of the seasons) in Greek characters. These few notes are not extensive. They do, however, hint at the wider cosmological interests of the scribe.

There are other horologia from the period. None, however, is like this one. It therefore shows some of the creativity involved at the intersections of early medieval religion and science in the eighth century.

Further Reading: Barbara Obrist, ‘The Astronomical Sundial in Saint Willibrord’s Calendar and its Early Medieval Context’, Archives d’histoire doctrinale et littéraire du Moyen Age 67 (2000), 71-118.