Thanks Bill and Bernd for your answers,

--- In guide-user@yahoogroups.com, Bill J Gray <pluto@...> wrote:

> When I set Guide's epoch to B1875, I see that Venus crosses
> RA=7h48m30s at 12:44:40 TT...
> I don't _think_ aberration would have any role in this. At least if ....

It is only a deduction from my calculations in excel (with vsop87d)
Instant calculated in excel from the mean position :
2557-08-14 12h44m51 (nearly like Guide)

Instant calculated in excel from the mean position but with aberration :
2557-08-14 12h26m31s (nearly like Horizon)

I did a lot of same calculations for the Sun they all agree with Guide without aberration ant with the JPL aberration included.
So I am pretty sure it has something to do with aberration, but I should rather agree with you that aberration should no be taken in account.

Luc