- to .
- Diurnal aberration.
- to .
- Refraction.

- 1.
- from civil time obtain the coordinated universal time, UTC (more later on this);
- 2.
- add the UT1-UTC (typically a few tenths of a second) to give the UT;
- 3.
- from the UT compute the Greenwich mean sidereal time (using sla_GMST);
- 4.
- add the observer's (east) longitude, giving the local mean sidereal time;
- 5.
- add the equation of the equinoxes (using sla_EQEQX).

Note that for very precise work the observer's longitude should
be corrected for *polar motion*. This can be done with
sla_POLMO.
The corrections are always less than about
, and
are futile unless the position of the observer's telescope is known
to better than a few metres.

Tables of observed and predicted UT1-UTC corrections and polar motion data are published every few weeks by the International Earth Rotation Service.

The transformation from apparent to *topocentric*
consists of allowing for
*diurnal aberration*. This effect, maximum amplitude
,
was described earlier. There is no specific SLALIB routine
for computing the diurnal aberration,
though the routines
sla_AOP *etc.* include it, and the required velocity vector can be
determined by calling
sla_GEOC.

The next stage is the major coordinate rotation from local equatorial
coordinates into horizon coordinates. The SLALIB routines
sla_E2H
*etc.* can be used for this. For high-precision
applications the mean geodetic latitude should be corrected for polar
motion.

Starlink User Note 67

P. T. Wallace

12 October 1999

E-mail:ptw@star.rl.ac.uk