Astroart Forum

In the manual there are a bunch of functions in Time and Date Coordinate that take an optional [jd] argument.

Firstly, am I correct in assuming that the default value for the optional [jd] is j2000?
Secondly what value type should [jd] be - a string? a number? and what format exactly?
Can I also just check that PrecessionJ2000 expects jnow coordinates and that PrecessionJNOW expects j2000 coords.

Thanks,
Andrew.

Hello, no.
JD, if not given, is the current date-time as read from the PC clock. It is used when converting from equatorial to altazimutal coordinates. It's the number of days passed since 4714 BC, see: http://en.wikipedia.org/wiki/Julian_day.
Precession functions work like you wrote.