逆ぎゃっ行こう運うん動どう

noun

retrograde motion; apparent retrograde motion; retrogradation

In astronomy, the apparent backward motion of a planet against the fixed stars. The two senses are merged here because they describe the same phenomenon; 'retrograde motion' and 'apparent retrograde motion' are essentially synonymous in modern usage, with 'retrogradation' being a less common variant.

See also: 順行運動

• 火か星せいの逆ぎゃっ行こう運うん動どうは約やく2か月げつ間かん続つづく。

  Mars's retrograde motion lasts about two months.

• 逆ぎゃっ行こう運うん動どうは、地ち球きゅうが外がい惑わく星せいを追おい越こすときに見みられる。

  Retrograde motion is seen when Earth overtakes an outer planet.