Finding North with the GMT-Master Why it works (longish) - WATCH TALK FORUMS - Best Watch
Filed under Uncategorized by kaef12 on 09-02-2010
Tags : Bedat & CO, Collection, Girard Perregaux, Gucci, Longines
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A recent question about the GMT-Master-II抯 ability to indicate North using its 24-hour hand finally got me thinking seriously about how this works. Here抯 what I came up with.
There are two key concepts to keep in mind: (1) the GMT-Master抯 hour hand revolves around the 24-hour hand once a day; and (2) in a standard time zone, 搉orth,?in the northern hemisphere, is for all practical purposes identical with the direction toward the sun at local midnight, when it is hidden on the opposite side of the earth.
Focus only on the GMT-Master抯 hour and 24-hr hands. The hour and 24-hr hands are aligned at the same time only once a day: at midnight. If both hands point toward the sun at midnight, and if I keep the 24-hr hand in that position, then the hour hand will track the sun for 24 hours and return to the 24-hr hand position at midnight. The 24-hr hand 搑emembers?where the sun was at midnight relative to the hour hand. (It does not matter that I have to rotate the watch case counterclockwise to keep the 24-hr hand pointing to where the sun was at midnight.) So, conversely, if I point the hour hand toward the sun at any other time of day, the 24-hr hand will point toward where both hands were at midnight, and midnight, by definition, is north. And there we have it.
In the southern hemisphere, it抯 a little trickier. I saw an old GMT-Master brochure that describes the north-pointing feature and then states that in the southern hemisphere it works the same but with the 24-hour hand pointing south. This is true, but only if the watch is flipped face down! Perhaps this little complexity is why Rolex doesn抰 discuss the truly fascinating compass feature in recent GMT-Master manuals.
For the north-pointing to be accurate, the watch has to be set to local standard time. If I抦 in DST, it throws my north-pointing 15 degrees west, which I can easily adjust for. (That抯 one hour on the 24-hr bezel scale.) After this, other sources of error are my location inside my standard time zone, and the equation of time. Overall, I estimate the worst-case pointing error to be about 11 degrees. Usually, it will be much less. For comparison, a magnetic compass can have a significant 搗ariation?that changes with location. Where I live, it抯 about 11 degrees west, but in the U.S. it can be more than twenty degrees, both east and west. If I used a compass without factoring that in, then my GMT-Master would show me true north at least as accurately, and usually more accurately, than a compass! But I can use the GMT-Master anywhere to find true north. All I have to know is what time zone I抦 in, which is easier to know than the magnetic compass variation! If I just use the quickset hour feature in different time zones, by the way, I have to estimate where the 24-hr hand would really be in local time.
This is one of the subtlest, yet most profound little things I抳e come across in a long while. It reaches right to the heart of astronomical timekeeping. I seriously doubt that Rolex designed the GMT-Master with this feature in mind, though it抯 possible that professional navigators were aware of it at the time. But it抯 yet another reason I think the remarkable GMT-Master is one of the most elegantly functional watches ever made.
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