Hi all,

I am an old Guide user but new to this forum. Forgive me if I do some mistakes in English as I am from Belgium and my mother tongue is french.

I try to accurately represent some calculations I made with excel in Guide 8.
For example I found that on 2557-08-14 at about 12h26m31s TD Venus crosses the limit Cnc/Cmi (a very rare event). This seems to be confirmed by the JPL Horizon web site which gives 12h27m28s for the same day. (The JPL uses an old precession formula for the period 2000/1875 to find the constellation, perhaps it could explain the one minute difference).
If I try to represent this in Guide, the best is to set the format to B1875 and the same for ticks, grids, labels etc ..., putting the north up for the sky, select TD for time-zone and geocentric for the position. Thanks a lot, Guide for all these rare options !
The limit being 7h48m30s in B1875, it is easy to see with a big zoom (unfortunately the boundaries disappears with big zooms, perhaps I missed here an option) that Venus crosses the limit at about 12h44m40s TD in Guide.
There is thus a difference of about 18 minutes in time between Guide and my calculation.
Looking at this more deeply I thought that it was related with aberration (not the time light, just the aberration like stars.
Actually to calculate the instant of the border's cross in excel I start from the apparent position of the date, I remove the nutation and I precess from 2557 to 1875. I suppose that the JPL starts with J2000 position add the aberration and precess from J2000 to B1875.
The 2 methods should give the same result and it seems to be nearly the case.
I realized that the grid in Guide is for the mean position. If I can ask apparent position in the formats menu, I cannot do the same for the grid, ticks etc .. and there is then a difference between the grid (mean) and the cursor position (apparent).
Actually it is not clear for me if the aberration has to be taken in account in this calculation, as the limits were build from the mean 1875's positions. I just can guess by comparison that the JPL seems to take it in account and also from a paper from french astronomer Bacchus that I found at :
http://adsabs.harvard.edu/full/1979LAstr..93..297B.
Perhaps could we just say that Guide give the instant of the real Venus and the JPL the instant of the apparent Venus ?
I should be grateful to get Bill's opinion on this.
Luc