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The Moon has another large effect on the Earth arising from its gravitational
pull on the Earth. The Moon raises ocean tides
and land tides on the Earth.
Here, we discuss ocean tides (the land tides
have smaller amplitude, less than a foot, while the ocean tides are
larger, ranging
from nearly a meter in the middle of the oceans to,
on average, several meters near coastlines).
Tides are raised because of what is known as
a Differential Force (tidal force).
The Sun also raises tides on the Earth but less so than does the Moon; the tidal force of the Sun on the Earth is 50 % smaller than the tidal force of the Moon on the Earth.
When the Sun, the Earth, and the Moon lie along the same
line (either new or full moon), the
tidal forces of the Sun and
the Moon add together, we get the highest high tides
and lowest low
tides of the month (spring tides).
When the line connecting the Sun, the Earth, and
the Moon forms a right angle (either first or third quarter moon),
the difference between high tide and low
tide is the smallest (neap tides)
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. On what date does the highest high tide occur during October, 2022 (see the tide tables)? For this date, what would we infer to be the rough phase for the Moon? What is the actual phase of the Moon on this date in October, 2022? (To find the Lunar phase)
The tidal bulge does not point directly toward the Moon; it leads the Moon slightly.
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. Because of the shorter day, 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. (Note that in 2.1 billion years the Earth's oceans will vaporize greatly reducing the tidal coupling between the Earth and Moon.) Also, remarkably, the Great Tohoku Earthquake of 2011 changed the shape of the Earth enough to shorten the length of the day by 1.8 microseconds and shifted the location of the spin axis of the Earth by 6.5 inches! }
