Definition

1. Noun:
   • A tidal day: The time interval between two successive meridian passages of the moon, approximately 24 hours and 50 minutes of mean solar time. This is the common meaning related to tides.
   • A rotational period: The period of time taken for the moon to make one full rotation on its axis relative to the stars, about 27.3 Earth days. This is also known as a sidereal day on the moon.

Usage

• The term "lunar day" is used in astronomy, tidal science, and space exploration.
• It is important to specify the context, as the term has two distinct meanings.

Examples

• Noun (Tidal Day):
  • The high tide is later each day because a lunar day is longer than a solar day.
  • Sailors must account for the lunar day when reading tide tables.
• Noun (Rotational Period):
  • A lunar day on the moon, from one sunrise to the next, lasts about 29.5 Earth days.
  • The Apollo astronauts experienced a long, hot lunar day during their surface missions.

Advanced Usage

• "Synodic lunar day": Often used to specify the period from one moonrise to the next on Earth (about 24.8 hours) or one sunrise to the next on the Moon (about 29.5 Earth days). This is tied to the Sun's position.
• "Sidereal lunar day": Used to specify the moon's rotation period relative to distant stars (about 27.3 Earth days).

Variants and Related Words

• Tidal day: Often used synonymously with the terrestrial meaning of a lunar day.
• Lunation: Refers to the cycle of lunar phases, approximately 29.5 days, which is the synodic month.
• Sidereal month: Approximately 27.3 days, equal to the moon's rotational period relative to stars.

Synonyms

• Tidal day (for the ~24.8-hour meaning).
• Moon day (informal).

Notes on Meaning

• The primary ambiguity of "lunar day" lies in the frame of reference:
  • As observed from Earth: A tidal day of ~24.8 hours.
  • As on the Moon itself: A full rotational cycle, which can be a sidereal day (~27.3 Earth days) or a synodic day (~29.5 Earth days from sunrise to sunrise). Context is essential for clarity.

Noun

1. the period of time taken for the moon to make one full rotation on its axis (about 27.3 sidereal days)