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An example of a three-body system
is the moon-Earth-sun. Newton
thought about this system but the
mathematical difficulties were
insurmountable, and human
knowledge of where the moon will
be in the distant future is still very
limited. Variations in the orbit of
Halley’s comet are another indicator
of the influence of the gravitational
fields of the planets operating in
addition to the gravitation of the
sun. Recent orbits have taken 76.0,
76.1, 76.3, 76.9, 77.4, 76.1, 76.5, 77.1,
77.8, and 79.1 years respectively
due to the combined gravitational
influence of the sun, Jupiter, Saturn,
and other planets on the comet.Shaping the planets
While Newton searched for religious
meaning in his scientific work, he
could find none behind his theory
of gravity. He did not discover the
hand of God setting the planets in
motion, but he had found a formula
that shaped the universe.
The action of gravity is key to
understanding why the universe
looks as it does. For instance,
gravity is responsible for the
spherical shapes of the planets.
If a body has sufficient mass, the
gravitational force that it exerts
exceeds the strength of the material
of the body and it is pulled into
a spherical shape. Astronomical
rocky bodies, such as the asteroids
between the orbits of Mars and
Jupiter, are irregular in shape if
they have a diameter of less than
about 240 miles (380 km) (the
Hughes-Cole limit).GRAVITATIONAL THEORY
Gravitation is also responsible for
the size of the deviations from a
sphere that can occur on a planet.
There are no mountains on Earth
higher than the 5.5 miles (8.8 km)
of Mount Everest because the
gravitational weight of a taller
mountain would exceed the
strength of the underlying mantle
rock, and sink. On planets with
lower mass, the weight of objects
is less, and so mountains can be
bigger. The highest mountain on
Mars, for instance, Olympus Mons,
is nearly three times as high as
Everest. The mass of Mars is about
one-tenth that of Earth, and its
diameter is about half Earth’s.
Putting these numbers into
Newton’s formula for gravitation,
this gives a weight on the surface
of Mars of just over one-third that
on Earth, which explains the size
of Olympus Mons.I have not been able
to discover the cause of
these properties of gravity
from phenomena, and I
frame no hypotheses.
Isaac NewtonIn his great work Principia, Newton
plotted the parabolic path of the Great
Comet by taking accurate observations
and correcting them to allow for the
motion of Earth.