Tropic of Cancer Marker Introduction
Connected to the World! On June 21, 2004, the summer solstice in the Northern Hemisphere, the six historical markers of the Tropic of Cancer located at the intersection of Bo’ai Road and Shih-Hsien Road in Chiayi, Taiwan, were officially inaugurated, making it the tenth location in the world to have a Tropic of Cancer marker. The Tropic of Cancer marker serves as a boundary between the temperate and tropical zones. Along with polar markers and equatorial markers, they are significant global landmarks that categorize the five climatic zones: the Arctic, the temperate North, the tropics, the temperate South, and the Antarctic. The group of Tropic of Cancer markers in Chiayi City is located at a major transportation hub in the southwest of the city and consists of a series of intersection landmarks attributed to four different years. Although small and delicate, they combine the significance of transportation tourism, historical culture, and earth science education. Currently, a "Graphical Monument" has been constructed on the central safety island to the northwest, featuring a marble inscription that briefly explains the key points of the marker, along with an illustrative diagram that helps people understand how the Tropic of Cancer quietly passed through this intersection in the late 18th century, which is closely related to the naming of Chiayi. The graphical monument records the movement of the Tropic of Cancer, marking it as a latitude line defined by the astronomical obliquity of the ecliptic. Three factors cause the variation of obliquity: 1. Precession: As the Earth revolves around the Sun, it is also affected by the gravitational pull of other planets, causing a change in the plane of revolution (the plane of the ecliptic) with a cycle of approximately 25,800 years, termed “precession.” 2. Nutation cycle: The Moon and other celestial bodies influence the Earth's rotation, causing the rotational axis to undergo complex oscillations in space, with a cycle of 18.6 years and an amplitude of 9.21", resulting in a north-south oscillation of the Tropic of Cancer by 0.28 kilometers. 3. Axial shift problem: The Earth's rotation axis is fixed within, and the movement of the geographic poles has a cycle of over a year. This long-term phenomenon of axial shift has been theoretically and observationally substantiated and accumulates to be quite significant over time. The aforementioned three factors directly affect the variation of the obliquity. (2) Geological factor - Plate tectonics: Taiwan is being pushed downward by the Philippine Plate against the continental plate, moving at an average speed of 1.5 centimeters per year, with violent instances reaching up to 14 centimeters. Interestingly, the marker of the Tropic of Cancer in Ruishui, Hualien, is moving in the direction towards the Ryukyu Islands of Japan.
