Perihelion

Celestial bodies in the solar system move in orbits that are quadratic curves with the sun as one of their foci, i.e., most of them are ellipses or parabolas, and the point on that orbit where the body is closest to the sun is called the perihelion. Conversely, the point where the body is farthest from the sun is called the aphelion. For celestial bodies that move in orbits that are close to circles, such as planets, the difference between perihelion and aphelion is small, but for elliptical orbits with large eccentricities, such as some asteroids and comets, and especially those that are close to parabolas, the difference is extremely large.

[Sadao Murayama]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

太陽系の天体は、太陽を焦点の一つとする二次曲線、すなわち多くは楕円(だえん)、放物線などの軌道を運行しているが、その軌道上でもっとも太陽に近づく点を近日点という。反対に太陽からもっとも遠ざかる点が遠日点である。惑星のように円に近い楕円軌道を描く天体では近日点と遠日点の差は少ないが、小惑星の一部や彗星(すいせい)のように離心率の大きな楕円軌道、とくに放物線に近いものでは、その差はきわめて大きくなる。

［村山定男］

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