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The hour angle watch ranks among the rarest horological creations of all time. First sketched by Charles Lindbergh after his famous transatlantic flight, it allows longitude to be determined quickly, making it a valuable navigational instrument. We shed light on this mysterious complication, explain exactly what it is used for, and show you how to read an hour angle watch.
Navigation on the Wrist: The Beginnings
Whenever the hour angle watch comes up, the Longines Hour Angle Watch is never far behind: it was this watch that emerged from Lindbergh's designs in 1931, and it was most recently reissued in 2017.
Although other manufacturers tried their hand at the complication over the course of history (such as Patek Philippe in 1936), the Longines remains the de facto monopolist among hour angle watches to this day. The idea behind it dates back to the 1920s, when aircraft were able to cover ever greater distances and therefore needed increasingly precise navigational instruments.
A pilot of that era relied on three essential devices: first, a compass; second, a sextant to determine latitude; and third, an accurate timepiece to calculate longitude.
Initially, this role was filled by so-called observation watches (B-Uhren for short) - large instruments that specially trained navigators had to read as quickly as possible because of the aircraft's movement. To reduce this effort, Charles Lindbergh – who knew the difficulties of complex navigation all too well – developed the hour angle watch together with Longines.
Its design is so ingenious because it allows the Greenwich hour angle to be read directly – the most important value for calculating your own longitude.
But what exactly is an hour angle?
The Hour Angle Explained
The hour angle of a location always refers to the angle between a celestial body and the local meridian. This value is expressed as time, with 15 degrees corresponding to one hour. Since it is a relative value, it can be either positive or negative: it is always counted positive from the south toward the west, but negative toward the east.
A simple example: in our Greenwich case, the sun serves as the reference point. If the hour angle is -1, the sun will reach its highest point over Greenwich in one hour (because the Earth rotates 15 degrees in one hour). If, on the other hand, it is 1, that highest point was reached an hour ago. Intuitively, you can think of "0" as the hour angle at which the sun reaches its highest point at a given location, because it stands in a perfectly "straight" relation to the local meridian.
Calculating Longitude with the Hour Angle: The Basics
The principle that 15 degrees correspond to one hour is reflected exactly in the dial of the hour angle watch. Its scale runs from 0 to 180 degrees, with 180 degrees corresponding to twelve hours - exactly half a rotation of the Earth on its axis. The minute and seconds hands allow an even more precise position fix: the Earth rotates by exactly 15 minutes of arc in one minute, and one minute of arc is nothing other than 15/60, or 1/4, of a degree.
Such precise readings are all well and good, but why does the hour angle watch show the Greenwich hour angle in particular, rather than that of any other place on Earth? It is purely a matter of definition, because the longitude of Greenwich is zero. An observer standing on the Greenwich meridian at exactly noon sees the sun due south, so its hour angle there is zero degrees. Building on this, the longitudes of other places on Earth can be determined precisely if you know the time.
Example: Reading an Hour Angle Watch
A simple example shows how to calculate the Greenwich hour angle using the Longines Hour Angle Watch. Let's assume the watch reads exactly 5 o'clock and zero seconds (in the afternoon). The hour angle at Greenwich therefore amounts to 15*5 = 75 degrees. From this, you can deduce that the Earth has already rotated 75 degrees since noon, which allows you to determine your own longitude precisely. So much for the theory.
In practice, however, there is a difference between apparent and mean solar time, which varies depending on the date between plus 16 minutes on November 3 and minus 14 minutes on February 11.
To correct for these inaccuracies, the hour angle watch features a special rotating bezel: if the offset between apparent and mean solar time is, for example, plus 10 minutes, we turn the bezel 10 minutes clockwise and obtain a corrected hour angle reading.
Are There Alternatives to the Hour Angle Watch?
The unique design of the hour angle watch makes it one of a kind, with no equal – at least in the world of traditional watchmaking. While pilots have relied on far more precise GPS technology for decades, lovers of traditional timepieces have to make do with the very limited offering from Longines. Only 90 pieces of the latest 2017 re-edition were made. If you are interested in navigational instruments in general, however, there are certainly alternatives:
For example, the Sinn 856 B observation watch or the high-end "Équation du Temps Marchante" from Blancpain's Villeret collection. Priced at over €150,000, the latter allows apparent and mean solar time to be determined precisely.
For the hour angle watch, however, we see no prospect of larger production numbers: its great fascination and rich history are offset by a complex mode of operation that probably offers no one any everyday benefit. It is its exotic rarity that makes it so desirable for some collectors. Nevertheless, we love the hour angle watch: it shows us once again just how diverse and historically significant the world of watches truly is.
