Hi Dario,

Yes, Guide can do this. (Note that the improvement
in position is _very_ small indeed!)

To do so, you first need to get the DE421 data,
in ASCII form, from

ftp://ssd.jpl.nasa.gov/pub/eph/planets/ascii/de421

You'll need to get the 'ascp1900.421' and 'header.421'
files. Next, you need to convert them to binary. There's
some discussion of how to do this at

ftp://ssd.jpl.nasa.gov/pub/eph/planets/README.txt

Once you have a binary ephemeris file, it's
easy to persuade Guide to use it. Let's say that you
have named that file 'ephem.421'. Edit the file
'guide.dat' in Notepad, and add this line:

JPL_EPH=ephem.421

Guide will then use that ephemeris for positions of
all planets (defined as Mercury through Pluto).

The weak point in all of this will become apparent
when you look at README.txt. The process for converting
the software is a little user-abusive, especially if you
are running in Windows. (For some DE ephemerides, there
are binary versions already made available. But I don't
think that's happened yet for DE-421.)

As it happens, I've played around a little with the
conversion utility, 'asc2eph', in hopes of making it a
little less abusive. You can get my version (with source
code included) at http://www.projectpluto.com/asc2eph.zip
(about 31 KBytes, just posted moments ago.)

UnZIP this into your Guide folder. Suppose you downloaded
'header.421' and 'ascp1900.421' to the folder c:\de421.
Go to the Guide folder and run

asc2eph c:\de421 421

Some data from the header will be shown, and then you'll
be asked, "Do you want to specify the subinterval for the
binary ephemeris (y/n)?" Hit 'n'.

Mere moments later, there should be a file called
'jpleph.421', about 13.9 MBytes, with data for the years
1900 to 2050. Add that line 'JPL_EPH=jpleph.421' to guide.dat,
and you should be off to the races. When you ask for 'more
info' on a major planet, you should see:

Position source: JPL DE-421

-- Bill