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Comment by eps | Hacker News Reader

There's one complication that I've seen in another watch and that puzzles me a lot.

On its back this watch has sunrise/sunset times. These times are derived from a so-called sunrise equation [1] which depends on the day of the year and the latitude, and is expressed via trigonometry functions.

I can see how this can be translated into some sort of irregularly shaped gear(s) if there were a single variable (the day of the year), but I have no idea how one could also allow for different latitudes.

Does anyone know or care to guess?

[1] https://en.wikipedia.org/wiki/Sunrise_equation

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Many european apartment houses have stairway lighting that is on around dawn and dusk, but at other times you push a button for about 10 minutes of light. The timing of these things is not static but follows the sun, and older such equipment was mechanical and used what I imagine was simply a sinusoidally shaped gear.

I used to manage a small apartment block in Stockholm and remember setting latitude and day-of-year into ours to get the timing to agree with the sun, and to stay there. Here's a photo of a similar device:

http://www.aew-electrical.co.uk/wp-content/uploads/2016/08/e...

Here is example of time switch with solar dial [1][2]. Big Clive explains its internals a bit. The dial seems to be made for specyfic latitude as cams are set and only input is day of the year.

I think that it can be made to use latitude input with some more complex set of cams and user setable input.

1. https://www.youtube.com/watch?v=3i_vfsuCpJY 2. https://en.wikipedia.org/wiki/Solar_dial

I thought of that video too. Before electronics, mechanically calculating the movement of celestial objects was pretty common, and goes back a long time:

https://en.wikipedia.org/wiki/Antikythera_mechanism

I don't know what watch you have seen, but here is an explanation of a similar watch: http://newatlas.com/krayon-everywhere-watch-sunrise-sunset/5...

TL;DR: The factors such as latitude are user-programmable.

Thanks, haven't seen this one. There is no explanation per se, but this confirms that it's doable. Knowing the answer exists is half of the solution :)

PS. And it's only 6.5 mm thick!

This clock is really impressive, but it took me nearly 10 minutes to find out what time it is on the pictures. The graphical display of sunrise and sunset on the other hand is genius.

The watch is build to function at the desired latitude of the buyer. These aren't mass produced models, so it's feasible. Especially considering the price tag. :)

I would think, that the customers who can afford those watches tend to travel between latitudes.

I suppose, as they can clearly afford the time to do so.

Wondering the process of changing the geo location.. Or is it even possible once the watch is finished.

Amazing peace of art though..

I recommend watching this video if you're interested in learning about multivariate analysis using mechanical computers. It's very approachable and informative. https://youtu.be/s1i-dnAH9Y4

As others have mentioned, the watches are customized to the owner's preferred location. https://www.hodinkee.com/articles/vacheron-constantin-les-ca...

It's not really feasible for a small pocket watch but for larger clocks you could use a cam where rotation is the day of the year and the position along it is the latitude. For the Long Now clock they're using something similar for their solar noon synchronizer to correct for drift of the main clock.

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