The Moon and the Earth revolve around their common center of mass in one month. Although their orbits are elliptical and the Earth’s equator and Moon’s orbit are inclined to the ecliptic. The mean distance between the Moon and the Earth is about 238,860 miles. The center of mass of the Moon-Earth system is located within the Earth about 2895 miles (mean) from the center of mass of the Earth, always toward the Moon. Figure 1.2 shows the gravitational force vectors, F, and the difference vectors, D. They are essentially the same as for the Sun-Earth system except for magnitude. The same development can be used for all points on the Earth as previously explained for the Sun-Earth system. Looking down on the North Pole in Figure 1.2, the Earth spins counter clockwise in one lunar day. The mean lunar day is the period of rotation of the Earth on its own axis referenced to the Moon and is 24.84 solar hours in length. Starting with the point on the equator toward the Moon, the point experiences a maximum in the lunar tide-generating force, 6.21 solar hours later, a minimum, then a maximum at 12.42 solar hours, a minimum again at 18.63 hours, and finally back to the maximum at the conclusion of the lunar day. Note that in one lunar day there are two complete tide generating force cycles, just as there were for the Sun. However, the solar cycle took 24 hours and the lunar, 24.84. The distended symmetrical egg-shaped envelope of the lunar tide-generating forces stays lined up with the Moon as the Earth and Moon go around their common center of mass each month. The Earth spins (slips) once within the envelope each lunar day.