In 1936, both Patek Philippe and Vacheron Constantin were commissioned to develop a limited series of hour angle wristwatches for aviation testing. Author quattro meticulously compiles details and images of these five exceptionally rare timepieces, presenting them together for the first time in this comprehensive article. This deep dive offers a unique opportunity to appreciate these historically significant instruments and the horological prowess they represent.
According to
Christie's, in spring of
1936 both
Patek Philippe and
Vacheron Constantin were "approached by a private to develop, produce and supply a very small series of
hour angle wristwatches for testing".
These five watches are never presented together, so I thought it would be interesting to do so.
What exactly an hour angle watch is and how it works is quite complex (to say the least), so I've placed some explanations at the end (just what I've been able to understand).
A/ VACHERON CONSTANTIN
1/ CASE NO. 260919
Diam. 57 mm. Thickness: 15 mm.
Victorin Piguet based calibre 20''' M73, frosted gilt brass, 17 jewels, straight line lever escapement, beryllium balance with gold timing screws, selfcompensating Breguet balance-spring, index regulator.
credit: Antiquorum, Perezcope & Christie's 2/ CASE NO. 260918 & 260920 No.
260218 is
similar to no. 260919 above.
But no.
260920 features a
splittable centre seconds activated by the
split-button in the band.
The
whereabouts of these two watches are unfortunately
unknown. The photo below comes from the book
The World of Vacheron Constantin Genève , by
Carole Lambelet and
Lorette Coen.
credit: zhuanlan.zhihu.com
B/ PATEK PHILIPPE
Both watches shown below now belong to the Patek Philippe Museum
1/ MOVEMENT NO. 170381
Diameter: 55.3 mm
Splittable centre seconds activated through the crown.
Cal. 19''' nickel-finished lever movement numbered twice and stamped with the Geneva seal, 21 jewels, Guillaume balance with gold poising screws, swan neck regulator, wolf's tooth winding.
credit: Hodinkee, Diveintowatches, Quill & Pad, Christie's & shutterstock
2/ MOVEMENT NO. 170383
Diameter: 56 mm.
Calibre 19"' "Extra", Geneva Quality Hall-mark, rhodiumed, "fausses-côtes" decoration, 21 jewels, lever escapement, "Guillaume", balance, Breguet balance-spring, micrometer regulator.
credit: Hodinkee, Diveintowatches, Worn & Wound and Antiquorum
NO. 170381 (LEFT) & 170383 (RIGHT) SIBE BY SIDE AT THE PATEK PHILIPPE MUSEUM 
credit: Hodinkee
WHAT IS AN HOUR ANGLE WATCH AND HOW DOES IT WORK ?
An hour angle watch is a watch designed to help aviators determine their longitude while in flight. It was famously developed by Charles Lindbergh in collaboration with Longines after his historic transatlantic flight in 1927.
Since the Earth rotates 360° in 24 hours, the "hour" hand rotates once in 24 hours, indicating the degrees of arc against the center circle divided into 360°. Each "hour" therefore corresponds to 15° of longitude.
The "minute" hand rotates once every 4 hours, corresponding to 60° of longitude. So each "minute" =1° of longitude.
The "second" hand revolves every 4 minutes, corresponding to 15° of longitude. So each "second"=0,25° of longitude.
For example, in the photo below (hopefully, my calculations are correct...):
- The left watch indicates 10h, 18min and 32 seconds=300° + 18° + (32 x 0,25)°=318° + 8°=326° of longitude.
- The right watch indicates 10h, 28min and 7 + 18 seconds=300° + 28° + (7 x 0,25)° + (18 x 0,25)°=328° + 1,75° + 4,5°=334,25° of longitude.
The watch being accurately set to Greenwich Mean Time (GMT), for example via a radio signal, it indicates the longitude of Greenwich.
In the cockpit, the pilot uses a sextant and a nautical almanac to calculate the local solar time. Then, by calculating the difference between the local solar time and Greenwich Mean Time, he can determine his exact longitude.
I hope this explanation is roughly correct. At least, that's what I managed to understand...
Thanks for reading.
Best, Emmanuel
