June 16, 2026
Description
Update (9-02-26): Fixed a tolerance issue between the hour plate & alidade. The fix is found under: PG_heliochronometer-hour_plate_assemblyv2.3mf & PG_heliochronometer-hour_platev2.stl
Update (31-03-26): (Optional Improvement) There is a new adjustable Knife-Edge design for the P&G Helio-Chronometer Cam Follower. It allows for small displacement adjustments between the follower knife-edge & cam to compensate for any wear or mechanical tolerance issues associated with the EoT Adjustment Mechanism. These small angular offsets can easily be calibrated out of the original design, or one can adopt this upgrade, which provides a means of applying a mechanical correction. Check out: https://www.printables.com/model/1657933-adjustable-knife-edge-for-the-pg-helio-chronometer
Update (01-04-26): A few corrections & clarifications needed to be made in the original Assembly Instructions. A big thank you goes out to https://www.printables.com/@DavidRoberts_4452875 for pointing those out to me. You will find the updated version under the file name: Assembly Instructions for the 3D Printed Replica of the Pilkington & Gibbs Helio-Chronometer v2
Update (03-04-26): The slots on the minute sector were slightly lengthened to enable full movement between one set of mounting holes and the next on the retaining ring. The updated files are: PG_heliochronometer-minute_sector_assembly_v2 & PG_heliochronometer-minute_sector_v2.
Update (12-05-26): (Optional Improvement) Removed the 5-day tick marks on the original month plate. They are not necessary & can create some confusion when lining up dates on the day sector. The new files are PG_heliochronometer-month_plate_assembly_v2.3mF, PG_heliochronometer-month_plate_v2.stl & PG_heliochronometer-month_plate_text_v2.stl.
The Pilkington & Gibbs Heliochronometer represented a technical pinnacle of early modern sundials, combining mechanical ingenuity with practical timekeeping capabilities. Its integration of latitude, longitude, and Equation of Time corrections via a mechanical cam assembly allowed users to determine mean time with remarkable accuracy; i.e., < 1 minute.
Note that this version is only compatible for use in the NORTHERN HEMISPHERE. I plan to publish a southern hemisphere version at a later date.
It is such an interesting sundial, but very few remain in existence today. My goal in this design was to promote its inner workings to future generations through a 3D-printable version. Also, check out the Reference Section at the end of this article for valuable design details.
Figure 1: Original Pilkington & Gibbs Helio-chronometer Advertisement
This Heliochronometer functions as a universal sundial, adjustable for latitude, longitude, and the annual Equation of Time (EoT). A cam mechanism applies the correction, enabling the conversion from solar to mean time without mental calculations. The replica measures approximately 210 x 210 x 300mm (L x W x H) and closely mimics the original design in features and stature. Under a PETG construction, it weighs in at approximately 730 grams.
Rotating Discs and Arms: Marked with hours on the larger disc and adjusted for the months on the smaller disc, light passing through holes at the Sight & aligned on the Screen indicates the exact time on the main Hour Plate;
Latitude Adjustment: Achieved via a quadrant on the base to set the upper disc elevation appropriately; Type-1 consists of a bowl mount, while Type-5 consists of a crescent mount. Both variants are provided in this design;
Meridian Alignment: With proper orientation to the standard time meridian, the sundial accurately indicates Local Mean Time;
Figure 2: Key Component Breakdown of Original with Type-1 Bowl Mount (from original)
EoT Adjustment System consisting of a cam, cam follower & spring mechanism: The system applies adjustments for seasonal and longitudinal differences. The cam is a mechanical replacement for the analemma curve engravings found on most heliochronometers. It was derived mathematically using spreadsheets, and a Python script was used to generate a 3D model of the cam. If you want to know how this was done, or develop your own custom cam, check out this link: https://www.printables.com/model/1647637-a-mechanical-cam-for-a-helio-chronometer-sundial
Figure 3: X-ray View of the Replica EoT Cam (kidney shape) situated beneath the Month Dial, also showing the Day Sector, which is fixed to the main hour plate. Once the Month Dial Index Mark is aligned to the day of the month on the Day Sector (Δdays), the proper EoT correction (Δt) is applied to the Alidade's Knife Edge through the cam profile.
