Testing a Heliochronometer

Pilkington & Gibbs Heliochronometer

Back in the early 1900s, heliochronometers were all the rage for those seeking some extraordinary piece for their garden or other vista-filled location. This particular instrument was built by the Pilkington & Gibbs company of London. Only about 1000 of these were made by the company – sadly much of the brass was melted down to support the war effort and the making of artillery shells. This unit is not only intact, but is in 100% working condition, another rarity, as many of these were left exposed to the elements where they would slowly get coated by a green oxidation patina.

Alignment with True North

Using these instruments was quite simple once it had been properly installed. Installation requires that their primary rotation axis be parallel to the rotation axis of the Earth, aimed at true north and with the correct angle to account for one’s latitude.  I accomplished this easily enough with the gentle use of a wrench, a screw driver and compass, being sure to dial in first the correct magnetic variation for the compass for our location.

Setting the date.

In use, one first sets the calendrical date on the smaller inner dial: June 16th. This slides the gnomon back and forth to account for the equation of time(!): quite a remarkable design!

Before and after alignment: Casting the pinhole image of the Sun onto the screen by rotating the main dial.

One then rotates the larger dial to project the dot of sunlight cast through a hole in the gnomon (sight vane) onto the opposing vertical line on the center of the screen vane. One then reads the hour and minute on the brass gauge on the outer edge of the main dial. Simple and accurate! In this case, it was within two minutes, likely because I have yet to adjust the minute readout-scale to its proper position for our longitude within the time zone.

Aligned and reading very close to the correct time… and yes, the watch is set to local daylight savings time, one hour ahead of the solar time read on the heliochronometer.
Reading the minute across from the hour. Very accurate!

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