RE: [find_orb] New development version of Find_Orb posted

Matson, Robert D. Apr 21, 2012

Hi Bill,

> ... To do it _really_ right, I'd have to modify Find_Orb's concept
> of the earth's orientation. It accounts for nutation and precession
> and Delta-T, of course, but not for "polar wander".

I'm not sure what additional effect you mean by polar wander.
Precession and nutation should completely account for our pole's
motion. (In reality, precession and nutation represent a somewhat
arbitrary separation of very long period, and shorter period
oscillations, respectively, in the earth's orientation. A single
set of equations can cover all polar motions, but for historical
reasons they tend to be treated as distinct from one another.)

How many terms are you using in your nutation equations? Or more
to the point, how small do the terms have to get before you no
longer care about them in FindOrb? 100 mas? 10 mas? Smaller?
Another factor you didn't mention is J2000 frame bias -- the
small correction that must be applied to J2000 catalog coordinates
to convert them to ICRS. Also, I assume you must be accounting for
DUT1 somewhere in your code, which requires periodic updates. Not
that important for MBAs, but it can affect astrometry for
fast-moving, nearby objects.

Best,
Rob


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