Using a GMT and other background info

[Nikrnic]

Originally Posted by holdtime

+ 1

Very nice timely thread I really enjoyed reading. Now I want one.

Sent from my SM-G900V using Tapatalk

[yuditracp]

Nice info. As a Pilot, setting the Green hand to UTC help me a lot. But its true when moving the bezel for 3rd timezone, it is quite difficult to read the UTC time. Have to multiply by 2, good excercise for the brain tough :)

[Andad]

If I travel with my old '71 datejust I can wind the date backwards or forwards so a GMT models is not the only Rolex model that can do this safely and without breaking.

Are the new ceramic GMT models a step backward in GMT functions as their bezel increments don't easily handle countries with a half hour time difference?

Some say that Rolex didn't bother because of the small population of South Australia but I'm sure there are a few more buyers in India and other countries.

[torrente]

First of all, thanks to the OP and further posters for the comprehensive information.
As practitioner in celestial navigation and amateur astronomer (not astrologist ;-) ) I had something to add:

Originally Posted by petespendthrift

The sun rises in the East and sets in the West. At 1200 GMT the sun is at it's highest point in the sky directly overhead Greenwich.

The sun is at highest point around 1200 but not always at that exact time due to the eccentric elliptic orbit of the earth around the sun, the difference is expressed with the so called “equation of time” and the results are these time differences during the year:

Originally Posted by petespendthrift

Knowing this actually made it possible to use measurement of the Suns angle in the sky at a given time to calculate where a ship was (in Longitude ie. East/West) so long as the ship had an accurate clock called a chronometer set to GMT.

Regarding this, measuring the angle of the Sun to the horizon at the highest point provides:
- practically inaccurate longitude, as it is really difficult to know the exact second of the meridian pass, and to give an idea the position error per each second difference is several miles.
- an accurate latitude, clock is not needed.

Anyway to calculate an accurate position at other time you need the clock, a nautical almanac and depending of the calculation method nautic tables or a scientific calculator. The way to do it is to measure the angle of a minimum of 2 celestial bodies (sun, moon, planets, stars), ideally if they are horizontally around 90 degrees apart. After some quite complex calculations you get 2 position lines that cross in your position.

Just for information, there is a quite complex procedure to calculate time without clock measuring the angle of the moon with other celestial body, it is very imprecise but could be useful in emergency scenarios.

[kimquyet88]

i like this story , it's very nice