Thank you very much Bill for your very helpful answer.
Looking forward for Guide 9...
Stefano

--- In guide-user@yahoogroups.com, Bill J Gray <pluto@...> wrote:
>
> Hi Stefano, all,
>
> Catching up a bit here...
>
> The problem is in the epoch.
>
> NEA.txt (and most of the MPCORB-formatted files, and most MPC orbital
> elements in general) are given for specific epochs, at 100 or 200-day
> intervals. For example, elements in the current NEA.txt file have an
> epoch of 2010 July 23 = Julian Day 2455400.5. Thus, if you ignore
> perturbations (which Guide does), you'll get exactly the right result
> for 2010 July 23.
>
> Set a date/time away from 2010 July 23, and things will gradually
> fall apart. It's not a big difference, usually, but if the object
> comes close to a planet, it can become one. And NEAs frequently do
> come close to a planet, namely ours.
>
> There are a couple of solutions to this problem:
>
> (1) Ignore it. For most uses, you can do this; the object will
> still be on the CCD frame pretty close to the right point. This will
> almost always be true for main-belt and further objects, or if the
> NEA doesn't make a close approach over the time in question. For
> example, if the object doesn't approach a planet between now and
> July 23, you could use the current NEA.txt without much trouble.
>
> Overwhelmingly, this is the most popular (and, in most cases,
> perfectly defensible) option. However, maybe the object does come
> close to the earth. Or maybe you're attempting to do a calculation
> requiring a truly exact answer, such as an asteroid occultation.
> In that case...
>
> (2) Use NEAm00.txt or a similar file. This will give you data with
> an epoch of "today". That usually works very nicely indeed. But perhaps
> your interest is not in where the object is today, or within a few
> days of today (MPC provides files running ten days into the past and
> future). Or maybe the object isn't an NEA. In that case...
>
> (3) Get MPCORB.dat, and run it through the Integrat software at
>
> http://home.gwi.net/~pluto/integrat.htm
>
> Full directions are given there.
>
> And, of course, these is option (4): Guide really ought to be
> bright enough to compute perturbed positions. I have some plans for
> this, and some of the work I've done for Find_Orb will make this
> a heck of a lot easier than it otherwise would be.
>
> However, I am currently focussed on what's needed to get Guide 9
> done. This mostly means "things that will appear on the DVD".
> Perturbation calculations would be a software change; I can post
> that as an update _after_ Guide 9 is shipped.
>
> -- Bill
>