Great question! Gravity is indeed a major force
that influences tides, but not the only one.
Newton postulated that the tides are related to
the gravitational attraction between the earth and the moon. His law of gravitation (summarized and without some values) states that the force of attraction between two objects is proportional to the product of their masses and inversely proportional to the square of their distance (that is:
d*d or d2, where d is the distance).
The result of this is that if the moon and/or earth were larger and closer together, the gravitational attraction would be larger.
However, while tides are based on this
principle,tidal generating forces are actually
inversely proportional to the cube of the distance
between the objects (that is d*d*d or
d3, where d is the
distance). Tidal generating forces are calculated
by taking into account the gravitational
attraction of the earth, moon, and sun and also
the rotation of these objects. Therefore, tidal
generating forces are more dependent on the
distance between the objects than the size of the
objects (like in the gravitational force).
This is why the moon dictates the tides of the
earth, not our sun. The National Ocean and
Atmospheric Administration has crunched the
"Although the gravitational attraction between
the Earth and the Sun is more than 177 times that
between the Earth and the Moon, the moon dominates
the tides. Our sun is 27 million times more
massive than our moon. If tidal forces were based
solely on comparative masses alone, the sun should
have a tide-generating force that is 27 million
times greater than that of the moon. However, the
sun is 390 times further from the Earth than is
the moon. Thus, its tide-generating force is
reduced by 3903, or about 59 million times compared to that of the moon. Because of these conditions, the sun’s tide-generating force is 27/59, or about half that of the moon (Thurman,
sources: here, and
Thanks for the question!
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