A well-traveled geophysicist recounts his curious findings at a prominent geographic benchmark in the Southern Hemisphere.
Most globes of Earth show four dashed lines that, aside from the equator, denote special latitudes related to the seasons. One of those, the Tropic of Capricorn, is the southernmost latitude at which the Sun shines directly overhead during the December solstice. It’s currently at 23° 26′ 12.3″ south, a latitude that’s decreasing (moving toward the equator) by 0.47 arcsecond (0.47″) per year.
Last month, while on a Sky & Telescope tour to see this year’s total solar eclipse, we stopped at a Tropic of Capricorn sign about 60 km south-southeast of San Pedro de Atacama, Chile, on Highway 23, and 50 km south-southwest of ALMA, the Atacama Large Millimeter/submillimeter Array of 66 radio telescopes high in the Andes.
The sign, shown at right, reads “Lat. 23° 26′ 16″ [south]”. It turns out that this was indeed the exact latitude of the Tropic of Capricorn in 2011. The sign is in good condition and might have been erected in 2011. So far, so good.
The GPS receiver on my phone, however, noted that the sign is actually located at about 23° 26′ 49.1″ south. That’s about 1.13 km south of the current Tropic line. (I also measured the longitude to be 67° 59′ 44.1″ west.)
You might ask how accurate my GPS receiver is. Since the U.S. Air Force removed the “dither” from the GPS signal in 2000, single fixes on hand-held devices have had a 50:50 horizontal uncertainty of a few meters. (That is, about half of the fixes are within a few meters of the true location.) On a Google satellite image, the latitude and longitude of the Tropic monument on Highway 23 are within a few meters of the values measured by my GPS receiver.
One uncertainty might be the geodetic datum, or global reference system, used by the Chilean government. In my experience working in various parts of the world (but not Chile), local geodetic datums rarely differ from the WGS84 ellipsoid by more than 100 meters.
So . . . I conclude that the Chilean highway department’s workers did a sloppy job of locating the sign.
They aren’t alone. In April 2019, I visited a Tropic of Capricorn marker in Namibia on road D1265, about 60 km west of Rehoboth. The sign, seen at upper right, was at 23° 28′ 0.3″ south, about 3.33 km south of the current tropic line. (The longitude was 16º 30′ 31.5″ east.)
In 2013, I visited another Tropic of Capricorn sign on highway C14 in Namibia, about 140 km west of Rehoboth. It was at 23° 30′ 0.4″ south, almost 7 km south of the Tropic line. (The longitude was 15º 46′ 19.8″ east.) I presume someone read that the Tropic of Capricorn was “about” 23½° south and hence put the sign there. The turnoff for the sign is visible on Google satellite images.
So, at signs marking the Tropic of Capricorn on highways in Chile and Namibia, neither sign was within a kilometer of the actual Tropic line. This is a bit disappointing, considering the ubiquity of hand-held GPS navigation devices for the past several years.
Moving to a slightly different topic (and back to the Northern Hemisphere), Wikipedia’s Tropic of Cancer article has a photo of highway markers on Carretera 83 in Mexico, showing the equatorward movement of the Tropic line over 5 years.