Objectives

SLALIB was designed to give application programmers a basic set of positional-astronomy tools which were accurate and easy to use. To this end, the library is:

• Readily available, including source code and documentation.
• Supported and maintained.
• Portable - coded in standard languages and available for multiple computers and operating systems.
• Thoroughly commented, both for maintainability and to assist those wishing to cannibalize the code.
• Stable.
• Trustworthy - some care has gone into testing SLALIB, both by comparison with published data and by checks for internal consistency.
• Rigorous - corners are not cut, even where the practical consequences would, as a rule, be negligible.
• Comprehensive, without including too many esoteric features required only by specialists.
• Practical - almost all the routines have been written to satisfy real needs encountered during the development of real-life applications.
• Environment-independent - the package is completely free of pauses, stops, I/O etc.
• Self-contained - SLALIB calls no other libraries.

A few caveats:

• SLALIB does not pretend to be canonical. It is in essence an anthology, and the adopted algorithms are liable to change as more up-to-date ones become available.
• The functions aren't orthogonal - there are several cases of different routines doing similar things, and many examples where sequences of SLALIB calls have simply been packaged, all to make applications less trouble to write.
• There are omissions - for example there are no routines for calculating physical ephemerides of Solar-System bodies.
• SLALIB is not homogeneous, though important subsets (for example the FK4/FK5 routines) are.
• The library is not foolproof. You have to know what you are trying to do (e.g. by reading textbooks on positional astronomy), and it is the caller's responsibility to supply sensible arguments (although enough internal validation is done to avoid arithmetic errors).
• Without being written in a wasteful manner, SLALIB is nonetheless optimized for maintainability rather than speed. In addition, there are many places where considerable simplification would be possible if some specified amount of accuracy could be sacrificed; such compromises are left to the individual programmer and are not allowed to limit SLALIB's value as a source of comparison results.