Tropic of Cancer Marker

Connected to the world! On June 21, 2004, the summer solstice in the Northern Hemisphere, six Tropic of Cancer markers, representing its historical positions, were officially inaugurated at the intersection of Bo'ai Road and Shixian Road in Chiayi, Taiwan. This was the tenth location globally to erect a Tropic of Cancer marker. The Tropic of Cancer marker serves as the boundary stone between the North Temperate Zone and the Tropics. Along with Arctic/Antarctic Circle markers and Equator markers, it belongs to a category of globally important landmarks that delineate the five major climatic zones: the North Frigid Zone, North Temperate Zone, Tropics, South Temperate Zone, and South Frigid Zone. The cluster of Tropic of Cancer markers in Chiayi City is situated at a strategically important traffic intersection in the southwest of the city. Essentially, they are a group of intersection landmarks, representing four different historical positions of the line. Although small and exquisite, they serve multiple purposes, including traffic guidance, tourism attraction, historical and cultural preservation, and Earth science education. Currently, a 'Graphic Inscription Stele' stands on the central median to the northwest. Its marble inscription briefly explains the significance of the markers, and a diagram further helps people fully understand how the Tropic of Cancer quietly drifted through this intersection in the late 18th century, and how it is closely related to the naming of Chiayi. • Graphic Inscription Stele: The Tropic of Cancer Drifting Southward The 'Tropic' (Tropic of Cancer/Capricorn) is a line of latitude determined by the equivalent value of the obliquity of the ecliptic in astronomy. There are three main astronomical factors that cause changes in the obliquity of the ecliptic: 1. Precession, 2. Nutation period, and 3. Polar motion. Additionally, plate tectonics can also cause the Tropic line to shift. Consequently, the Tropic of Cancer moves southward at a speed of over 14 meters per year, and in 1788, it passed near the northern end of the intersection of Bo'ai Road and Shixian Road in this city. This intersection is a major transportation hub in southwestern Chiayi City. In conjunction with intersection landscape improvement projects, six historical Tropic of Cancer markers were erected here, carrying profound historical, cultural, and astronomical science education significance. • Reasons for the Tropic Line's Shift (I) Astronomical Factors 1. Precession: While orbiting the sun, Earth is also influenced by the gravitational pull of other planets, causing changes in Earth's orbital plane (the ecliptic plane). This cycle, lasting approximately 25,800 years, is known in astronomy as 'precession'. 2. Nutation Period: The Moon and other celestial bodies influence Earth's rotation, causing its rotational axis to perform complex oscillations in space. This period is 18.6 years, with an amplitude of 9.21 arcseconds, leading to a north-south oscillation of the Tropic line with an amplitude of 0.28 kilometers. 3. Polar Motion: Earth's rotational axis is not fixed within the Earth; the geographic poles move with a period of slightly over one year. This has been confirmed theoretically and through observation, revealing a long-term phenomenon of polar motion, which accumulates to a significant extent over time. The three factors mentioned above directly influence changes in the obliquity of the ecliptic. (II) Geographical Factors Plate Tectonics: Taiwan is being squeezed by the Philippine Sea Plate moving towards the continental plate. On average, it moves at a speed of 1.5 centimeters per year, reaching up to 14 centimeters during intense periods. Interestingly, another Tropic of Cancer landmark in Taiwan, the one in Ruisui, Hualien, is experiencing plate movement in a different direction, towards Japan's Ryukyu Islands.