Finding North with the GMT-Master: Why it works (longish)

[Jeff B]

A recent question about the GMT-Master-II’s ability to indicate North using its 24-hour hand finally got me thinking seriously about how this works. Here’s what I came up with.

There are two key concepts to keep in mind: (1) the GMT-Master’s hour hand revolves around the 24-hour hand once a day; and (2) in a standard time zone, “north,” 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’s 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 “remembers” 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’s 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’t 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’m in DST, it throws my north-pointing 15 degrees west, which I can easily adjust for. (That’s 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 “variation” that changes with location. Where I live, it’s 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’m 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’ve 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’s possible that professional navigators were aware of it at the time. But it’s yet another reason I think the remarkable GMT-Master is one of the most elegantly functional watches ever made.

[JBHII]

Jeff - great post, and long live the GMT!

[Jeff B]

Thanks, John! This was something I needed to do to find out how much confidence I could put in it. It looks like it's pretty good indeed!

[Sea-Dweller]

Jeff...brilliant explanation! Thank you very much!

[CJCJR]

Jeff,

I had heard about the GMT working as a compass, but never understood it until now. Thanks

[bullandvodka]

Jeff,

I like your style. Thanks for writing up such a nice explanation for everyone.

[timefinder]

Awesome write up! Thanks!

[Planet X]

This is a fantastic way to think about it, and to explain it. Are you a physicist?

Just one kind suggestion --- In your sentence,

"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,"

I think you meant to say, "If both hands point toward the sun at midnight, then the 24-hour hand will track the sun for 24 hours and return to the 24-hr position at midnight."

This makes a lot of sense. I live quite far north, and I also have a vantage point from my home that essentially allows me to see what the sun is doing all day --- and even into the night during the summer months, when it provides a visible glow over the horizon after sunset. With your explanation, and what I have observed the sun doing, I can now nicely visualize the GMT's compass function.

I must say I "understand" the GMT better now. I've been a diver guy for some time, but this really ups the ante for the GMT! At least for me, the compass aspect is way more cool than the "I can track two timezones at once" aspect.

Thanks again for a fantastic post!

Kindest Regards,
Planet X

[TimeRider27]

I bought my first Rolex which was a GMT Master II in 2004. I think the booklet still discussed the compass aspect, although I can't be sure.
They should use your explanation and put it back in the booklet.

I may have to go looking for that GMT again!

[gotleib1]

Now I have another reason to buy that GMT Master!

Thanks for the great post gentleman!