Tropic of Cancer Marker Introduction
Connecting with the world! On June 21, 2004, the summer solstice in the Northern Hemisphere, the six historical Tropic of Cancer markers at the intersection of Poai Road and Shih-Hsien Road in Chiayi, Taiwan, were officially inaugurated, making it the tenth location in the world to build a Tropic of Cancer marker. The Tropic of Cancer marker serves as a boundary between the northern temperate zone and the tropics. Along with polar markers and the equator marker, they are globally significant landmarks that differentiate the five zones: the northern frigid zone, northern temperate zone, tropics, southern temperate zone, and southern frigid zone. The Tropic of Cancer marker group in Chiayi City is located at a crucial southwestern transportation hub, essentially comprising a series of intersection landmarks, divided into four different years. Although small and exquisite, these markers embody significant meanings in transportation tourism, historical culture, and earth science education. Currently, a "Graphic Inscription" monument has been erected on the central safety island to the northwest, featuring a brief description of the focus of the markers carved in marble, along with a diagram that helps people fully understand how the Tropic of Cancer quietly passed through this intersection in the late 18th century and its close connection to the naming of Chiayi. The "Tropic of Cancer Drift" graphic inscription states that the "Tropic of Cancer" is a latitude line defined by the angle of the celestial equator, influenced by three factors: 1. precession, 2. nutation, and 3. polar motion. Additionally, plate tectonics also affects the position of the Tropic of Cancer. As a result, the Tropic of Cancer moves southward at a rate of more than 14 meters per year and crossed close to the northern end of the intersection of Poai Road and Shih-Hsien Road in 1788. This intersection is crucial for southwestern road traffic in Chiayi City, and in conjunction with the landscape improvement project, six historical Tropic of Cancer markers have been established here, holding profound historical, cultural, and astronomical educational significance. The reasons for the drift of the Tropic include: (1) Astronomical factors: 1. Precession: As the Earth revolves around the Sun, it is also influenced by the gravitational pull of other planets, causing the plane of Earth's revolution (the ecliptic) to change, with a cycle of about 25,800 years, known as "precession." 2. Nutation: The Moon and other celestial bodies affect the Earth's rotation, causing the rotation axis to wobble in complex patterns, with a cycle of 18.6 years and an amplitude of 9.21 seconds, resulting in a north-south oscillation of about 0.28 kilometers along the Tropic of Cancer. 3. Polar motion: The Earth's rotation axis is fixed internally, while the pole moves, with a cycle of just over a year. This has been confirmed both theoretically and through measurement, revealing a long-term polar motion phenomenon that accumulates over time. These three factors directly influence changes in the angle of the celestial equator. (2) Geographical factors: Plate tectonics: Taiwan is pressed by the Philippine plate moving down toward the continental plate at an average speed of 1.5 centimeters per year, with peaks reaching up to 14 centimeters. Interestingly, the Tropic of Cancer marker in Ruisui, Hualien, while located in Taiwan, is directed towards Japan's Ryukyu Islands due to the different movement of tectonic plates.
