"Obliquity" is what gives rise to the seasons. It is the angle between the equatorial plane, which is determined by Earth's rotation, and the ecliptic plane, which is the Earth's orbital plane around the Sun. The Tropic of Cancer in the Northern Hemisphere and the Tropic of Capricorn in the Southern Hemisphere mark the limits of highest latitudes that the subsolar point reaches at solstices each year. The present-day obliquity of the ecliptic of the Earth, about 23.5° , is decreasing by about 0".5 a year. This article reports a "concrete" terrestrial testimony to this time variation, which plays a major part in the Milankovitch cycles.
It has long been recognized that rather than staying constant, obliquity varies slowly with time as a result of external gravitational influences. The Moon and Sun's tidal torques on Earth's ellipticity give rise to the familiar 26,000-year astronomical precession, while the gravitational pull of other planets, primarily Jupiter and Venus, slowly perturbs the orientation of the ecliptic plane in space. The combined effect observed by Earth dwellers is an ~41,000-year oscillation in the obliquity with overall amplitude typically of about 2°. This oscillation is one of the three Milankovitch cycles that ultimately affect our long-term climatic system and serve as the pacemaker of ice ages. The present-day obliquity happens to be close to the mean value, and we are in the middle of a downswing (see figure (1)). In terms of real distance on the Earth's surface, one should see a slow equatorward shift of the tropics by 14.4 m a year—well over 1 km in a century!
This is exactly what has happened on Taiwan, an island 400 km in length that straddles the Tropic of Cancer in the western Pacific. In 1908, to commemorate the completion of the north-south-running island railroad, the colonial Japanese government built a monument beside the railroad on the contemporary Tropic of Cancer. The monument is in Jia-Yi County. The figure (2) shows a rare historical photograph of the monument, with the clear inscription of the then-latitude of the Tropic, N 23 degrees 27' 4".51, written in Chinese; this is in remarkable agreement with modern theory.
The Japanese and later the Republic of China governments of Taiwan maintained the tradition of building generations of monuments, mostly of concrete, to commemorate the location of the Tropic of Cancer as old monuments fell victim to typhoons, earthquakes, and exposure to the elements. The newer monuments are conveniently situated near the original site and still bear the old latitude inscription. The site is now a small dedicated park, with Monuments #4 and #5 standing side by side.
But unable to acquire more land, the park administration faces a dilemma. The true latitude of the Tropic has by now moved southward by as much as 1.27 km (see the figure (1))! Worse still, the Tropic will continue to move south for another 90 km before it swings back some 9300 years later! There are many monuments and landmarks throughout the world that mark the two Tropics. Over time, they all will face the same dilemma as those in Taiwan.
By the same token, the Arctic Circle and the Antarctic Circle are currently moving poleward at the same rate. The world's temperate zone is expanding at the expense of the tropical and Arctic zones at the rate of some 1500 km2 per year! This is the Milankovitch cycle happening right before our eyes.
It should be mentioned that historical observations of the Sun's angle at solstices, especially those by ancient Chinese records, and more recent astrometric measurements of the Sun and planets, have been analyzed for the change in obliquity for years. The results essentially agree with the theoretical prediction within 1%. Although small, the discrepancies are believed to be geophysically significant.
Finally, it is interesting to compare this large motion with the much smaller but perhaps better known phenomenon of the polar drift. The polar drift refers to a slow drift of Earth's mean rotation pole away from the true "North Pole." The polar drift has been systematically monitored since 1900. Believed to be caused by large geophysical mass redistributions such as postglacial rebound, a total of only ~10 m of polar drift has been observed in the direction of ~80°W, corresponding to a maximum drift of ~0".3 per century in the apparent latitude along that meridian. This rate is 2 orders of magnitude smaller than the rate of obliquity variation discussed here.
Source: Eos, October 29, 1996, p. 433.
I grew up on the beautiful subtropical island of Taiwan, Republic of China, where I completed my college education in physics from the National Taiwan University in 1973. I entered the graduate school of the Scripps Institute of Oceanography of the University of California in San Diego in 1975, and earned a Ph.D. degree in Earth Sciences in 1981. I then joined the NASA workforce as a geophysicist, working on exciting research problems at the forefront of global dynamics related to Earth's rotation and gravity.
I was in high school when I realized that things in our physical world do not "just happen"— rather, there are reasons and explanations behind every thing and every phenomenon, and there are some very simple physical laws that past great minds have taught us. That was such a thrilling discovery that I wanted to explore and learn more about the physical sciences. I have since been greatly rewarded by the sense of satisfaction that I get from understanding the wonders around us (and by getting paid to do it!). Two facets played important roles in the process: being curious and analytical.
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