Tropic of Cancer Marker Introduction
Connected to the world! On June 21, 2004, the summer solstice in the Northern Hemisphere, the six North Tropic markers located at the intersection of Bo'ai Road and Shih-Hsien Road in Chiayi, Taiwan were officially inaugurated, making it the tenth place in the world to construct a North Tropic marker. The North Tropic markers serve as boundary stones between the temperate zone and the tropics, similar to important global landmarks such as the Arctic Circle markers, the Equator markers, etc., defining the five zones: the Northern Frigid Zone, Northern Temperate Zone, Tropics, Southern Temperate Zone, and Southern Frigid Zone. The North Tropic marker group in Chiayi City is situated at a significant transportation hub in the southwestern area of Chiayi City and comprises a set of landmarks from four different years. Although small and exquisite, they hold significance for transportation tourism, historical culture, and Earth science education. Currently, there is a "Text and Image Monument" built on the central safety island to the northwest, featuring a brief marble inscription explaining the key points of the markers, along with diagrams that help visitors fully understand that the North Tropic line quietly crossed this intersection in the late 18th century, which is closely related to the naming of Chiayi. The North Tropic line, designated based on the astronomical angle of the ecliptic and equator, is subject to shifts due to three main astronomical factors: the precession of the equinoxes, the nutation period, and the axial shift, along with tectonic movement affecting the line's location. Therefore, the North Tropic line is moving southward at an average rate of over 14 meters per year and passed north of the intersection of Bo'ai Road and Shih-Hsien Road around 1788. This intersection is a critical transportation junction in southwestern Chiayi City, and in conjunction with urban landscape improvement projects, six North Tropic historical markers were established here, carrying profound historical, cultural, and astronomical education significance. The causes for the shift of the North Tropic line are as follows: 1. Astronomical factors: - Precession of the equinoxes: While the Earth revolves around the Sun, it is also influenced by the gravitational pull of other planets, causing changes in the plane of Earth’s revolution (ecliptic plane) over a period of approximately 25,800 years, known as "precession." - Nutation period: The Moon and other celestial bodies affect Earth's rotation, causing the rotation axis to wobble in space with a period of 18.6 years and an amplitude of 9.21 arc seconds, resulting in a north-south wobble of approximately 0.28 kilometers of the North Tropic line. - Axial shift: The Earth's rotation axis is fixed within the Earth itself, with pole movement occurring over a period of slightly more than a year. This phenomenon has been verified theoretically and through observation, showing a significant long-term axial shift. The above three factors directly affect the variation in the angle between the ecliptic and the equator. 2. Geographic factors: - Tectonic movement: Taiwan is pushed downward by the Philippine Plate toward the continental plate, moving at an average speed of 1.5 centimeters per year, with extreme cases reaching up to 14 centimeters. Interestingly, in Taiwan's North Tropic markers, the one located in Ruisui, Hualien, is experiencing tectonic movement toward the direction of the Ryukyu Islands in Japan.
