A well-traveled geophysicist recounts his curious findings at a prominent geographical benchmark in the southern Hemisphere.
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Most globes of Earth display four dashed present that, aside from the equator, signify special latitudes concerned the seasons. Among those, the Tropic of Capricorn, is the most southern latitude at which the sunlight shines directly overhead during the December solstice. It"s at this time at 23° 26′ 12.3″ south, a latitude that"s decreasing (moving towards the equator) through 0.47 arcsecond (0.47″) every year.
Last month, when on a Sky & Telescope tour to check out this year"s total solar eclipse, we quit at a Tropic that Capricorn sign about 60 kilometres south-southeast of mountain Pedro de Atacama, Chile, ~ above Highway 23, and 50 kilometres south-southwest the ALMA, the Atacama huge Millimeter/submillimeter range of 66 radio telescopes high in the Andes.
The sign, presented above, reads "Lat. 23° 26′ 16″
The gps receiver on mine phone, however, detailed that the sign is actually situated at around 23° 26′ 49.1″ south. That"s about 1.13 kilometres south that the existing Tropic line. (I also measured the longitude to it is in 67° 59′ 44.1″ west.)
You might ask just how accurate my gps receiver is. Because the U.S. Air force removed the "dither" indigenous the gps signal in 2000, solitary fixes on hand-held gadgets have had actually a 50:50 horizontal uncertainty of a few meters. (That is, about fifty percent of the fixes room within a few meters of the true location.) ~ above a Google satellite image, the latitude and also longitude of the dry monument ~ above Highway 23 space within a couple of meters of the values measured through my gps receiver.
One uncertainty can be the geodetic datum, or an international reference system, supplied by the Chilean government. In my endure working in assorted parts of the human being (but not Chile), neighborhood geodetic datums seldom differ from the WGS84 ellipsoid by more than 100 meters.
So . . . I conclude that the Chilean highway department"s employees did a sloppy project of locating the sign.
They aren"t alone. In April 2019, I checked out a Tropic of Capricorn mite in Namibia on road D1265, around 60 km west the Rehoboth. The sign, checked out at top right, to be at 23° 28′ 0.3″ south, about 3.33 kilometres south that the present tropic line. (The longitude was 16º 30′ 31.5″ east.)
In 2013, ns visited another Tropic that Capricorn sign on highway C14 in Namibia, about 140 km west that Rehoboth. It to be at 23° 30′ 0.4″ south, nearly 7 km south of the tropic line. (The longitude to be 15º 46′ 19.8″ east.) ns presume someone review that the Tropic the Capricorn was "about" 23½° south and hence put the authorize there. The turnoff because that the authorize is clearly shows on Google satellite images.
So, at three signs marking the Tropic the Capricorn ~ above highways in Chile and also Namibia, no one to be within a kilometer of the yes, really Tropic line. This is a little disappointing, considering the ubiquity of hand-held gps navigation gadgets for the past several years.
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Moving to a slightly various topic (and back to the north Hemisphere), Wikipedia"s Tropic the Cancer write-up has a picture of highway mite on Carretera 83 in Mexico, mirroring the equatorward motion of the tropic line end 5 years.