Equator

Japanese: 赤道 - せきどう

The line that connects the points at a 90-degree angle from the point where the Earth's rotation axis passes through the Earth (the North and South Poles) is called the Earth's equator, and the line that intersects with the celestial sphere when this equator is extended across the celestial sphere is called the celestial equator. This name comes from the fact that the celestial sphere's equator was drawn as a red line on ancient Chinese star charts. The Earth is an oblate, three-axis unequal ellipsoid with an equatorial radius of 6,378 km and a radius in the pole direction that is 1/294.118 shorter than this. The plane that is perpendicular to the Earth's rotation axis and passes through the center of the Earth is called the equatorial plane. The definitions of the Earth's equator and equatorial plane are also applied to the Sun, Moon, planets, and other celestial bodies with a certain size to define the equator (plane) of each celestial body. The flattening ratio of the Earth's equatorial ellipse is 1/90,000, and the major axis of the equatorial ellipse is oriented in the direction of 15 degrees west longitude.

[Kojiro Wakao]

Geography

The line of intersection between the Earth's surface and a plane that passes through the center of the Earth's axis and cuts it at a right angle to the axis. It is the standard for latitude and corresponds to latitude 0 degrees. The equatorial plane is tilted at an angle of 23 degrees 26 minutes with respect to the Earth's orbital plane. The equator receives direct sunlight twice a year (on the vernal and autumnal equinoxes), and because the sun's altitude is high even on the summer and winter solstices, the area near the equator is heated, causing updrafts and forming an equatorial calm zone or equatorial low pressure zone. As a result, there is generally a lot of rain, with many areas receiving more than 2,000 millimeters of rain per year and showers occurring almost every day, resulting in a hot and humid tropical rainforest climate, and the vegetation in this area forms tropical rainforests.

[Masami Ichikawa and Kojiro Wakao]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

Japanese:

地球の自転軸が地球を貫く点（南北極）から角度90度の点を結んだ線を地球の赤道といい、この赤道を天球に広げて天球と交わった線を天の赤道という。古代中国の星図で、天球の赤道を赤線で描いたことに由来する。地球は扁平(へんぺい)な三軸不等の楕円(だえん)体で、赤道半径は6378キロメートル、極方面の半径はこれより294.118分の1短い。地球自転軸に垂直でかつ地球中心を通る面を赤道面という。太陽、月、惑星など大きさをもつ天体にも、地球の赤道、赤道面の定義を準用して、それぞれの天体の赤道（面）を定義している。地球の赤道面楕円の扁平率は9万分の1で、赤道面楕円の長軸は西経15度の方向を向いている。

［若生康二郎］

地理

地球の地軸の中心を通り、地軸に対して直角に切る平面と地表との交線をいう。緯度の基準となり、緯度0度にあたる。赤道面は地球の公転軌道面に対して23度26分の傾きをもっている。赤道は年2回（春分の日と秋分の日）、太陽の直射を受けるほか、夏至(げし)や冬至(とうじ)の日でも太陽高度が高いので、赤道付近は熱せられて、上昇気流が生じ、赤道無風帯あるいは赤道低圧帯を形成する。そのため一般に雨量が多く、年降水量2000ミリメートル以上の地域が多く、毎日のようにスコールがあり、いわゆる高温多湿の熱帯雨林気候となり、その地域の植生は熱帯雨林を形成する。

［市川正巳・若生康二郎］

[参照項目] | 緯度 | 赤道座標 | 赤道低圧帯 | 地軸 | 熱帯雨林気候 | 無風帯

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