Naval Observatory Vector Astrometry Software

C Edition C3.1 (31 March 2011)

This repository is an import of the USNO NOVAS C v3.1 library, with the simple goal of making an easy to find, self-contained, turn-key version of this library on Linux.

The official library is none of these things, to put it mildly.

See README.txt for the official readme documenation.

The following changes have been made:

1. The source code was modified to remove warnings. The most noticeable of these changes is to not pass names as arrays to novas.c which was a well intentioned change but is poor practice. The modified functions all check the length of the passed strings before copying.
2. Makefiles have been added to handle building a separate static library (libnovas.a) as well as the self-check applications with appropriate dependency checking. There are two Makefiles:
   1. Makefile builds the library to use solsys1.c and eph_manager.c which expects to be passed the path to a binary ephemeris file.
   2. Makefile3 builds the library to use solsys3.c which is self-contained for use with just the Sun and stars.
3. Makefile automatically fetches an x86 (little-endian) binary ephemeris file from its current location at https://ssd.jpl.nasa.gov/ftp/eph/planets/Linux/.
4. The test output files for the checkout self-tests have been converted from DOS to unix line-endings and I updated the first line to reflect the different date range associated with the above ephemeris file.
5. Add C linkage guards so the library can be linked with C++ applications.
6. After creating this library I stumbled across https://github.com/toddcarnes/novas.git which is another copy of NOVAS C on github. This repo has made a few bugfixes and added support for many more ephemeris versions, so I have incorporated those fixes here also.

Building using an ephemeris file

If you are interested in building an application to perform astronomical operations involving any of the planets, then you can build the library by:

• Running make

Run the self-check tool or example application by:

• Running make check or
• Running make example

To build your own application, you should have it include eph_manager.h and novas.h and link with lib/libnovas.a. For simple applications, it would be trivial to just add a Makefile target for your application, similar to the example target.

For applications in another location, add this repo directory as an -I include path in your compiler options and as a prefix to lib/libnovas.a. Your application will be responsible for calling ephem_open() with a path to the ephemeris file (you may wish to make a copy or link in your application directory or a well-known location), as well as ephem_close().

Building without an ephemeris file

If your application only requires data for the Sun and stars, then you can build a library without any dependency on an external ephemeris file by.

• Running make -f Makefile3

Run the self-check tool by:

• Running make check

The steps to build your own application are the same as above except there is no need to make any calls to ephem_open() or ephem_close() and no dependency on an ephemeris file.

A word on binary ephemeris files

The NOVAS library originally claimed that ephemeris files could only be distributed in ASCII form because binary data is not portable between machines. This is really a legacy from the dark ages of using Fortran for this library. In C, or any modern language, this statement is only true if your software makes no effort to serialize and deserialize your data correctly. That's a solved problem, as demonstrated by every network application that exchanges binary data.

Because the NOVAS library still depends on ephemeris files in native binary format, there are conversion tools that convert the ASCII files into binary for a given machine architecture. However, the official version of this tool is only provided in Fortran. My attempts to use it generated ephemeris files that didn't pass the self-check test (the self-check test itself aborts with a floating point exception even on the official binary DE405 files). Others have tried to rewrite it in C but I found those implementations to also have bugs that caused the self-test checks to fail even after I fixed the format bugs that prevented this library from parsing those files.

Fortunately, JPL does host some pregenerated binary files for both big and little-endian architectures, which is what this project uses instead (although these are not mentioned in any of the documentation that I came across). The downside is that there is less flexibility with the date ranges provided and the files are larger than needed for many applications. I think a tool for trimming binary files would be straightforward, so I may add that at some point.

If you are building for a big-endian Unix platform, then you will want to modify the ephemeris URL (ie https://ssd.jpl.nasa.gov/ftp/eph/planets/SunOS/de405/unxp1900.405) in Makefile.