Extreme

Japanese: 極 - きょく

A term used in the fields of earth and celestial bodies and physics.

Physics

The part where the magnetic force is concentrated, such as the two ends of a bar magnet, is called a magnetic pole. Today, (electro)magnetic phenomena are usually considered based on the concept of a field, where magnetic poles only interact with a magnetic field, and the nature and strength of the magnetic poles can be considered based on the way in which this interaction occurs. The Coulomb force acts between two magnetic poles through the magnetic field created by each magnetic pole, and this defines the strength of the magnetic pole. The strength of a magnetic pole is also called magnetic quantity. Therefore, it can be said that a magnetic pole is a part where magnetic quantity is concentrated. There are two types of magnetic poles, positive and negative. In any substance or elementary particle confirmed to date, magnetic quantity always appears in pairs of magnetic poles of equal strength, positive and negative. Therefore, the total magnetic quantity in a substance is always zero. Among magnetic pole pairs, those that are idealized to be infinitely small in size (although it is often useful to think of them as real entities, it is an abstract concept for accurate handling) are called magnetic dipoles. All magnetic poles of macroscopic size in reality can be considered to be a collection of these dipoles. Magnets are what we are familiar with in our daily lives. The distinction between positive and negative magnetic poles originated from the interaction between magnets and the Earth's magnetic field. The magnetic pole facing north is usually called the N pole (positive pole), and the magnetic pole facing south is called the S pole (negative pole).

[Hiroshi Yasuoka]

Earth

(1) The poles of the Earth. The intersections of the Earth's axis and the Earth's surface are called poles, and there are the North Pole and the South Pole. The North Pole is at 90 degrees north latitude, and the South Pole is at 90 degrees south latitude. The Earth is an ellipsoid that bulges out by about 1/300 in the equatorial direction. If the axis of symmetry in the direction where it is most flat (i.e., the north-south direction) is taken as the Earth's axis, this is called the "geometric axis." In contrast, the axis of the Earth's rotation in space is called the "rotation axis." The rotation axis and the geometric axis are almost the same, but in reality there is a deviation of about 10 meters, and the pole of the rotation axis moves counterclockwise in an irregular circle around the pole of the fixed geometric axis. This is called polar motion, which was theoretically predicted by Euler in the 18th century and confirmed observationally at the end of the 19th century. This phenomenon continues to be observed through an international cooperative project called the International Earth Rotation Service (IERS).

(2) Geomagnetic poles. The Earth is a huge magnet, and like a normal bar magnet, it has north and south poles. The distribution of magnetic field lines around the Earth is well approximated by the distribution of magnetic field lines when a bar magnet is placed at the center of the Earth. In this case, the north and south poles of this bar magnet point toward the Earth's south and north poles, respectively, opposite to the compass needle. The points where the direction of the magnetic poles intersect with the Earth's surface are called the geomagnetic south pole and geomagnetic north pole, respectively. These geomagnetic poles do not coincide with the Earth's poles, and currently there is a deviation of about 1,100 kilometers. This deviation is also fluctuating at a speed of about 100 kilometers per century. In addition, there are points near the north and south poles where the direction of the magnetic field lines on the Earth's surface is perpendicular to the ground, and these are called the magnetic south pole and magnetic north pole, respectively. These do not coincide with the Earth's poles or the geomagnetic poles, and the speed of the deviation is several times faster than that of the geomagnetic poles. As of 2000, the geomagnetic north pole is located near 79.6 degrees north latitude and 71.6 degrees west longitude, and the magnetic north pole is located near 81.0 degrees north latitude and 109.7 degrees west longitude.

[Masami Ichikawa and Koichi Nakajima]

astronomy

Just like the North and South Poles on Earth, two poles can be defined on the celestial sphere. In that case, there are various poles depending on how the axis is chosen.

(1) The points where the extension of the Earth's rotation axis intersects with the celestial sphere are called the "North Celestial Pole" and the "South Celestial Pole." These are the basis for "right ascension and declination" (equatorial coordinate system), which show the position of stars on the celestial sphere.

