Tropic of Cancer Marker Introduction
Connected to the world! On June 21, 2004, the summer solstice in the northern hemisphere, the group of six Tropic of Cancer markers located at the intersection of Bo'ai Road and Shih Hsien Road in Chiayi, Taiwan, was officially inaugurated, making it the tenth location in the world to have Tropic of Cancer markers. The Tropic of Cancer marker serves as a boundary between the temperate and tropical zones, alongside other important global landmarks such as the Arctic Circle marker, the Tropic of Capricorn marker, and the Equator marker, which distinguish the five climate zones: the 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 situated at a transportation hub in the southwest of the city, consisting mainly of a collection of intersection landmarks, categorized by four different years. Although small and delicate, they hold significant meaning in terms of transportation tourism, historical culture, and Earth science education. Currently, the "Text and Image Monument" has been built on the central safety island to the northwest, featuring a marble inscription that briefly explains the key points of the markers. Accompanied by a diagram, it illustrates how the Tropic of Cancer quietly passed through this intersection in the late 18th century and its close relationship with the naming of Chiayi. The "Tropic of Cancer" is a latitude line determined based on the equatorial inclination of celestial mechanics. There are three factors causing the change in this inclination: (1) Precession of the equinoxes, (2) Nutation, and (3) axial shift. Additionally, plate tectonics also contribute to the movement of the Tropic of Cancer. As a result, the Tropic of Cancer moves southward at a speed of more than fourteen meters each year, having passed near the northern end of the intersection of Bo'ai Road and Shih Hsien Road in 1788. This intersection is a vital roadway in the southwest of Chiayi City; combined with landscape improvement projects, six historical Tropic of Cancer markers were established here, holding significant historical, cultural, and astronomical educational value. Causes of the changes in the Tropic of Cancer: (1) Astronomical factors: 1. Precession of the equinoxes: As the Earth orbits the Sun, it is also influenced by the gravitational pull of other planets, which causes changes in the plane of Earth's orbit (the ecliptic). This cycle lasts about 25,800 years and is referred to in astronomy as "precession." 2. Nutation: The influence of the moon and other celestial bodies on the Earth's rotation leads the rotation axis to oscillate in space, with a period of 18.6 years and an amplitude of 9.21", resulting in a north-south oscillation of approximately 0.28 kilometers around the Tropic of Cancer. 3. Axial shift: The Earth's rotation axis is fixed internally, and the movement of the poles occurs on a cycle of just over a year. This has been confirmed theoretically and through measurement, revealing an observable long-term axial shift. The cumulative effect of these changes can be significant. The three aforementioned factors directly affect the variation in the equatorial inclination. (2) Geographical factors: Plate tectonics: Taiwan is subjected to the pressure of the Philippine Plate moving downward toward the continental plate, moving at an average rate of 1.5 cm per year, and during intense periods, this can reach up to 14 cm. Interestingly, the Tropic of Cancer marker in Ruisui, Hualien, is also influenced by plate motion, but its direction heads toward the Ryukyu Islands in Japan.
