Nutation - Shodo

A relatively short, regular change, other than precession, in the variation of the Earth's axis of rotation caused by the gravitational forces of the Sun and Moon. The force acting due to the slight difference in the gravitational forces of celestial bodies such as the Sun and Moon depending on the location on Earth is called tidal force. Tidal force acts to stretch the Earth in the direction of the Moon and Sun. When tidal force acts on the bulge at the Earth's equator, it becomes a couple force that tries to change the Earth's direction. This causes the Earth's axis of rotation to describe a cone with its center of gravity at its apex, like the oscillating motion of a top about to stop. This is called precession, and the longest period of this variation (about 26,000 years) is called precession, and the shortest period is called nutation. The strength and direction of the gravitational forces exerted by the Sun and Moon on the Earth change regularly in periods such as half a month, half a year, or 18.6 years, reflecting the nature of their revolution. In response to this, the Earth also undergoes various periodic wobbly movements, but these short-period movements are called nutations. Note that unlike polar motion, nutations do not change the position of the Earth's poles (the intersections of the Earth's surface and the axis of rotation).

Nutation is made up of many periodic components, the most representative of which are the semiannual and semilunar nutations caused by the 23°26′ tilt of the equatorial plane (the plane perpendicular to the Earth's axis of rotation) and the ecliptic plane (the plane along which the Earth revolves); the 18.6-year nutation caused by the ecliptic plane (the plane of the Moon's orbit) maintaining a tilt of about 5° with respect to the ecliptic plane and changing its direction every 18.6 years; and the annual and lunar nutations caused by the elliptical orbits of the Earth and Moon. Even the maximum 18.6-year nutation is a very small motion with an amplitude of only 9 arcseconds, but since its discovery by J. Bradley in 1747, it has been closely observed as a subject of astronomy research and compared with theoretically predicted phases and amplitudes. Nutation is strongly influenced by the Earth's internal structure, particularly the fluid core deep inside the Earth, and its properties are useful in research into the viscosity of the fluid core and the shape of the core-mantle boundary. The famous annual z term discovered by Kimura Hisashi through latitude observations during the Meiji period was also due to the effect of the fluid core on nutation. In order to predict the apparent positions of stars, it is necessary to take the effect of nutation into account in advance, so the theoretically calculated values ​​of each periodic component of nutation are one of the astronomical constants defined by the International Astronomical Union.

[Kousuke Hioki]

"Modern Astronomy Lectures Vol. 1: Earth Rotation" edited by Wako Kojiro (1979, Koseisha Kouseikaku)" ▽ "Can the Extent of the Universe be Measured?" by Yoshida Shotaro (1985, Chijin Shokan)" ▽ "Handbook for Using the Astronomical Yearbook" edited by the Astronomical Yearbook Editorial Committee (1987, Seibundo Shinkosha)" ▽ "Astronomical Calculation Classroom" new edition by Saida Hiroshi (1998, Chijin Shokan)

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

太陽と月の引力によっておこる地球の自転軸の変動のうち、歳差以外の、比較的短い周期の規則的変化。太陽や月などの天体が及ぼす引力が地球上の場所によってわずかに異なる分の差によって働く力は潮汐力(ちょうせきりょく)とよばれる。潮汐力は地球を月や太陽の方向に引き伸ばす働きをする。潮汐力が地球の赤道部分の膨らみに作用すると、地球の向きを変えようとする偶力となる。これによって地球の自転軸は、止まる寸前のこまの首振り運動のように重心を頂点とする円錐(えんすい)を描く。これを歳差運動というが、このうちもっとも長い周期（約2万6000年）の変動を歳差とよび、短い周期のものを章動とよぶ。太陽と月が地球に及ぼす引力の強さと向きは、公転運動の性質を反映して半月、半年、18.6年などの周期で規則的に変わる。それに応じて地球もさまざまな周期の首振り運動を行うが、このような短い周期のものが章動である。なお章動では極運動と異なり地球上の極（自転軸と地表の交点）の位置はほとんど変わらない。

　章動は多数の周期成分からなっているが、代表的なものは赤道面（地球の自転軸に垂直な平面）と黄道面（地球が公転運動を行う平面）の23度26分の傾きを原因とする半年周と半月周の章動、白道面（月の軌道面）が黄道面に対して約5度の傾きを保ち、しかもその向きが18.6年の周期で変化することによる18.6年周章動、地球と月の軌道が楕円(だえん)形であるために生じる年周と月周の章動などである。最大の18.6年周章動でさえ振幅が9秒角にすぎない微小な運動であるが、1747年にJ・ブラッドリーによって発見されて以来、位置天文学の研究対象として精密観測され、理論的に予測される位相や振幅との比較が行われてきた。章動は地球の内部構造、とりわけ地球深部の流体核の影響を強く受けるため、その性質を利用して流体核の粘性や核マントル境界の形状などを調べる研究に役だっている。明治期の緯度観測によって木村栄(ひさし)が発見した有名な年周z項も章動に及ぼす流体核の影響が原因であった。星の視位置を予測するには、あらかじめ章動の効果を考慮に入れておく必要があるので、章動の各周期成分の理論計算値は、国際天文学連合が定める天文定数の一つになっている。

［日置幸介］

『若生康二郎編『現代天文学講座　第1巻　地球回転』（1979・恒星社厚生閣）』▽『吉田正太郎著『宇宙の広さは測れるか』（1985・地人書館）』▽『天文年鑑編集委員会編『天文年鑑活用ハンドブック』（1987・誠文堂新光社）』▽『斉田博著『天文の計算教室』新装版（1998・地人書館）』

出典　小学館　日本大百科全書(ニッポニカ)日本大百科全書(ニッポニカ)について　情報 | 凡例