Hi John,

The only way I can suggest to compute an opposition time is this.
Right-click on the object, then generate an ephemeris with (say) a
one-day spacing. Make sure that the elongation (and/or distance to
the object) is included.

If it covers the time of opposition, you'll see that the elongation
reaches a maximum in the ephemeris. If it covers the time of minimum
distance, you'll see that, too. If you're so inclined, you can
then create another ephemeris covering the day or two around that
instant.

Incidentally, be warned that the concept of "opposition" is a
little fuzzy. The above method will tell you when the object is at
its maximum elongation from the sun, and I'd defend that as being
the best choice. Some people define "opposition" to be the time when
the object's ecliptic longitude is 180 degrees from the sun; others
would say it's when the right ascension is 180 degrees from the sun.
Neither of these makes a lot of sense to me. But both definitions
see at least some use.

-- Bill