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Re: JPL Planetary Ephemeris DE405

Francesco Poli <invernomuto@paranoici.org> writes:
> I wrote an
> [essay](https://www.inventati.org/frx/essays/softfrdm/whatissource.html)
> on this topic.
> [...]
>> Is it the preferred form of the work for making modifications (GPL def)?
>> Clearly not. The preferred form would be to repeat the observations.
> I don't agree: repeating the observations is the preferred *method*
> (not *form of the work*) for *re-creating* the work from scratch.
> On the other hand, *making modifications* to the work requires some
> form of the work itself.

You are right here.

> You have to imagine the desire to alter the data (even in ways that
> make the data no longer be an accurate scientific representation of a
> phenomenon) and ask yourself: which is the best form I would start
> from, in order to make modifications?

Sure. And this is not so simple as for source code: It heavily depends
on your goal, and on the data.

> I think this is much like digital photographs.
> A digital photograph represents a scene with objects and/or living
> beings.
> But you can modify it to create an image that no longer represents a
> real scene (think about special effects in movies).
> See the last FAQ in my above-mentioned essay...

Citing from there:
| Depending on which format is extracted from the digital camera and on
| which form one starts with when making modifications, the source may
| be in raw format, JPEG, or some other form...

This would be what I call "raw data", so the original measurements,
which were used for the several analysis steps that finally lead to the

Some use cases where one would like to change the ephemeris:

 - I have an improvement in one of the processing steps, and I want to
   make use of it

 - I have additional data that I want to merge in an early step of the
   processing chain

I can't do this, since I don't have the software available that produced
the final data (even when the method is described in a paper). And I
also don't have the raw data available.

The idea that one could legally "just change some numbers" is not
realistic at all: the numbers in the ephemeris data depend on each
other, and changing them by hand will just make the data
inconsistent. Having only this right gives only a formal freedom, but no
real advantage, since you need more information about the inner
structure of the data.

Or to use another part of your essay:

| [...] preferred by whom?
| The person whose preference should be taken into account is the one
| who last modified the version of the work under consideration. If
| he/she prefers to modify the work in a given form, then that form is
| the source code for the work.

If we take JPL, they will probably not just modify the data. They will
regenerate them with modified input. This is at least the case for the
astrophysics databases I am connected with: they don't ever "modify" the
published data, they create a new release with improved analysis: The
Gaia mission provided a data release 1 in 2016 and will publish a new
data release 2 in a month or so, with better accuracy, more stars, and
more data for each star. The (preferred) "modification" is not done in
the final data, but in the data processing chain.

Would you really consider these data releases as "source"?

Best regards


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