Figure 4: X-ray View of the EoT Mechanism showing the Positions of the Sight (at the Fixed Pivot) & Screen (at opposite end of the Alidade). Also shown are the Month Plate with Cam, Hour Plate and Vernier Minute Sector (minus Day Sector). The Alidade's Cam Follower Knife Edge rides against the cam under constant tension provided by the Spring.
George James Gibbs (1866 – 22 February 1947) was an astronomer, engineer, inventor, and public science lecturer. In 1906, Gibbs applied for a patent for a novel sundial design—the Universal Equinoctial Mean Time Helio-chronometer (or Heliochronometer), commonly known as the Gibbs Heliochronometer—capable of indicating mean time to within about one minute at any latitude and season. The instrument addressed a practical need that arose after the standardization of time in the 1880s, a problem not fully resolved until radio time signals became widespread in the 1920s. Lacking the capital to commercialize the device on his own, Gibbs partnered with his employer, William Renard Pilkington, and together they established Messrs. Pilkington & Gibbs Ltd. The helio-chronometers were sold for prices ranging from £7 10s to £15 15s (approx. $1K USD today) and were marketed to an elite clientele, with promotional material noting commissions from prominent patrons, including HM The King.
The instruments were promoted both as precision timekeepers and as ornamental features for large country estates, aligning well with early twentieth-century landscape design trends and appearing in gardens designed by figures such as Mawson and Lutyens. Tensions later emerged within the partnership when Pilkington sought patents for similar devices in 1911 and 1913, prompting Gibbs to note that these efforts were attempts to avoid paying him royalties. Manufacture ceased with the outbreak of the First World War in 1914, and there is no evidence that the partnership continued after 1923. Although approximately 1,000 helio-chronometers were produced before the war, many were subsequently lost due to metal recycling drives during the Second World War. Unfortunately, only about 50 are known to exist today, with roughly only 30 to 38 of those likely to be in working order (or easily made working) today.
(The following text is from the original P&G instruction booklet.)
(Refer to Figure 4) Mounted on a base, but free to slide round on it, is the face or hour plate A, on which there are two vertical arms called a sight & screen at E and F, respectively. On sight E are two (or three) small holes, and the instrument is placed such that the sun may always cast a beam of light through one of these holes, according to the season, onto the other screen F. The larger plate A has the hours engraved around its circumference, and the smaller plate B, mounted on plate A, carries the months. Minutes and days are found on the fixed sectors, C and D, respectively.
(The following text is also from the original P&G instruction booklet.)
(Refer to Figure 4) The method for reading the time is relatively simple. Plate B is rotated until the correct day of the month is shown on sector D. Using the thumb grips, the large plate A is then rotated to get a beam of light through one of the holes at sight E to shine onto the center line inscribed on screen F. When the spot of light is bisected, the true clock time can be read from sector C, a 5-minute magnifying Vernier with a 30-second resolution.
Figure 4: Plan-View Schematic of the Heliochronometer (from original)
Definitions:
A — main hour plate or dial;
B — inner rotating month plate or dial with cam located underneath;
C — fixed minute sector on the periphery;
D — fixed day sector on the main hour plate;
E — moving sight with dual (or triple) seasonal pinholes;
F — fixed screen with engraved bisecting line.
Incoming sun rays pass through E and form a spot on F. Multiple seasonal sun positions are indicated, labelled by season; i.e., summer, equinox, winter, with rays converging toward the pinhole screen E, illustrating how different solar elevations still produce a usable illuminated sun projection.
Note: The dials & crescents have two print options: i.e.
