The Moon has another large effect on the Earth; it raises ocean tides and land tides. Here, we discuss only the ocean tides (the land tides have much smaller amplitude, on the order of inches). Tides arise because of what is known as a Differential Force (tidal force).
What is a tidal force?


Tides are actually more complicated than I just described. The amplitudes and even the number of high tides per day, depend on the locality, the wind, and other factors. As a mild example of this, let's look at the tidal table for Yaquina Bay. What is the phase of the Moon today, January 15th, 2008? Can we tell from the tides? Answer: Lunar Phases.

The Sun also affects the tides, but less so than does the Moon. When the Sun, the Earth, and the Moon lie along the same line so that the tidal forces of the Sun and the Moon add together, we get the highest high tides and the lowest low tides (spring tides) and when the line connecting the Sun, the Earth, and the Moon forms a right angle, the difference between high tide and low tide is the smallest (neap tides).

Actually, the tidal bulge does not point directly toward the Moon, it leads the Moon slightly. Why is this so? Are there consequences of this effect?

Yes, the Earth's spin is slowing by ~0.0015 s per century and the Moon's orbit is increasing in size by ~4 cm per year. At this rate of slowing, the Earth probably spun with a period of 22 h half a billion years ago. In addition, the year was then 397 days long. This process will continue until the spin of the Earth and the orbital period of the Moon are the same. This will occur when the spin period for the Earth is ~ 47 of the current days and the size of the Moon's orbit is ~1.43 times its current size. The process is slow and will take many billions of years.