Quote:
Originally Posted by morls
Just out of interest, has anyone or any theory described or modelled the motion of the earth around the sun in such a way that includes the sun's movement as part of the milky way and the milky way's movement as part of the local group?
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Hi Stephen,
What is referred to as the secular aberration drift (SAD) is fancy wording that
describes the solar system barycentre's rotational acceleration around the
centre of the galaxy, a point heavily weighted by the supermassive black
hole designated Sagittarius A.
In the year 2000, we went from using the positions of approximately 1500
stars to using the positions of distant quasars for defining RA and dec.
This system we now use is referred to as the International
Celestial Reference Frame (ICRF).
This capability came about through the now routine use of Very
Long Baseline Interferometry (VLBI) where a global network of
radiotelescopes uses the distant quasars as references.
In recent years, they are measuring angles down at the phenomenally
small microarcsecond (μas) level.
So the SAD (our rotation around the centre of the Milky Way) has
been measured at around 5 to 6 μas per year.
The theories of precession and nutation are now refined to mindboggling
levels of precision, as they need to be for everything else to be
measured so accurately, so there is not much that is not accounted for with
respect the Earth's own motion.
They even use VLBI to measure plate tectonic motions down to
one millimetre per year!
In turn the proper motions with respect the Local Group are being
measured and refined as well.
Years ago I had a customer buy from us and I recognised his name
as one of the leading authorities in planetary position theory.
He worked at JPL and I had a brief email exchange with him.
Turned out he was an amateur observer too in his spare time.
He was one of the people responsible for refining the ephemeris for
the Earth, Moon and planets so that, for example, the JPL landers
could more accurately hit their reentry corridors through the
Martian atmosphere.
I told him I had read several of his papers and that we used a subset
of that very emphemeris to compute the positions of the planets.
In turn he was happy to use our device in his spare time to look at the
planets.
So I was chuffed by the thought his work had gone through
a full circle.
These ephemeris in their full form will compute the position of any of
the planets to no less 25 metres accuracy and to the Moon within a metre!