Jaeger-LeCoultre Gyrotourbillon

Part 2 - The Equation of Time

Text and photos by Ron DeCorte

March, 2005

The Jaeger LeCoultre "Gyrotourbillon" is truly a Grand Complication of significance. Including multi-axis tourbillon, instantaneous perpetual calendar with double retrograde date indication, equation of time, and 8-day power reserve with up and down indication. This watch is a tour de force that deserves a closer look to better understand the magic that makes it so special.

 In part #1 of JLC Gyrotourbillon we explored the technical aspects of the multi-axis tourbillon in this new JLC Grand Complication. If you haven't read part #1, I suggest you visit the article here. In part #2 we'll explore the equation of time complication.

Equation Of Time

Equation of time is the difference between mean time (the time we see indicated on our watches and clocks) and solar time (the actual time that the sun is at its zenith each day). In very simple terms if you place a stick in the ground and measure the length of the shadow cast by the stick, the shadow will be shortest at noon (mid-day) solar time.  But only about 4 times per year will your watch or clock indicate 12:00 coinciding with the solar time indication; late December, mid April, mid June, and late August. The true difference between mean time and solar time varies from about -14 minutes in late January to about +16 minutes in late October.

One might think that the four times per year when mean time and solar time coincide should be equally spaced through the year, about every three months. Careful examination reveals otherwise. In actuality three of these coincidences occur within a 41/2-month period, while only one occurs during the other 71/2 months. The longest period between coincidences is about 4 months, late August to late December. And the shortest is about 2 months, mid April to mid June. Why aren't these coincidences equally spaced throughout the year? First of all the Earth doesn't make a truly circular orbit around the Sun, in simple terms it's an-egg-shaped orbit, and the Earth's tilt is not consistent. Other variables such as the Earth's proximity to other planets, leap year, your physical location within the time zone your watch is set to, etc, all have an influence on the -exact- difference between mean time and solar time, mean/solar difference (M/SD). Exact M/SD tables are available for each year and are quite complex. Hence all mechanical watches with equation of time use a single cam that is calculated for an -average- and will rarely be absolutely exact. Any questions so far?

Obviously the equation of time is not a simple task for a watch to track. On early watches this was accomplished via a single hand operating in a sector, controlled by a kidney shaped cam making one revolution per year, indicating the difference between mean time and solar time in minutes. This worked fine for pocket watches with their large dial space but isn't easily readable on a wristwatch. JLC has devised a very clever mechanism for their Gyrotourbillon (Gyro) that transfers the M/SD from a kidney shaped cam to an extra minute (solar) hand showing the M/SD in direct comparison to the regular minute hand.

The thin solar minute hand with sun shaped pointer indicating a M/SD of about +6 minutes. In this particular case the solar noon will occur 6 minutes before mean time noon. This incidence can only occur two times each year, mid September and early December. Notice the perpetual calendar shows a date of December 5th.

Here we see the kidney shaped equation of time cam (and month cam for the perpetual calendar that we will explore in part #3 of this series) mounted on a wheel assembly that makes one revolution per year.

Click here to enlarge the drawings for greater detail

How it works:

Equation of time cam "A" makes one rotation per year.

Arm "B", with jeweled contact beak "C", follows cam "A". Spring "L" keeps the beak in constant contact with the cam.

Teeth on the internal perimeter of arm "B" (look closely) transmit small amounts of correction from cam "A" to planetary wheel-pinion "D" meshing with pinion "E" that carries the solar hand "F".

Wheel "G" and platform "H" are fitted together forming a carriage for planetary wheel/pinion "D". This carriage also carries the hour hand and makes two rotations per day.

Minute wheel-pinion "J" meshes with minute pinion "K", creating a 12/1 ratio between the mean time minute hand and hour hand. (There are12 revolutions of the minute hand for each revolution of the hour hand).

Three wings "M", on arm "B", are for planar support (see below)

In summary:

Planetary wheel-pinion "D", carried by carriage "G"/"H" making two revolutions per day, creates a 12/1 ratio in conjunction with pinion "E" for the solar minute hand "F", with solar minute corrections supplied via the equation of time cam "A".

A technical note:

In the above description I refer to "planetary wheel-pinion "D". This is another way of saying planetary-gear. In planetary-gearing one gear has a stationary axis of rotation (in the above case the internal teeth of arm "B") while another gear has an axis of rotation that orbits around it (in this case wheel-pinion "D").

The fact that the rotation of arm "B" is irregular (being controlled by cam "A") we create a variable differential for very precise corrections of the solar minute hand.

A detail of the assembly, from the bottom.

Note: The three "wings" on Arm "B" ride on convex ruby jewels for planar support, while 6 ruby pins supply lateral support.

A few facts:

• Over the course of a year the solar minute hand and mean time minute hand will make the same amount of revolutions (about 8,760).

• The solar minute hand will deviate from the mean time minute hand a total of 30 minutes per year (from -14 to +16 minutes).

• There are more than 50 components in the equation-of-time assembly.

• The total length of the entire equation-of-time assembly is about 20mm!

Stay tuned for Part 3, where we will explore the perpetual calendar mechanism.