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Thanks, James. That's a great article.

So if any of you were in charge of planting a monument in Taiwan to marke the Tropic of Cancer today, where would you put it? Not difficult at first glance: just look up the obliquity in a new Astronomical Almanac or equivalent and find the spot. But what about the gravitational deflection of the vertical?? Should we worry about that? As you can see in the map here, the DOV is substantial on the island of Taiwan. Wherever you see a bit of yellow, that's 60" of deflection. Bright red is 48". Deep aqua is about 30", which is common along that latitude in Taiwan. That's significant and comparable to the long-term changes described in the article. Should you place the monument based on astronomical latitude (which includes the deflection of the vertical) or should it be placed using true latitude (geodetic latitude, such as would be produced by a GPS/GNSS device)? Which choice makes the most sense for a public monument? Me?? I'm not sure!

Frank Reed

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