Return line


July 5, 2022

The regression line (tropic) is the latitude line of the obliquity angle (23 degrees 26 minutes on the earth). Also known as a tropic. It can be defined for celestial bodies (planets, satellites, etc.) that orbit a star, but the return line of the earth will be described below. The angle at which the sun illuminates the surface of the earth (solar zenith angle, elevation angle) changes depending on the season and time. In the case of the largest angle around noon (the angle of the sun at this time is called the south-middle altitude), it illuminates vertically on the equator on the spring and autumn equinox days. The sun shines vertically at latitude 23 ° 26'N at the summer solstice in the Northern Hemisphere and at latitude 23 ° 26'S at the winter solstice in the Northern Hemisphere. This latitude is the highest latitude and the sun comes to the zenith. The former is called the Tropic of Cancer and the latter is called the Tropic of Capricorn. In European languages ​​such as English, the word meaning return line (eg English: tropic) is derived from the Greek word tropos, which means "turn". The name "return line" is a translation of that. The red parallels projected from the celestial sphere, that is, the celestial sphere, were once drawn on the celestial sphere, but are not included in modern star charts. The latitude of the regression line changes between 22.1 degrees and 24.5 degrees in a cycle of about 41,000 years as the earth's equatorial tilt angle changes, and changes in an amplitude of 9 seconds in a cycle of about 18.6 years due to nutation. These cause changes on the order of seconds per year. Therefore, it is not meaningful to accurately represent the seconds, but at the mid-2005 value (2005.5), it is 23 degrees 26 minutes 18 seconds. It is often rounded to 23 degrees 26 minutes, 23.4 degrees, etc. by an appropriate digit. The latitude in 2000.0 (Epoch), which is the base year, is 84 381.406 seconds, that is, 23 degrees 26 minutes 21.406 seconds (standard uncertainty is 0.001 seconds). The area between the two return lines, north and south, is often exposed to radiation from the sun (radiant heat), so the temperature is high throughout the year and it is tropical. The area near the return line is dry all year round due to the influence of the subtropical high pressure zone, and it is easy to have a desert climate. The amount of rainfall is about 10 mm. This happens because the updraft that occurred near the equator dries in the sky and descends near it. The fact that the sun comes directly above once a year is not the cause of the subtropical high pressure zone.

Reference item

Subsolar point


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