On Fri, Sep 17, 2010 at 09:20:39AM +0100, [log in to unmask] wrote:
>
> > From previous studies the more correct method was found to correct
> > the arrival times to TDB - solar system Bary centre.
>
> You may be confusing two things here. The TDB time scale is
> barycentric in the sense that it removes from terrestrial time
> barycentric phenomena, namely the orbital speed of the Earth and
> the gravitational potential of the Sun and planets. But that is
> not the same as correcting the observations for light time. People
> sometimes think that TDB is to do with a clock at the barycentre,
> but it isn't: it's just as local to the observer as TAI or TT.
>
> The way you allow for light time is to express the direction to
> the source as a unit vector in your choice of spatial coordinate
> system and to take its dot product with the barycentric spatial
> coordinates of the observer in the same spatial coordinate system,
> and divide by c. You can make this as complicated as you like,
> but for your accuracy requirements I should think that a
> geocentric correction would be good enough. Call sla_EPV (or
> sla_EVP) to get the barycentric coordinates of the geocentre, and
> sla_DCS2C to get the unit vector corresponding to AE Aqr's ICRS
> (~J2000) direction, then sla_DVDV to get the dot product, and
> multiply by 499.004783836.
There recently appeared a Wikipedia article on BJD (the lighttime
correction from observatory to solar system barycentre), which
has a reference to a paper in PASP on the subject and on using TT
or TDB with it and not UT.
--
Horst Meyerdierks Royal Observatory Edinburgh
Linux/Network Manager [log in to unmask]
http://www.roe.ac.uk/~hme/ +44-131-6688-309
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