Bill J Gray wrote:

> 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.

"See at least some use" is indded an understatement - the standard way of
define opposition is a 180 deg difference in ecliptic longitude.

Your way of defining opposition as the moment of greatest elongation
has one serious drawback: it doesn't tell whether an object ever reaches
opposition or not. Not all greatest elongations are oppositions. Venus
for instance reaches maximum elongation from the Sun on 3 November -- but
I think a lot of people would object if you called this an "opposition" of
Venus. So you'd have to add some extra condition, e.g. that the greatest
elongation had to exceed, say, 150 degrees. But then, what about e.g. Pallas,
which has an orbit inclined more than 30 degrees and therefore will be closer
to the Sun than 150 degrees at some of its oppositions - would such an
opposition not really be an opposition?

To define opposition as the time when the difference in ecliptic longitude
is 180 degrees works fine, and it does not differ strongly from the moment
of greatest elongation unless the celestial body moves very quickly across
the sky far away from the ecliptic. For an object which is stationary in
the ecliptic coordinate system, the moment of 180 deg difference in ecliptic
longitude from the Sun will be exactly the same as the moment of greatest
elongation from the Sun, and either of these could then be used to define
"opposition". However, the "180 deg longitude difference" method has a
practical advantage: it is easier to compute.


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