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Hello Paul,

I feel like i'm getting to the core of this.

Bu t i need your confirmation on my numbers before i move on. Here is what i've got for the conditions at the first post of this chain (time:18-03-1977 00:02:17 UT lat: -27.583333 , lon: 144.895000 ):

> The first thing you need is the phase angle: the angle between a vector
from Venus (for example) to the Sun, and from Venus to the observer.

1- phase angle = 139.313052 degrees

> The other illumination variable is position angle: the "azimuth" on the
celestial sphere from Venus to the Sun.

2- position angle = 50.820597 degrees

> Semidiameter of a body is computed with a function in the SofaJpl Angle
class.

3- sd = 0.42 minutes

> George Huxtable's formula for the offset between the center of
a body and the center of light

4- offset = 0.312019 minutes

my good old windows xp pc won't install the .net framework, so i got these numbers from currently installled NOVAS-C routines.

I also gave it a try and went ahead with the phase correction formula from the old almanac ( k*cos theta ) , using the position angle as theta and offset as k. The result is 0.19711 minutes to be added to the original uncorrected hc.

did i go too far?

Have a great weekend...

Mehmet