自転(じてん)

自転(じてん) is the standard astronomical term for a body's rotation about its own axis. It is paired with 公転(こうてん) ("orbital revolution"), and the contrast between the two is one of the first things taught in Japanese science classes when planetary motion is introduced.

Forms

• 自(じ) ("self, on one's own") + 転(てん) ("turn, rotate") → 自転(じてん)

Common collocations

• 自転(じてん)する: to rotate (on its axis)
• 自転(じてん)周期(しゅうき): rotation period
• 自転(じてん)軸(じく): axis of rotation
• 自転(じてん)速度(そくど): rotation speed
• 地球(ちきゅう)の自転(じてん): the Earth's rotation
• 自転(じてん)と公転(こうてん): rotation and revolution

CONTRAST WITH 公転(こうてん):

• 自転(じてん): rotation on its own axis (e.g., Earth turns once every ~24 hours)
• 公転(こうてん): revolution around another body (e.g., Earth orbits the Sun once a year)
• These are typically taught and used as a pair in introductory astronomy.

Similar / related terms

• 回転(かいてん): rotation, spinning (general term, not limited to celestial bodies — wheels, motors, dancers)
• スピン: spin (loanword; common in everyday and physics contexts, e.g., a figure-skating spin or particle spin)
• 歳差(さいさ)運動(うんどう): precession (the slow wobble of a rotation axis)

Usage

• For non-astronomical "rotation," use 回転(かいてん): タイヤの回転(かいてん) ("the rotation of the tires"), モーターの回転(かいてん)数(すう) ("the motor's RPM"). 自転(じてん) sounds odd for everyday objects.
• 自転(じてん) is also part of the everyday word 自転車(じてんしゃ) ("bicycle"), where the meaning is more like "self-turning vehicle."
• Beware homophones: 辞典(じてん) ("dictionary"), 事典(じてん) ("encyclopedia / reference work"), and 時点(じてん) ("point in time") all share the reading じてん but use entirely different kanji.