27.3-day and Average 13.6-day Periodic Oscillations in the Earth's Rotation Rate and Atmospheric Pressure Fields Due to Celestial Gravitation Forcing

Variation in length of day of the Earth (LOD, equivalent to the Earths rotation rate) versus change in atmospheric geopotential height fields and astronomical parameters were analyzed for the years 1962--2006. This revealed that there is a 27.3-day and an average 13.6-day periodic oscillation in LOD and atmospheric pressure fields following lunar revolution around the Earth. Accompanying the alternating change in celestial gravitation forcing on the Earth and its atmosphere, the Earths LOD changes from minimum to maximum, then to minimum, and the atmospheric geopotential height fields in the tropics oscillate from low to high, then to low. The 27.3-day and average 13.6-day periodic atmospheric oscillation in the tropics is proposed to be a type of strong atmospheric tide, excited by celestial gravitation forcing. A formula for a Tidal Index was derived to estimate the strength of the celestial gravitation forcing, and a high degree of correlation was found between the Tidal Index determined by astronomical parameters, LOD, and atmospheric geopotential height. The reason for the atmospheric tide is periodic departure of the lunar orbit from the celestial equator during lunar revolution around the Earth. The alternating asymmetric change in celestial gravitation forcing on the Earth and its atmosphere produces a ``modulation to the change in the Earths LOD and atmospheric pressure fields.