What is NOVAS?
--------------

NOVAS is an integrated package of functions for computing various
commonly needed quantities in positional astronomy.  The package can
supply, in one or two function calls, the instantaneous coordinates of
any star or solar system body in a variety of coordinate systems.  At a
lower level, NOVAS also provides astrometric utility transformations,
such as those for precession_, nutation_, aberration_, parallax_, and
gravitational `deflection of light`_.  The computations are accurate to
**better than one milliarcsecond.** The NOVAS library is an easy-to-use
facility that can be incorporated into data reduction programs,
telescope control systems, and simulations.  The U.S. parts of
*The Astronomical Almanac* are prepared using NOVAS.

This Python package includes both the NOVAS library and the Python
wrapper that are available from the `NOVAS home page`_ at the United
States Naval Observatory.  This version includes a few bug fixes and
packaging adjustments that are not in the most recent June 2011
release of the software from the Naval Observatory itself.  You can
find these changes described at the bottom of this page in the
Changelog, and you can also review them yourself at the `project
repository on GitHub`_.

This package has been uploaded to the Python Package Index by Brandon
Rhodes <brandon@rhodesmill.org>.  Please contact me, and not the busy
folks at the Naval Observatory, about any problems you encounter when
trying to install it — any problems with how it has been packaged are
my fault, not theirs!  For questions about how to use the library, you
can also ask for help on Stack Overflow, where I watch for questions
that involve Python and astronomy.

Quick Examples
--------------

Importing the library and opening the planetary ephemeris:

>>> from novas import compat as novas
>>> from novas.compat import eph_manager
>>> jd_start, jd_end, number = eph_manager.ephem_open()

Converting a calendar date to a Julian date:

>>> jd_tt = novas.julian_date(2012, 10, 2, 12.0)
>>> jd_tt
2456203.0

Asking where Mars is located in the sky on a given date,
in “astrometric” coordinates of the kind that are used
in printed sky atlases:

>>> mars = novas.make_object(0, 4, 'Mars', None)
>>> ra, dec, dis = novas.astro_planet(jd_tt, mars)
>>> print 'R.A. %d:%02f' % (ra, abs(ra) % 1. * 60.)
R.A. 15:36.176177
>>> print 'dec. %d:%02f' % (dec, abs(dec) % 1. * 60.)
dec. -20:11.951841
>>> print 'distance %f AU' % (dis,)
distance 1.947674 AU

There is more information at the `NOVAS home page`_
and in particular a
`full PDF manual that includes a “Sample Calculations” chapter
<http://aa.usno.navy.mil/software/novas/novas_c/NOVAS_C3.1_Guide.pdf>`_.
Even though the manual is for the C version,
you can generally puzzle out how to make the same calls from Python
if you compare the sample code
to the way that similar calculations are done
in the ``test`` package included inside of ``novas``!

Installation
------------

Like other packages listed here on the Python Package Index, this
package can be installed with the ``pip`` command. You will need to
install both the library itself as well as a high-accuracy ephemeris
data set, with the DE405 ephemeris being the current default::

    $ pip install novas
    $ pip install novas_de405

Note that the second command may take several minutes to run, depending
on your Internet connection, because the JPL ephemeris that it has to
download is 55 MB in size!

If you are managing a Python project that has a ``setup.py`` or a
``requirements.txt`` file, then instead of running these ``pip``
commands manually you can simply list these two package names alongside
the other packages that you depend on, and let them be installed as part
of your normal project install.

Sanity check: running the tests
-------------------------------

Once the package is installed, you can run its tests with the new
test-discovery feature built-in to Python 2.7. If the tests pass to
extremely high accuracy, then the result should be::

    $ python -m unittest discover novas
    ........................................
    ........................................
    ...........
    ----------------------------------------
    Ran 191 tests in 0.022s

    OK

If you are using an older version of Python, then you can run the tests
with the ``unittest2`` compatibility package instead::

    $ pip install unittest2
    $ unit2 discover novas

Running the tests this way should also result in a pretty field of dots,
followed by the message “OK.”

