Skyfield

------------------------

* Attempted to speed up Earth satellite calculations by caching a single
time scale object instead of creating a new one each time.

* Fixed a possible divide-by-zero error when applying deflection to an
apparent position.

---

* The ``observe()`` method of an observer on the Earth’s surface now
correctly accounts for the way that the Earth’s gravity will deflect
the apparent position of objects that are not exactly overhead,
bringing Skyfield’s agreement with the Naval Observatory’s NOVAS
library to within half a milliarcsecond.

* The time method ``tt_calendar()`` method no longer raises a
``TypeError`` when its value is an array.

* Running ``repr()`` on a ``Time`` array now produces a more compact
string that only mentions the start and end of the time period.

* The ``api.load()`` call no longer attempts to animate a progress bar
if the user is running it under IDLE, which would try to accumulate
the updates as a single long line that eventually hangs the window.

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* Added an `api` document to the project, in reverent imitation of the
`Pandas API Reference`_ that I keep open in a browser tab every time I
am using the Pandas library.

* New method `ICRF.separation_from()` computes the angular separation
between two positions.

* Fixed ``==`` between `Time` objects and other unrelated objects so
that it no longer raises an exception.

---

* 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.

---

* 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.