Of course, the gravitational field of the rest of observable universe is also present, but that is an extremely UNIFORM EXTERNAL gravitational field over the Solar System that it has NO effect on the INTERNAL motions of the Solar System.
In fact, whenever one describes a particular gravitational field as on or over some particular system of astro-bodies, it is always understood that gravitational field of the rest of observable universe is also present, but that gravitational field is an extremely UNIFORM EXTERNAL gravitational field over the particular system, and so has NO effect on the INTERNAL motions of the particular system.
The center of mass of the Earth would then be the origin of an ideal inertial frame with coordinate axes attached to the unrotating Earth. The inertial frame can also be described as the center-of-mass (CM) inertial frame of the Earth.
In this counterfactual case, every point on the surface of the Earth is NOT accelerated with respect to the ideal inertial frame attached to the Earth's center of mass so also defines an ideal LOCAL inertial frame (i.e., an inertial frame at the point and nearby surroundings).
You do NOT need for the point to be the center of mass of any object and usually you would NOT call such Earth surface inertial frames CM inertial frames even they were actually CM inertial frames.
For an example of a picturesque piece of Earth surface (but NOT an ideal one), see the figure below (local link / general link: alpine_tundra.html).
The gravitational field
of the Solar System
is NOT exactly UNIFORM over the
Earth.
The Moon's
gravitational field
and secondarily the
Sun's
gravitational field
vary across the
Earth.
Note the Moon's
gravitational field is
weaker than the Sun's, but its
variation is greater.
And it is the variation in the
gravitational field
that causes the tidal force
on the Earth
(see Mechanics files:
tide_earth.html and
The Tidal Force and the Earth).
The stretching causes the Earth
tides
(i.e., the water tide)
and also the Earth's
land tide
and
atmospheric tide.
Now the tidal forces
of the Moon
and the Sun
have virtually NO effect on
small-scale
everyday life
and small-scale laboratory
experimentation
and so can be neglected for most purposes, but NOT all purposes as discussed below.
For more on the
tides,
see Mechanics files:
tide_earth.html
and
frame_basics.html:
The Tidal Force and Earth.
The Earth
actually has the
Earth's daily axial rotation.
This means that every point on the
surface of the Earth is in
acceleration.
Now, in principle, for all calculations you could just use
the inertial frame defined
Earth's
center of mass
as an
origin
with coordinate axes
unrotating relative to the observable universe.
For brevity, let's call this
inertial frame, the UNROTATING FRAME.
Therefore, for Earthly purposes,
we use a
rotating frame that
rotates with the Earth
and use
rotating frame
inertial forces
to convert said
rotating frame in an
inertial frame.
The rotating frame
inertial forces
are the centrifugal force,
the Coriolis force,
and the Euler force.
The Euler force is for
accelerating
rotation
and is NOT needed for the
case of the Earth.
We explicate the convertion and the needed
rotating frame
inertial forces in
frame_basics.html: Rotating Frames and the Centrifugal Force and the Coriolis Force.
But the fact is that the
acceleration of the
surface of the Earth is actually
so low (⪅ 0.03 m/s**2:
see Wikipedia: Gravity of Earth:
Latitude) that
for most small-scale purposes (but NOT all purposes), you can treat
every surface point as defining a LOCAL
inertial frame
(i.e., an inertial frame
at the point and nearby surroundings)
without using
the centrifugal force
and the Coriolis force.
How small is small scale?
Small-scale everyday life
and small-scale laboratory
experimentation.
For more on these uses and
rotating frames in general,
see
Mechanics files:
frame_basics.html: Rotating Frames and the Centrifugal Force and the Coriolis Force,
frame_basics.html: Rotating Frames Explicated,
frame_basics.html: The Centrifugal Force of the Earth's Rotation,
and
frame_basics.html: The Coriolis Force of the Earth's Rotation.
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Note a bound orbit is
a rotation
relative to the observable universe
of an astro-body
about a center of mass
for the
gravitationally bound system
containing the astro-body.
Such a
gravitationally bound system,
of course, constitutes a
CM inertial frame.
Note that unless one says
rotating frame,
one usually means an
inertial frame
NOT rotating relative to the
observable universe.
In fact, the
UNROTATING FRAME
is super inconvenient for all purposes since
almost all Earthly things (including us) mostly rotate with the
Earth.