Figure: The daily rotation of the celestial sphere westward as seen from Earth reference frame
Caption:
An animation of the
Earth rotating
eastward within the
celestial sphere
marked with lines of right ascension (RA)
and declination (Dec or δ),
the celestial equator,
and the ecliptic.
Features:
- The celestial sphere is
not-to-scale.
It should be quasi-infinite compared to the Earth---but that is
undrawable.
- If one took the Earth as an
at-rest
reference frame as we do
often in everyday life
and as an observational perspective in
astronomy,
then the
celestial sphere rotates
westward
as, in fact, we usually consider it to be doing.
- Remote astronomical objects
(that we project onto to the celestial sphere
for location purposes)
must have enormous
rotation velocities
RELATIVE to the
at-rest
Earth
reference frame.
At some point far enough from the Earth, those
rotation velocities become
superluminal.
See the calculations below.
- However, these enormous
rotation velocities
are
geometrical velocities,
NOT physical velocities,
since NO
information (which includes all forms of
mass-energy)
is propagating at
faster than the
vacuum light speed c = 2.99792458*10**5 km/s ≅ 3*10**5 km/s ≅ 1 ft/ns
RELATIVE to
local inertial frames:
"local" meaning at the place the information is at any time.
So they do NOT violate special relativity.
Recall, special relativity is based on
an axiom that the
vacuum light speed
to be the highest physical velocity.
- NO physical thing
(such as a light signal) traveling
in a local inertial frame
can move at a
superluminal velocity: i.e.,
NO physical velocity
is superluminal.
- Thus, superluminal
geometrical velocities
are NOT a violation of
special relativity and they
are very common as our example with the
Earth
reference frame
and remote astronomical objects shows.
- In fact, most of the remote
astronomical objects
with enormous
rotation velocities
RELATIVE to the
at-rest
Earth
reference frame
are moving at far, far less than
the vacuum light speed
relative to their
local inertial frames.
- To be specific to the case of the animation,
rotation relative to the
observable universe
(i.e., to the bulk mass-energy
of the observable universe)
is considered absolute
rotation
in our modern understanding of
cosmology.
If a physical body with rest mass
is rotating relative to the
observable universe,
every bit of it must be moving
with a physical
velocity
(necessarily less than the
vacuum light speed relative
to a local inertial frame)
and the body is just said to have a
physical
rotation.
So the Earth rotates physically
relative to the
observable universe
and NOT
vice versa.
- To illustrate the enormous
geometrical velocities,
in the
at-rest
Earth
reference frame, note that
rotation velocity
v = r * (2πf) ,
where r is radius
perpendicular to the
rotation axis
and f is the
frequency of
rotation.
For the
at-rest
Earth
reference frame,
f=1/( 86164.1 s) where 86164.1 s = 1
sidereal day.
We have the special cases:
v = 0.4651 km/s = 1.551*10**(-6) * c for r = 6378.1 km/s = Earth equatorial radius,
v = 1.091*10**5 km/s = 0.03639 * c for r= 1.495978707*10**8 km =
1 astronomical unit,
v = 7506 * c for r= 3.0856775814671900*10**13 km = 1 parsec.