(2) The "ecliptic poles" are defined by axes perpendicular to the ecliptic on the celestial sphere. These are in the constellations Draco and Tabula gracilis, and serve as the basis for the "ecliptic longitude and latitude" (ecliptic coordinate system) used to indicate the positions of planets.

(3) The direction perpendicular to the galactic plane is sometimes called the "galactic pole." This defines the "galactic coordinate system."

Because the pole serves as the reference point for various coordinate systems, determining it precisely is an important research topic in astronomy.

[Koichi Nakajima]

[References] | Magnetic dipole | Magnets | Geomagnetic field

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

Japanese:

地球や天体、および物理学の分野での用語。

物理学

棒磁石の両端のように磁力の集中した部分を磁極とよぶ。今日では（電）磁気現象は場の概念に基づいて考えるのが普通で、磁極は磁場とのみ相互作用し、この相互作用の仕方によって磁極の性質や強さを考えることができる。二つの磁極の間にはそれぞれの磁極のつくる磁場を通してクーロン力が働き、これによって磁極の強さを定義できる。磁極の強さは磁気量ともよばれる。したがって磁極は磁気量の集中した部分であるといいかえることもできる。磁極には2種類あって正と負で区別する。今日までに確かめられているどんな物質や素粒子においても、磁気量はかならず正負等量の強さの磁極が対になって現れる。したがって物質中の全磁気量はかならずゼロになる。磁極の対のうち、大きさを無限に小さくする理想化を行ったもの（実体として考えることもしばしば有益であるが正確な取扱い上は抽象的概念）を磁気双極子という。現実の巨視的な大きさの磁極はすべてこの双極子の集合体と考えることができる。日常なじみのあるのは磁石である。磁極の正負の区別の仕方は、磁石と地球磁場との相互作用で決められたのが起源である。北に向くほうの磁極をN極（正極）、南に向くほうの磁極をS極（負極）とよぶのが普通である。

［安岡弘志］

地球

(1)地球の極。地球の地軸と地球表面との交点を極といい、北極と南極とがある。北極は北緯90度にあたり、南極は南緯90度にあたる。地球は、赤道方向に約300分の1だけふくらんだ回転楕円(だえん)体の形をしているが、そのもっともつぶれた方向（すなわち南北方向）の対称軸を地軸とするとき、これを「形状軸」という。これに対し、宇宙空間内での地球の自転運動の軸を「自転軸」という。自転軸と形状軸はほぼ一致するが、実際には約10メートルのずれが生じ、不動の形状軸の極の周りを、自転軸の極が不規則な円を描いて反時計回りに運動している。これを極運動とよび、18世紀にオイラーによって理論的に予言され、19世紀末に観測的に確認された。またこれは、国際地球回転観測事業（IERS：International Earth Rotation Service）とよばれる国際協力事業により観測が続けられている。

(2)地磁気の極。地球は一つの巨大な磁石であり、通常の棒磁石のようにNSの極をもつ。地球の周りの磁力線の分布は、地球中心に1個の棒磁石を置いたときの磁力線の分布としてよく近似される。この場合、この棒磁石のN極、S極は、方位磁針とは反対にそれぞれ地球の南極、北極の方向を向く。この磁極の方向線が地表と交わる点を、それぞれ地磁気南極、地磁気北極とよぶ。この地磁気極は地球の極とは一致せず、現在の時点で約1100キロメートルほどのずれがある。またこのずれは100年で100キロメートル程度の速度で変動している。これとは別に、地表での磁力線の方向が地面に垂直方向となる地点が南北極付近にあり、それぞれ磁南極、磁北極とよばれる。これは地球の極、地磁気極とも一致せず、また変動の速度も、地磁気極の変動よりも数倍大きい。2000年現在の地磁気北極は北緯79.6度、西経71.6度付近、また磁北極は北緯81.0度、西経109.7度付近にある。

［市川正巳・中嶋浩一］