Tropic of Cancer Marker Introduction
Connecting with the world! On June 21, 2004, the summer solstice in the Northern Hemisphere, the group of six Tropic of Cancer markers was officially inaugurated at the intersection of Boai Road and Shih Hsien Road in Chiayi, Taiwan, making it the tenth location globally to establish a Tropic of Cancer marker. The Tropic of Cancer serves as a boundary between the Northern Temperate Zone and the Tropical Zone, alongside other significant global landmarks like the Arctic Circle and the Equator, which distinguish the five climatic zones: the Northern Cold Zone, Northern Temperate Zone, Tropical Zone, Southern Temperate Zone, and Southern Cold Zone. The Tropic of Cancer markers in Chiayi City are situated at a vital transportation hub in the southwest of the city, and they are essentially a collection of intersection landmarks from four different years. Though small and exquisite, they hold significance for traffic tourism, historical culture, and Earth science education. Currently, there is a "graphic inscription monument" on the central safety island to the northwest, which provides a brief marble engraving that explains the importance of the markers, accompanied by a simple diagram that helps people understand that the Tropic of Cancer quietly passed through this intersection in the late 18th century and is closely related to the naming of Chiayi. The marker "Tropic of Cancer" is defined as a latitude line determined by the celestial equator based on the obliquity of the ecliptic. There are three factors that cause variations in the obliquity of the ecliptic: 1. Precession, 2. Nutation, and 3. Axial shift. Additionally, plate tectonics can also cause changes to the Tropic of Cancer. Therefore, the Tropic of Cancer moves southward at an average speed of more than 14 meters per year, having crossed near the northern end of the intersection of Boai Road and Shih Hsien Road in 1788. This intersection is a crucial transportation junction in the southwest of Chiayi City, and in conjunction with the intersection landscape improvement project, six historical markers of the Tropic of Cancer have been established here, holding profound historical, cultural, and astronomical educational significance. The reasons for the changing positions of the Tropic of Cancer are as follows: (1) Astronomical factors: 1. Precession: While the Earth revolves around the sun, it is also influenced by the gravitational forces of other planets, causing the ecliptic plane to shift, with a cycle of about 25,800 years, referred to in astronomy as "precession." 2. Nutation: The Moon and other celestial bodies affect the rotation of the Earth, causing the rotation axis to perform complex oscillations in space, with a cycle of 18.6 years and an amplitude of 9.21 seconds, resulting in a North-South oscillation of 0.28 kilometers at the Tropic of Cancer. 3. Axial shift: The Earth's rotation axis is fixed within the Earth, yet the geographic poles experience movement with a cycle of over a year. Both theoretical and observational studies have confirmed prolonged phenomena of polar movement, which accumulates to a notable extent over time. These three factors directly influence the variations in the obliquity of the ecliptic. (2) Geographical factors: Plate tectonics: Taiwan is being squeezed by the Philippine Plate towards the continental plate at an average annual movement of 1.5 centimeters, with extreme cases reaching up to 14 centimeters. Interestingly, the marker of the Tropic of Cancer in Taiwan, located in Ruisui, Hualien, is moving in the direction towards Japan’s Ryukyu Islands.
