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Connect with the world! On June 21, 2004, the summer solstice in the Northern Hemisphere, six North Tropic Line monuments, located at the intersection of Bo'ai Road and Shi Xian Road in Chiayi City, Taiwan, were officially inaugurated. This marked the tenth location worldwide to construct a North Tropic Line monument. The North Tropic Line monument serves as a boundary marker between the Northern Temperate Zone and the Tropics. Along with the Arctic Circle monument, Tropic Line monuments, and the Equator monument, it belongs to a group of world-renowned landmarks that distinguish the five zones: the North Frigid Zone, North Temperate Zone, Tropics, South Temperate Zone, and South Frigid Zone. The North Tropic Line monument group in Chiayi City is situated at a crucial transportation hub in the southwest of the city. Although small and exquisite, it serves multiple purposes: traffic and tourism guidance, historical and cultural significance, and earth science education. Currently, a "Graphic Monument" stands on the central safety island to the northwest. Made of marble, it briefly explains the significance of the monument and, with simple diagrams, helps people understand how the North Tropic Line quietly passed through this intersection in the late 18th century and its connection to the naming of Chiayi. **The North Tropic Line's Southward Movement Graphic Monument** The "Tropic Line" is a latitude line determined by the Earth's axial tilt. Three factors contribute to changes in the Earth's axial tilt: 1. **Precession**: The Earth's orbit around the Sun is influenced by the gravitational pull of other planets, causing the Earth's orbital plane (ecliptic) to change over a period of approximately 25,800 years. 2. **Nutation**: The Moon and other celestial bodies affect the Earth's rotation, causing the rotational axis to wobble in space over an 18.6-year cycle with an amplitude of 9.21". This results in a north-south oscillation of approximately 0.28 kilometers along the Tropic Line. 3. **Polar motion**: The Earth's rotational axis does not remain fixed within the Earth; instead, it shifts over a period of over a year. This polar motion has been confirmed through observations and measurements. These three astronomical factors directly impact changes in the Earth's axial tilt. **(b) Geographical Factors** * **Plate Tectonics**: Taiwan is subject to the interaction between the Philippine Sea plate and the continental plate. The island moves at an average rate of 1.5 cm/year, with a maximum rate of 14 cm/year. Interestingly, another North Tropic Line monument in Taiwan, located in Hualien's Ruisui, is moving in a different direction, towards the Ryukyu Islands in Japan.