Tropic of Cancer Marker Introduction
Connected to the World! On June 21, 2004, the summer solstice in the Northern Hemisphere, the six historical Tropic of Cancer markers standing at the intersection of Bo’ai Road and Shih-Hsien Road in Chiayi, Taiwan, were officially inaugurated, making it the tenth place in the world to establish a Tropic of Cancer marker. The Tropic of Cancer marker serves as a boundary between the northern temperate zone and the tropical zone, and alongside the Arctic Circle marker, Equator marker, and Tropic of Capricorn marker, they are all significant landmarks that distinguish the five climatic zones: the North Frigid Zone, North Temperate Zone, Tropical Zone, South Temperate Zone, and South Frigid Zone. The Tropic of Cancer marker group in Chiayi City is located at a southwestern traffic hub and primarily consists of a group of landmark markers belonging to four different years. Although small and exquisite, they hold significant meaning in terms of transportation tourism, historical culture, and Earth science education. Currently, on the central safety island to the northwest, there is a "graphic memorial stone" that provides a brief marble inscription explaining the importance of the markers, along with a diagram to enhance understanding of how the Tropic of Cancer quietly passed through this intersection in the late 18th century, closely related to the naming of Chiayi. The "Tropic of Cancer" is a latitude line defined by the astronomical inclination of the ecliptic. There are three factors that cause changes in the obliquity: first, precession, second, nutation, and third, polar motion; additionally, tectonic plate movements also cause variations in the Tropic of Cancer. Thus, the Tropic of Cancer shifts south at a speed of over 14 meters per year, having passed through the northern end of the intersection of Bo’ai Road and Shih-Hsien Road in the year 1788. This intersection is a key transit point in the southwest of Chiayi City, and with the landscape improvement project, six historical Tropic of Cancer markers were established here, embodying significant historical, cultural, and astronomical scientific educational meaning. Reasons for changes in the Tropic of Cancer: 1. Astronomical Factors a. Precession: As Earth orbits, it is also influenced by the gravitational pull of other planets, causing changes in the plane of Earth's orbit (the ecliptic), a cycle of about 25,800 years, referred to in astronomy as "precession." b. Nutation: The moon and other celestial bodies affect the rotation of Earth, causing its rotation axis to perform complex oscillations in space, with a cycle of 18.6 years and an amplitude of 9.21", resulting in a north-south oscillation of approximately 0.28 kilometers along the Tropic of Cancer. c. Polar Motion: The rotation axis of Earth is fixed inside the planet, while the poles shift, with a cycle of more than a year. This phenomenon has been confirmed theoretically and through measurements, revealing a long-term polar motion that accumulates significantly over time. These three factors directly impact changes in the obliquity. 2. Geological Factors Tectonic Plate Movement: Taiwan is squeezed by the Philippine Plate moving down towards the continental plate at an average speed of 1.5 centimeters per year, with significant instances reaching up to 14 centimeters. Interestingly, while the Tropic of Cancer marker in Hualien Ruishui moves in the direction of the Ryukyu Islands in Japan.
