HORIZONS help

[posfan12]

A question regarding JPL's HORIZONS ephemeris databases.

How do I get the direction a planet's rotational axis points toward with respect to the J2000 reference plane?

[posfan12]

BTW, the J2000 reference axis is Earth's mean equinox IIRC.

[John Van Vliet]

--- edit ---

[posfan12]

Yeah, it's too bad.

[stro]

What is the difference between horizons's astrometric and apparent ephemerides?

[selden]

The quote below is taken from
ftp://ssd.jpl.nasa.gov/pub/ssd/Horizons_doc.pdf

Geometric coordinates
Geometric coordinates are the true, or instantaneous states of a body at a particular ephemeris time.

Astrometric coordinates
Accounts for the finite but varying amount of time it takes light to travel from the target to the observer.

Apparent coordinates
Takes into account factors which appear to change target position with respect to the background stars and inertial
coordinate system: light-time, stellar aberration, the relativistic deflection of light. Usually, a final rotation to some
"of-date" coordinate system is performed, such as precession-nutation to Earth true-equator and equinox-of-date

[stro]

Thanx for document.
I found there another one definition:
Observer table coordinates, such as RA and DEC, may be with respect to two possible coordinate systems:

Earth mean equator and equinox of reference epoch (astrometric coordinates):
Reference epoch: J2000.0 or B1950.0
xy-plane: plane of the Earth's mean equator at the reference epoch
x-axis : out along ascending node of the instantaneous plane of the Earth's orbit and the Earth's
mean equator at the reference epoch
z-axis : along the Earth mean north pole at the reference epoch

Earth true equator and equinox of date (apparent coordinates)
Reference epoch: "of date"
xy-plane: plane of the Earth's true equator at the reference epoch
x-axis : out along ascending node of instantaneous plane of the Earth's orbit and the Earth’s true equator
plane at the reference epoch
z-axis : along the Earth's true north pole at the reference epoch

Comment to the generated ephemeris table is like the same:

R.A._(ICRF/J2000.0)_DEC =
J2000.0 astrometric right ascension and declination of target center.
Corrected for light-time.

R.A._(a-apparent)__DEC. =
Airless apparent right ascension and declination of the target center with
respect to the Earth true-equator and the meridian containing the Earth true
equinox of date. Corrected for light-time, gravitational deflection of light,
stellar aberration, precession & nutation.

Why I am asking this...
In the HORIZONS I choose time span around equinox on march 20, 2000. In the generated table I look at the moments, when RA and DEC are zero - for astrometric and apparent data. The difference in time for astrometric and apparent is ~ 10 min. What is the reason? Two coordinate system coinside, because true equinox coinsides with the j2000 axis.

[selden]

Check to see what time standards are being used. The default time standards are likely to be different for the different types of ephemerides.

Usually astrometric ephemerides use TDC, which does not include leap-seconds, for example.
Often ground-based ephemerides use UTC, which includes leap-seconds in order to stay synchronized with the Earth's varying rotation. There are other differences between them, too.