Contents and Documentation
--------------------------

Successful installation will produce a ``novas`` package that contains
several namespaces full of functions:

``novas.compat``
    Main NOVAS functions.

``novas.constants``
    Important constants.

``novas.nutation``
    Nutation models.

``novas.compat.eph_manager``
    Functions from the NOVAS ``eph_manager.c`` module.

``novas.compat.solsys``
    Functions from the NOVAS ``solsys1.c`` module.

``novas.compat.nutation``
    Functions from the NOVAS ``nutation.c`` module.

You can find more information and documentation on the project's
official `NOVAS home page`_ at the Naval Observatory.

Changelog
---------

Version 3.1.1.2 — 2013 July 31
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

* The top-level ``__init__.py`` now attempts to load the NOVAS C library
  from several different filenames under Python 3, instead of only
  trying filenames with an ABI identifier.  (On my Ubuntu 13.04 laptop,
  Python 3.3 is *not* in fact including such an identifier, which was
  causing an ``ImportError: cannot import name novaslib`` failure.)

Version 3.1.1.1 — 2013 March 2
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Further fixes have been provided by users, so I am making this interim
release while the official version still sits at 3.1.

* Leo Singer fixed the wrappers for ``cal_date()`` and ``limb_angle()``
  so these two NOVAS functions can now be used from Python code.
* The example test file ``checkout-stars-full.py`` now uses syntax
  compatible with Python 3, eliminating a warning during install.
* The ``setup.py`` now specifies the encoding for this readme, so the
  package can install on systems where UTF-8 is not the default.

Version 3.1.1 — 2012 November 25
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The Naval Observatory has not released a new version of NOVAS since
June 2011, but reports and requests from users convinced me to make
several small fixes to the code and make an interim release here on
the Python Package Index:

* The library is now Python 3 compatible!
* Python code can now access the ``ephem_close()`` routine inside the
  ``eph_manager`` module.
* Bugfix: the ``eph_manager.state()`` function was always raising an
  exception if invoked.
* Bugfix: to ``eph_manager.c`` as recommended in the `NOVAS FAQ`_.
* Bugfix: to ``novas.c`` as recommended in the `NOVAS FAQ`_.

To examine the code changes yourself, you can visit the `project
repository on GitHub`_.

Version 3.1 — 2012 September 19
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Initial release of the library on the Python Package Index.

License and Citation
--------------------

This software was produced by the United States Naval Observatory at the
expense of United States taxpayers, and is therefore not suseptible to
copyright, because a copyright would place taxpayer property under
private ownership. Since it is not copyrighted, it cannot be licensed;
it is simply free.

To credit the authors, you are invited to cite their work as follows:

**Barron, E. G., Kaplan, G. H., Bangert, J., Bartlett, J. L., Puatua, W., Harris, W., & Barrett, P. (2011)** `"Naval Observatory Vector Astrometry Software (NOVAS) Version 3.1, Introducing a Python Edition," <http://aa.usno.navy.mil/software/novas/novas_py/novas.pdf>`_ **Bull. AAS, 43, 2011.**

The authors of NOVAS ask that if you use their software in your work,
that you let them know at help@aa.usno.navy.mil since a record of who is
using their software helps them justify the excellent work that they are
doing by making the software available to the public.

.. _precession: http://asa.usno.navy.mil/SecM/Glossary.html#precession
.. _nutation: http://asa.usno.navy.mil/SecM/Glossary.html#nutation
.. _aberration: http://asa.usno.navy.mil/SecM/Glossary.html#aberration
.. _parallax: http://asa.usno.navy.mil/SecM/Glossary.html#parallax
.. _deflection of light: http://asa.usno.navy.mil/SecM/Glossary.html#deflection-light
.. _webpage: http://ssd.jpl.nasa.gov/?planet_eph_export
.. _unittest2 module: http://pypi.python.org/pypi/unittest2
.. _NOVAS home page: http://aa.usno.navy.mil/software/novas/novas_py/novaspy_intro.php
.. _NOVAS FAQ: http://aa.usno.navy.mil/software/novas/novas_faq.php
.. _project repository on GitHub: https://github.com/brandon-rhodes/python-novas
