章動
Chinese edit
| cycle of 19 tropical years | motion; movement | ||
|---|---|---|---|
| trad. (章動) | 章 | 動 | |
| simp. (章动) | 章 | 动 | |
Etymology edit
So called because the principal period of terrestrial nutation, driven by the same gravitational dynamics responsible for the precession of the lunar nodes, is approximated by the length of a Metonic cycle, a calculation device that helps keeping the calendar aligned closely with both the synodic month and the tropical year. In early Chinese calendars, the concept of 章 (zhāng) refers to the length of one such cycle, namely 19 years. Despite this connection, the Earth's nutation is conceptually distinct from that of lunar procession, and in general the periods of nutation do not conform to simple multiples or fractions of calendrical periods.
The word appears to have been coined in the translation of John Herschel's Outlines of Astronomy into Chinese by Alexander Wylie and Li Shanlan:
- 地軸除歲差外，別有搖旋之動，十九年一周，名章動。若無歲差，則十九年中，赤極必行成一小橢圓，長徑十八秒五，短徑十三秒七四。 [Classical Chinese, trad.]
- From: John Herschel (侯失勒約翰), 1849. Outlines of Astronomy, Chapter 5. Translated into the Chinese by Alexander Wylie (偉烈亞力) and Li Shanlan (李善蘭) as《談天》in 1859.[1][2]
- Dìzhóu chú suìchā wài, bié yǒu yáoxuán zhī dòng, shíjiǔ nián yī zhōu, míng zhāngdòng. Ruòwú suìchā, zé shíjiǔ nián zhōng, chìjí bì xíng chéng yī xiǎo tuǒyuán, chángjìng shíbā miǎo wǔ, duǎnjìng shísān miǎo qī sì. [Pinyin]
- The nutation of the earth's axis is a small and slow subordinate gyratory movement, by which, if subsisting alone, the pole would describe among the stars, in a period of about nineteen years, a minute ellipsis, having its longer axis equal to 18".5, and its shorter to 13".74.
地轴除岁差外，别有摇旋之动，十九年一周，名章动。若无岁差，则十九年中，赤极必行成一小椭圆，长径十八秒五，短径十三秒七四。 [Classical Chinese, simp.]
Pronunciation edit
Noun edit
章動