Latin America

STARGAZER'S CORNER: ADVENTURES UNDER THE NIGHT SKY

WHERE IS THE TROPIC OF CAPRICORN?

BY: ROBERT KIECKHEFER AUGUST 20, 2019




A well-traveled geophysicist recounts his curious findings at a prominent geographic benchmark in the Southern Hemisphere.

Tropic of Capricorn in Chile 2019Member's of Sky & Telescope's 2019 total solar eclipse tour pose with the Tropic of Capricorn marker in Chile's Atacama Desert. The author is at far right.
Sean Walker / Sky & Telescope

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.



A well-traveled geophysicist recounts his curious findings at a prominent geographic benchmark in the Southern Hemisphere.

Tropic of Capricorn in Chile 2019Member's of Sky & Telescope's 2019 total solar eclipse tour pose with the Tropic of Capricorn marker in Chile's Atacama Desert. The author is at far right.
Sean Walker / Sky & Telescope
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 above, 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.

More:
https://skyandtelescope.org/observing/stargazers-corner/where-is-the-tropic-of-capricorn/