Lunar Day

The lunar day, or tidal day, is the 24-hour 50-minute interval between two successive transits of the Moon over a fixed meridian, about 24.84 hours. It is longer than the 24-hour solar day because the Moon advances roughly 12.2 degrees eastward in its orbit each day, so Earth must rotate extra to catch up. The lunar day sets the timing of the tide: a semidiurnal port sees two highs and two lows within it, spaced about 12 hours 25 minutes apart, the period of the principal lunar constituent M2. Successive high waters therefore arrive about 50 minutes later each calendar day, the lag a navigator builds into daily tide planning.