Slide rule on my watch - Seiko Prospex SBDR001

I just bought a Seiko Prospex SBDR001. I bought it mainly for the well integrated digital world clock and the analog watch. It allows you to easily transfer the time from the world clock to the analog watch and the navigation in the world clock is simple. The daylight savings is manual, but I've finally decided after being screwed by daylight saves one too many times that many I should keep my head around it, rather than trying to get my watch to keep track.

This watch has a slide rule. It allows you to multiply, divide, solve simple equations and do conversions very fast. I started fiddling it with it during take-off and landing when you can't use electronic devices and I discovered that I really enjoyed my first experience with a slide rule.

It also explained in the instructions that if you point the little hand at the sun, the point between the 12:00 mark and the little hand is South. (To the left in the morning and to the right in the afternoon.) That makes sense in retrospect, but I didn't know that.

Comments

[Marcus Saw]

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[MostlyVowels]

It also explained in the instructions that if you point the little hand at the sun, the point between the 12:00 mark and the little hand is South. (To the left in the morning and to the right in the afternoon.) That makes sense in retrospect, but I didn't know that.

A few caveats:

• The direction of the South so determined will be accurate only if you happen to be near the local timezone's base meridian (e.g. London or Greenwich or Cambridge for the 0° meridian, Kobe for Japan's 135° meridian etc.)

• In Western continental Europe, there will usually be a significant offset between the official time and the solar time. For example, Sevilla in Spain is in the CET timezone (GMT+1), whose base meridian is 15° East, even though the city's longitude is about 6° West. The direction of the South determined on a watch display, based on the Sun's position, will thus deviate by 15+6=21° towards the East relative to the true (geographical) South.

• When summer time is in effect, a timezone's base meridian is virtually shifted 15° towards the East (i.e. 30° East for continental Europe). A watch set to the local (summer) time in Sevilla will thus indicate a computed “South” that is offset by a whopping 21+15=36° relative to true South !

• When you take into account the Equation of Time effects (caused by the tilting of Earth's rotation axis relative to the Ecliptic plane, and the elliptic shape of Earth's orbit around the Sun), true solar time – i.e. the Sun's actual position — can lag the local mean sun time – computed e.g. with an atomic clock — by as much as 6 minutes — i.e. about 1°30' — in the summer.
In late July, the total deviation (or error) between the South indicated on a watch's dial and true South could thus be about 37°30' in Sevilla, 29° in Paris, 26°30' in Amsterdam, 19° in Rome, 18° in Berlin etc.

What you thus need to determine the South's direction, based on the Sun's position, is not an electronic gizmo like the Seiko, but rather a decent mechanical watch from the likes of Jaeger-LeCoultre, Breguet, Audemars Piguet, Patek Philippe etc. with an ”Equation of Time” display, as well as a map or a GPS receiver showing your place's exact longitude ;-)

Those who suggest that a magnetic compass might be a simpler and less expensive way to determine the South's direction, that would be usable even when the Sun isn't visible, are but philistines unable to appreciate the ingenuity, artistry and care put into the production of complicated mechanical watches ;-)

[Chris Hamilton]

Hi Joi,

I manage an Australian Mobile Phone Network and I am hoping you would be open to a link exchange? Please email me at the above address so we can discuss it further. :)

Looking forward to hearing from you.

Chris

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