Hi All,

Did you read right?
Did you really see this sentence in the last Bill's message?

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.

That's the greatest new I have seen in Guide users list for a long time! Not only Guide 9 is focused on, but moreover, it is question of shipping!!!
However April 1st is no longer under way?

Clear clear skies for all of us!

Bertrand




From: Bill J Gray
Sent: Saturday, April 03, 2010 2:22 PM
To: guide-user@yahoogroups.com
Subject: Re: [guide-user] Re: NEAs or NEAs today?



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





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