Hoptroff No.10
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Hoptroff No.10 Hoptroff No.10 Hoptroff No.10

Hoptroff No.10

The First Atomic Timepiece

The No.10 is one of the most remarkable timepieces in the history of horology: the first watch to be regulated by its own atomic clock.

It is accurate to one and a half seconds per thousand years. Not just marginally better than other watchmaking movements, but thousands of times so.

Not content with that, Hoptroff has created one of the most complicated watches ever conceived. The front dial alone has 28 pointers, ranging from celestial functions such as sidereal time and tidal harmonics to reports from on-board sensors for temperature, pressure, humidity and compass heading. The rear dial is due for completion in 2015 and is expected to contain a further 20 moving indicators.

The golden-drum case echoes the classical styling of Harrison’s Sea Watch No.1 or Patek Philippe’s Calibre 89. “If Huygens, Harrison or Breguet were alive today,” says managing director Richard Hoptroff, “I’m convinced they would have made the same technology and design choices.”

The No.10 was first conceived as a navigational aid. With only this extraordinary timepiece and a sextant, it is possible to determine one’s latitude and longitude to within a nautical mile – a traditional system, and keystone of horological accuracy, updated for the 21st century with the atomic precision of the Hoptroff No.10. It is a shame, then, that only twelve pieces will ever be made.

Currently sold out.  Interested parties should contact us about the second and final production run in 2016.

Dial Guide


The current 12-hour time (local and UTC selectable), as measured by the atomic clock.

  1. Hour, 1 – 12
  2. Minute, 0 – 59
  3. Seconds (atomic), 0 – 59


Sidereal time measures the rotation of the earth relative to the stars, rather than the sun.  Although it uses a “24-hour” clock dial, each sidereal “day” is actually 23 hours, 56 minutes 4 seconds long.  This is why the seconds hands (3) and (6) are not in sync.  Stars rise and set at the sidereal time every day.

  1. Sidereal hours, 0 – 23 (retrograde), as measured by atomic clock
  2. Sidereal minutes, 0 –  59 (retrograde)
  3. Sidereal seconds, 0 –  59 (pendulum)

LONGITUDO (Longitude)

Longitude measures the angular position east or west of Greenwich, UK.  This value is set with a sextant using the noon sight method, in order to compare local noon with a chronometer reference time, just as Harrison did, except with an atomic clock and without the need to refer to admiralty tables.

  1. Longitude – coarse degrees, 180°E – 180°W
  2. Longitude – fine degrees, 0° – 180°  (retrograde)
  3. Longitude – minutes, 0’ – 59’ (retrograde)

VIS RELIQA (Power Remaining)

  • Battery power remaining, 100% – 0%

ERROR HORÆ (Time Error)

From moment the time is set, timekeeping slowly drifts.  (In the case of atomic timekeeping, extremely slowly.)  The Time Error is a measure of the maximum degree to which timekeeping could have drifted. 

  • Time error, 10-9s – 10s (logarithmic)

ÆTAS LUNÆ (Moon Age)

The traditional moon age indication, in days since the new moon.

  • Moon age in days, 0 –  29½  (retrograde)

MERIDIES LUNÆ (Lunar Transit)

The local time when the moon will be highest in the sky.  Moonrise will be six hours before lunar transit, moonset will be six hours after.

  • Transit time, 00:00 – 23:50 (retrograde)


The local tide height.  Tidal harmonics vary throughout the globe, based on the relative positions of the sun and moon, and the local geography.  First modelled by Sir William Thomson in 1860, these eight tidal harmonics are stored for three thousand ports worldwide.  The closest port is selected for the indication.  It is the most complex complication in the timepiece.

  • Tide height, -½m – 15m

UMOR (Humidity)

  • Relative humidity, 0% – 100%, as measured by on-board instrument, with 24-hour historic animation


TEMPERIES (Temperature)

  • Temperature, -40°C – 50°C, as measured by on-board instrument, with 24-hour historic animation.  (Note: Usage below -35°C is not recommended due to battery chemistry.)


PRESSUS (Pressure)

  • Atmospheric pressure, 955mbar – 1045mbar, as measured by on-board instrument, with 24-hour historic animation


ANNUS (Annual wheel)

The annual wheel smoothly completes a rotation once per year.

  • Month, Jan – Dec.  Common constellation of solar transit, ^ – i (Aries – Pisces)


LATITUDO (Latitude)

Latitude is the angular position north or south of the equator.  After a noon sight is taken, the noon sun height is entered on this dial, and then the actual latitude is determined relative to the declination of the sun.

  1. Latitude – minutes, 0’ – 59’ (retrograde)
  2. Latitude – degrees, 90°S – 90°N 


DIES (Date)

  • Day of the month in the local time zone, 1 – 31 (perpetual).


AXIS SEPT (Due North)

  • Due north, as measured by on-board instrument, with 24-hour historic animation.  Selectable correction for magnetic deviation from true north


The most accurate ‘pendulum’ yet discovered is the wavelength of light emitted or absorbed during an electron’s transition between higher and lower energy states.  The No.10’s physics package measures the atomic transitions of Caesium gas held in a tiny vial.  An oven heats it to a constant 130°C to ensure that temperature changes do not affect the sensors.  A laser pumps the electrons into transition, and a microwave resonator then locks onto the transition frequency.  A digital signal processor then counts 4,596,315,885 such transitions to determine that a second has elapsed. 

In watchmaking terminology, it ticks at a rate of 1.6 x 1013 (16 million million) vibrations per hour, and the resulting tick is accurate to one and a half seconds per thousand years.

Why is this accuracy necessary?  For marine navigation.  Even the most accurate temperature compensated quartz movements drift by several seconds per year.  Every second inaccuracy equates to a nautical mile in navigational terms.

FURN (Oven)

  • Oven status:  Off, On, Stable.

LUX (Laser)

  • Laser status:  Off, On, Stable.

μUNDA (Microwave)

  • Microwave resonator status:  Off, On, Stable.

FLUX (Power)

  • Atomic clock power:  Charging, External Power, Discharging.


  • Timekeeping mode:  Pure Atomic (A), Atomic Disciplined Quartz (ADQ), Temperature Compensated Crystal Oscillator (TCXO).


  • Atomic resonance detection:  Off, On, Locked.



Control pushers.  Functionality to be finalized.



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