For dual-color parts like the dials & crescents, use the 3MF files. They superimpose the main dials/crescents with text as separate STL's such that different print colors can be assigned;
If you prefer engraved parts only, just use the main parts without the separate text;
Infill: 15-20% is sufficient, except for the alidade arm, cam follower & spring, which should be at 100% for added strength & durability;
Layer Height: 0.20mm is sufficient;
Infill: Grid pattern;
Material: PETG. For added outdoor protection, consider applying a transparent UV-resistant protective paint over the printed parts. If you are getting delamination, or too much stringing, or blobs on your prints, then try using the following settings on your printer:
Refer to the instructions provided in the file section.
(The following text is from the original P&G instruction booklet, including additional alignment details found at reference 2)
Ensure that the pedestal P (refer to Figure 5) is level and stable and that the central rag bolt R (without the nut) is securely fitted in the pedestal. If the rag bolt feels loose, refit it using two-part epoxy resin, protecting the threads with insulation tape during this operation. Using an engineering-quality spirit level, check the cardinal directions on the pedestal.
Figure 5: Pedestal Installation Cross-Section (from original)
Using the 3 holes in the webs of the lower base plate G disposed at 120° (refer to figure 6), prepare three stainless-steel M5 x 15 mm long grub screws.
Figure 6: Grub Screw Locations on Lower Base Plate (from replica)
Mount the lower base plate G loosely on the rag bolt and adjust the three grub screws sitting on copper pads (or old coins) until the base plate is perfectly level. In practice, the grub screws would be nearly flush with the top of the base plate to prevent interference when installing the upper base plate. Fit the rag bolt nut and tighten cautiously: check the level and use the adjusting screws to correct it. This is a precise method of levelling with good control, superior to the use of shims that tend to skid at final tightening. If this is to be a permanent installation, mark the angular position of the base plate on the pedestal, then remove it. Protect the rag bolt threads and the six tapped clamping holes with insulation tape on both sides. Prepare some hard mortar or, preferably, epoxy, put appropriate quantities in the required places on the pedestal, then refit the lower plate in the marked position, pressing down until it bottoms on the levelling screws and pads. Fit the rag bolt nut and tighten cautiously, checking the levels. Ensure that no epoxy or mortar sets on the visible machined surfaces. As the epoxy sets, a small additional turn will transfer the load from the levelling screws onto the greater area of epoxy.
Place the pedestal or support firmly in position to receive maximum sunshine. The two or three hours about noon are the most valuable, and care should be taken that the low winter sun is not obstructed;
Place the upper base clamp with the instrument on the base plate G and put the six screws loosely connecting both, such that the instrument can rotate freely;
Adjust the instrument's elevation for the latitude at which it will be installed;
Adjust the minute sector circle C + or - the required number of minutes to compensate for your longitude offset from your standard time meridian. You will need to remove the screws & re-secure the minute sector using the series of holes located along the edge of the retaining ring;
Set the month dial B (refer to figure 4) to the current date and the hour circle A to the true clock time, then turn the instrument bodily round from the base clamp until the sun throws a spot of light through one of the holes at sight E, exactly on the centre line of screen F. Tighten the six screws to secure the upper base clamp;
Adjust your minute sector to account for any Daylight Saving Time.
1. M. Nadeau: Making a Working 3-D Printable Replica of the Pilkington & Gibbs Helio-Chronometer, BSS Bulletin, 38(i), 8-12 (March 2026).
2. British Sundial Society Bulletin, Volume 18(ii), June 2006, A REVIEW OF THE HELIOCHRONOMETERS BY PILKINGTON & GIBBS, Part 1. The design and accuracy of the Gibbs Helio-Chronometer, GRAHAM ALDRED.
3. British Sundial Society Bulletin, Volume 18(iv) December 2006, MAKING REPLACEMENT SPRINGS FOR A PILKINGTON & GIBBS HELIO-CHRONOMETER, TONY MOSS.
4. Photographs from: https://equation-of-time.info/cams
License:
Creative Commons — Attribution — Noncommercial — Share Alike
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