Hi Luc,

I'll have to dig into this a bit more, but here's a summary of
what I've found so far:

Horizons and Guide produce essentially identical J2000 ephemerides
for Venus at this time. So that's not the source of the problem.

When I set Guide's epoch to B1875, I see that Venus crosses
RA=7h48m30s at 12:44:40 TT... which is exactly what you found. The
constellation readout in the legend does not change at exactly that
point. I need to investigate this, since it's a chunk of Guide that
has probably remained untouched for about fifteen years.

I don't _think_ aberration would have any role in this. At least if
it were up to me, I'd say that the constellation boundaries should
remain fixed in the B1875 (and therefore the J2000) frame of reference.
Otherwise, stars within about 20" of the border would switch constellations
on an annual basis.

I'll dig into this a bit more, probably this afternoon.

-- Bill