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 located at the intersection of Boai Road and Shih-Hsien Road in Chiayi, Taiwan, were officially inaugurated, making it the tenth location worldwide to establish a Tropic of Cancer marker. The Tropic of Cancer marker serves as a boundary between the northern temperate zone and tropical zone and is part of significant global landmarks, including the Arctic Circle marker, the Tropic of Capricorn marker, and the Equator marker, which delineate the five climatic regions: Northern Cold Zone, Northern Temperate Zone, Tropical Zone, Southern Temperate Zone, and Southern Cold Zone. The Tropic of Cancer marker group in Chiayi City is located at a major transportation hub in the southwest of the city, essentially functioning as a cluster of intersection landmarks categorized by four different years. Despite their small size, these markers carry substantial significance in transportation tourism, historical culture, and earth science education. Currently, on the central safety island to the northwest, there is a “Text and Image Monument” made of marble that briefly explains the key points of the markers and includes a diagram to help people fully understand that the Tropic of Cancer quietly drifted through this intersection in the late 18th century and is closely related to the naming of Chiayi. The “Tropic of Cancer” is defined by the equatorial coordinate of the celestial obliquity based on astronomy. There are three primary factors that cause variations in the celestial obliquity: 1) Axial precession, which is the change in the Earth's rotation plane (ecliptic plane) influenced by the gravitational pull from other planets, with a period of about 25,800 years; 2) Nutation cycle, where the moon and other celestial bodies affect the Earth’s rotation, causing the axis to wobble in a complex manner with a period of 18.6 years and an amplitude of 9.21”, resulting in a south-north fluctuation of 0.28 kilometers at the Tropic of Cancer; 3) Axial shift, where the axis of Earth’s rotation is fixed internally, with a gradual movement of the poles over a period of more than a year. This phenomenon of pole shift has been confirmed both theoretically and empirically, revealing significant long-term effects. The above three factors directly influence variations in the celestial obliquity. As for geographical factors, tectonic movements affect Taiwan, causing it to be pressed down by the Philippine Plate towards the continental plate, moving at an average rate of 1.5 cm per year, and in extreme cases, up to 14 cm. Interestingly, another Tropic of Cancer landmark in Taiwan, located in Ruisui, Hualien, is moving towards the direction of Japan's Ryukyu Islands.
