Precession - Trivial

There are two types of precession:

(1) The precession of the Earth is called precession for short. The Earth's equatorial plane and ecliptic plane (the mean orbital plane of the Earth-Moon center of gravity) are not mechanically constant. The Earth is shaped like a flattened spheroid in the north-south direction, and its equatorial plane is tilted at about 23.4 degrees to the ecliptic plane. For this reason, the couple of the Sun and the Moon causes the Earth's polar axis to rotate in space. This is called precession. In other words, the North Celestial Pole moves around the North Celestial Pole in the ecliptic plane with a radius of 23.4 degrees in a period of about 26,000 years. This is called the precession of the sun and moon, and the thing that moves in this way is called the mean pole. A complex movement (called nutation) with an amplitude of about 12 arc seconds and a main period of 18.6 years is compounded around this mean pole, and it continues to change direction in space. The polar axis of the Earth here refers to the Earth's dynamically predictable geometric axis (the intersection of this axis with the celestial sphere is called the ephemeris pole), which is very close to the instantaneous axis of rotation. The Earth's actual geometric axis has a rotational motion called polar motion, which has an unpredictable amplitude and period, and its direction is constantly changing. The ecliptic plane is constantly moving due to the gravitational forces of the planets, and this motion is called planetary precession. The vernal equinox moves from east to west on the ecliptic at a speed of about 50 arc seconds per year due to the precession of the sun and moon, and moves from west to east along the equator at a speed of about 0.11 arc seconds per year due to the precession of the planets. The combination of the precession of the sun and moon and the precession of the planets is called general precession.

A top spinning on a table rotates its spindle due to the force of the Earth's gravity, but this is also precession. The direction of the Earth's precession is opposite to the direction of the Earth's rotation, but in the case of a top, the direction of the top's rotation and the precession are the same.

(2) The difference between the solar year and the sidereal year caused by the precession of the Earth. Sidereal time is defined as the "hour angle of the vernal equinox," but because the vernal equinox moves from east to west every year due to precession, the period in which the sun returns to the same position relative to the vernal equinox is shorter than the period in which the sun returns to its original position relative to the stars. The former is called the tropical year (solar year) and has a period of 365.24219 days, while the latter is called the sidereal year and has a period of 365.25636 days. This difference is called precession. This phenomenon was observed by the Greek Hipparchus around 150 BC.

[Kojiro Wakao]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

歳差とよばれるものには次の二つがある。

(1)地球の歳差運動を略して歳差という。地球の赤道面と黄道面（地球・月重心の平均軌道面）とは、力学的に不変ではない。地球は南北に扁平(へんぺい)な回転楕円(だえん)体に近い形をしており、かつ赤道面は黄道面に対して約23.4度傾いている。このため、太陽と月の偶力によって、地球の極軸は空間に対して回転する。これを歳差運動という。すなわち、天の北極は約2万6000年の周期で黄道面の北極の回りを、半径23.4度で動く。これを日月歳差(じつげつさいさ)とよび、このような動きをするものを平均の極という。この平均の極の周りに、その振幅が約12秒角で、おもな周期が18.6年の複雑な動き（これを章動という）が合成されて、空間に対して向きを変え続ける。ここでいう地球の極軸とは、力学的に予測可能な地球の形状軸（この軸と天球の交点を暦表極という）をいい、瞬間自転軸にたいへん近いところにある。実際の地球の形状軸は、これに、極運動とよばれる、振幅・周期とも予測不可能な旋回運動があって、その方向を絶えず変えている。黄道面は惑星の引力によって絶えず動いており、その動きを惑星歳差という。春分点は日月歳差によって、黄道上を毎年約50秒角の速度で東から西へ動き、また惑星歳差によって、赤道に沿って毎年約0.11秒角の速度で西から東へ動く。日月歳差に惑星歳差を加えたものを一般歳差という。

　机上で回転しているこまは、地球の重力によって芯棒(しんぼう)の首回し運動をするが、これも歳差運動である。地球の歳差運動の方向は、地球の自転方向と反対方向であるが、こまの場合は、こまの回転方向と歳差運動の方向は同じである。

(2)地球の歳差運動によって生ずる太陽年と恒星年の差をいう。恒星時は「春分点の時角」と定義されるが、この春分点は歳差運動によって東から西へ毎年動いているため、春分点に対して太陽が同じ位置に戻る周期は、太陽が恒星に対して元の位置に戻る周期より短くなる。前者は回帰年（太陽年）とよばれ、周期は365.24219日、後者は恒星年とよばれ、周期は365.25636日である。この差を歳差という。この現象は紀元前150年ごろにギリシアのヒッパルコスによって観測された。

［若生康二郎］

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