Skyfield

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* Introduced the ``Timescale`` object with methods ``utc()``, ``tai()``,
``tt()``, and ``tdb()`` for building time objects, along with a
``load.timescale()`` method for building a new ``Timescale``. The
load method downloads ∆T and leap second data from official data
sources and makes sure the files are kept up to date. This replaces
all former techniques for building and specifying dates and times.

* Renamed ``JulianDate`` to ``Time`` and switched from ``jd`` to ``t``
as the typical variable used for time in the documentation.

* Deprecated timescale keyword arguments like ``utc=(…)`` for both the
``Time`` constructor and also for all methods that take time as
an argument, including ``Body.at()`` and ``Topos.at()``.

* Users who want to specify a target directory when downloading a file
will now create their own loader object, instead of having to specify
a special keyword argument for every download::

load = api.Loader('~/ephemeris-files')
load('de421.bsp')

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* Users can now supply a target ``directory`` when downloading a file::

load('de421.bsp', directory='~/ephemerides')

* Fix: removed inadvertent dependency on the Pandas library.

* Fix: ``load()`` was raising a ``PermissionError`` on Windows after a
successful download when it tried to rename the new file.

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* Skyfield now generates its own estimate for ``delta_t`` if the user
does not supply their own ``delta_t=`` keyword when specifying a date.
This should make altitude and azimuth angles much more precise.

* The leap-second table has been updated to include 2015 July 1.

* Both ecliptic and galactic coordinates are now supported.

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* Skyfield has dropped the 16-megabyte JPL ephemeris DE421 as an install
dependency, since users might choose another ephemeris, or might not
need one at all. You now ask for a SPICE ephemeris to be downloaded
at runtime with a call like ``planets = load('de421.bsp')``.

* Planets are no longer offered as magic attributes, but are looked up
through the square bracket operator. So instead of typing
``planets.mars`` you should now type ``planets['mars']``. You can run
``print(planets)`` to learn which bodies an ephemeris supports.

* | Ask for planet positions with ``body.at(t)`` instead of ``body(t)``.

* Per IAU 2012 Resolution B2, Skyfield now uses lowercase *au* for the
astronomical unit, and defines it as exactly 149 597 870 700 meters.
While this API change is awkward for existing users, I wanted to make
the change while Skyfield is still pre-1.0. If this breaks a program
that you already have running, please remember that a quick ``pip``
``install`` ``skyfield==0.4`` will get you up and running again until
you have time to edit your code and turn ``AU`` into ``au``.

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* To prevent confusion, the :meth:`~skyfield.timelib.Time.astimezone()`
and :meth:`~skyfield.timelib.Time.utc_datetime()` methods
have been changed to return only a ``datetime`` object.
If you also need a leap second flag returned,
call the new methods
:meth:`~skyfield.timelib.Time.astimezone_and_leap_second()`
and :meth:`~skyfield.timelib.Time.utc_datetime_and_leap_second()`.

---

* The floating-point values of an angle
``a.radians``, ``a.degrees``, and ``a.hours``
are now attributes instead of method calls.


.. _Pandas API Reference: http://pandas.pydata.org/pandas-docs/stable/api.html