S1

S1 is the solar diurnal tidal constituent with a period of exactly 24.000 hours and a speed of 15.000 degrees per hour. Its gravitational forcing is weak, so the S1 seen in tide-gauge records is mostly radiational, driven by the daily cycle of solar heating, atmospheric pressure, and land-sea breeze rather than by the Sun’s gravity alone. This thermal origin makes S1 hard to model from astronomical theory and means it varies geographically with local meteorology. S1 is small at most ports but matters for precise tide prediction and for separating the radiational from the gravitational solar tide in long records used for geodesy and Earth-rotation studies.