Instruments · Mean-time sundial
Ferguson solar chronometer, Serial Number: 183 Registered No. 576942
- Maker / attribution
- Ferguson, James; J H Steward (maker); James Ferguson (maker); J H Steward (maker)
- Date
- 183
- Historical period
- Classical Antiquity (500 BCE – 500 CE)
- Country of origin
- England
- Place
- England, United Kingdom, England
- Object type
- Mean-time sundial
- Dimensions
- overall: 90 mm x 100 mm x 120 mm,
- Holding institution
- Science Museum Group, London
- Accession / object no.
- 1951-302
- Credit line
- Miss E. Court
- Catalogue title (original)
- Ferguson solar chronometer
Record summary
Held and catalogued by Science Museum Group, London (object number 1951-302), this is a mean-time sundial by Ferguson, James; J H Steward, made in England, United Kingdom, England, dated 183. The Atlas places it in the Classical Antiquity period (500 BCE – 500 CE) of its Instruments collection. Recorded dimensions: overall: 90 mm x 100 mm x 120 mm. The record is illustrated with 1 photograph.
Assembled from the source record's structured fields. Dates and attributions are given as the source states them; where a field is missing the Atlas leaves it blank rather than estimating.
Catalogue description
The source record does not include a narrative description. The structured fields above are reproduced from it.
Historical context
Classical Antiquity (500 BCE – 500 CE)
Greek and Roman engineers refine the clepsydra; Ctesibius adds float regulation; the Antikythera mechanism models the heavens.
Greek astronomy produced the theory behind sundials and the astrolabe; the Antikythera mechanism (second or first century BCE) shows the sophistication of Hellenistic geared astronomical models.
Read the period in the timelineMore instruments from this periodEarlier: Ancient Civilizations
Atlas type guide
Sundials and solar time-finders
A sundial tells time from the position of a shadow cast by a gnomon or style. Because the sun's apparent motion depends on latitude, every fixed dial is laid out for a particular latitude, and portable dials either carry a latitude scale or are made for one region. Sundials show local apparent solar time, which drifts against mean (clock) time by up to about sixteen minutes over the year — the equation of time.
Portable dials were the pocket instruments of the sixteenth to eighteenth centuries. Nuremberg and Augsburg workshops produced folding ivory diptych dials with inset compasses; English and French makers sold brass horizontal dials, ring dials and the Butterfield type; Japanese and Chinese makers had their own traditions of compass dials. After mechanical watches became reliable, dials remained in use for setting them, and nineteenth-century heliochronometers were built to correct directly for the equation of time.
What to look for
- The latitude engraved on the dial plate or a latitude arc on the gnomon — it shows where the dial was meant to be used.
- The type of hour lines: equal (equinoctial) hours, Italian or Babylonian hours counted from sunset or sunrise, or unequal temporal hours on early and Islamic instruments.
- An inset magnetic compass and whether a magnetic-declination mark is drawn on its card — a useful dating clue because declination changes over time.
- Maker's signatures and town marks: Nuremberg compass-makers (Kompassmacher) often struck a personal mark and the town's punch.
- Additional scales such as lunar volvelles, wind roses, perpetual calendars or lists of cities with their latitudes.
Dating clues
- Ivory diptych dials are characteristic of Nuremberg between about 1500 and 1700.
- Butterfield-type dials with a bird-shaped adjustable gnomon point to Paris workshops from the late seventeenth century onward.
- Equation-of-time tables engraved on the plate became common after the mid-seventeenth century, once pendulum clocks made the difference between solar and mean time obvious.
General guidance written by the Atlas editors about this kind of object. It is not a statement about this particular piece; for that, rely on the source record above and on physical examination.
Read next:The history timeline
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