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Epact Unpacked: A Self-Orienting Crucifix Dial

Spring, 2000

EPACT contains a number of sundials made in the shape of a cross. As devotional and practical objects, these crucifix dials had attributes of both the sacred and – literally – the secular. In an age when people were more sensitive to such meanings, it may be that the popularity of this form of dial was due to its combination of the temporal and the eternal. While the dial was used to find the time, it obliged the user to contemplate things eternal.

Crucifix dials can be of different designs and operate in different ways, but the most common form is equinoctial, that is, the plane of the dial is positioned parallel to the equator, and the gnomon, pointing to the pole, is at right angles to this. It must therefore be possible to set the dial at an angle which depends on the latitude (the angle with the horizontal plane is made equal to the complement of the latitude) and to align the gnomon with the plane of the meridian.

In the crucifix dial the corner edges at right angles to the plane of the cross are used to cast shadows onto the surfaces that join these edges. This leaves the front face of the cross free to receive an image of Christ. There are, in effect, six parallel gnomons and six surfaces marked with hour lines. The plane of the cross is elevated to the angle appropriate to the latitude and its main axis aligned with the meridian. Normally the instrument is hung as a crucifix from the neck or a belt but, when used as a dial, a hinge at the bottom allows one face of the hollow cross to be opened out to form a base, while a strut pivoted from the main section is set to a latitude scale on the base.

For the meridian alignment, most crucifix dials are equipped with small magnetic needles in round compass boxes that fit into the hollow body of the cross when it is closed. In these cases the registering faces of the cross are simply engraved with hour lines parallel to the gnomons. Depending on the time of day, the shadow cast by one of these corner lines will register the time on one face, with twelve and six o’clock defined by the axis and cross-piece respectively.

One crucifix dial in Epact, an Oxford instrument, has no compass, but has additional engraved lines on the faces registering the hours. The additional lines run along these faces at different angles, crossing the hour lines, and are marked with the signs of the zodiac; they follow the annual sequence according to the solar declination and are marked in pairs except for those at the extreme angles of declination at the solstices. The dial is by the Flemish maker Adriaan Zeelst, signed at Louvain and dated 1588.

At least one other instrument by Zeelst has these additional lines, which can be used to orient the instrument without using a compass. The user identifies the lines appropriate to the time of year, determines the relevant corner vertex formed by three faces at one end of the cross-piece (this requires a knowledge of whether it is morning or afternoon) and turns the dial, previously set for latitude, until the shadow defined by this vertex falls on the appropriate line. This can occur only when the cross is in the plane of the equator. At the equinoxes the target lines are defined by the edges of the main axis of the cross.

The same principle of self-orienting was used in the later, and much more common, equinoctial ring dial, but it can also be applied to the astronomical rings devised by the 16th-century Louvain mathematician and instrument maker Gemma Frisius.

It is not clear whether or not this self-orienting feature of some crucifix dials has been noticed before. It seems at least to be a novelty in Oxford, since the old label for this instrument states that ‘The compass is missing’. There is indeed a little square box, now empty, mounted on the base, but compass needles were generally housed in round boxes. It now seems more likely that this box brings us back to the devotional aspect of the instrument, as it could have been used to hold a tiny, sacred relic. In another crucifix dial in Oxford, included in Epact, the remains of such relics have even survived in a compartment within the cross.

A third Oxford crucifix dial in Epact – made by Markus Purmann and dated 1596 – is very similar in overall design to the Zeelst dial, but combines a magnetic compass with simple hour lines. An additional feature of this dial reminds us again of the symbolic importance of such objects for 16th-century users, and the connections they could make between their features.

On the front of the crucifix dial by Purmann is the usual image of Christ on the cross, but the back has a less common engraving of the brazen serpent Moses was told to present to the Israelites dying from snake bites in the desert. This is often understood today as a symbol of the physician, since the Israelites were healed by looking at the serpent. On the dial, however, it reminded the user that for the Christian, spiritual healing or redemption comes from Christ’s sacrifice on the cross and that this was prefigured in the Old Testament story.

J. A. B.