Skyfield

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

* Replaced the old historic ∆T table from the United States Naval Observatory
with up-to-date splines from the 2020 release of the extensive research by
`Morrison, Stephenson, Hohenkerk, and Zawilski <Morrison, Stephenson, et al>`
and also adjusted the slope of Skyfield’s near-future ∆T estimates
to make the slope of ∆T much less abrupt over the coming century.

* Added a full reference frame object
for the :class:`~skyfield.sgp4lib.TEME` reference frame
used by SGP4 Earth satellite elements.

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

* Added a :meth:`~skyfield.positionlib.ICRF.frame_latlon_and_rates()` method
that can compute the rates at which angles like altitude and azimuth,
or right ascension and declination,
are changing.

* Accepted a contributor’s helpful fix for a rounding error
that had slightly shifted a few constellation boundaries.
`548 <https://github.com/skyfielders/python-skyfield/issues/548>`_

* The :class:`~skyfield.timelib.Time`
tuple :data:`~skyfield.timelib.Time.utc`
and method :meth:`~skyfield.timelib.Time.utc_strftime()`
are now backed by the same math,
so they always advance to the next calendar day at the same moment.
This makes it safe to mix values returned by one of them
with values returned by the other.
`542 <https://github.com/skyfielders/python-skyfield/issues/542>`_

* Vector subtraction now returns the position subclass
specific to the resulting vector’s center.
`549 <https://github.com/skyfielders/python-skyfield/issues/549>`_

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

* Tweaked several lines of code that build NumPy arrays
to avoid a new deprecation warning
``Creating an ndarray from ragged nested sequences
(which is a list-or-tuple of lists-or-tuples-or ndarrays
with different lengths or shapes) is deprecated``.
NumPy no longer wants to accept a simple constant like ``0.0``
where the resulting array needs a whole row of zeros.
`536 <https://github.com/skyfielders/python-skyfield/issues/536>`_

* Added an :meth:`~skyfield.positionlib.ICRF.hadec()` position method that
returns hour angle and declination.
`510 <https://github.com/skyfielders/python-skyfield/issues/510>`_

* The default ``str()`` and ``repr()`` strings
for geographic positions have been streamlined,
and no longer raise ``ValueError`` when elevation is an array.
They now show simple decimals
instead of splitting degrees of longitude and latitude
into minutes and seconds;
always show elevation, even if zero;
properly format NumPy arrays;
and abbreviate long arrays.
`524 <https://github.com/skyfielders/python-skyfield/issues/524>`_

* Fixed
:meth:`Angle.dstr() <skyfield.units.Angle.dstr>`
and
:meth:`Angle.hstr() <skyfield.units.Angle.hstr>`
to return an array of strings when the angle itself is an array.
`527 <https://github.com/skyfielders/python-skyfield/issues/527>`_

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

* Deprecated the old ``Topos`` class,
which not only featured a clunky interface
but hid from users the fact that Skyfield was generating IERS2010 positions
from latitude and longitude
when in fact nearly all users want WGS84 positions.
Users are now encouraged to supply latitude and longitude
to the :meth:`~skyfield.toposlib.Geoid.latlon()` method
of either the :data:`~skyfield.toposlib.wgs84` object
or the :data:`~skyfield.toposlib.iers2010` object.
Related discussion:
`372 <https://github.com/skyfielders/python-skyfield/issues/372>`_

* The two new geoid objects :data:`~skyfield.toposlib.wgs84`
and :data:`~skyfield.toposlib.iers2010`
have also provided a happy new home
for the :meth:`~skyfield.toposlib.Geoid.subpoint()` method —
which was previously stranded
over on the :class:`~skyfield.positionlib.Geocentric` class,
where it couldn’t be used with positions of other classes
that might be centered at the geocenter.
(The old method will remain in place to support legacy code,
but is discouraged in new applications.)

* The effects of :ref:`polar-motion` — if configured — are now included
both when computing the position in space of an Earth latitude and longitude,
and when determining the latitude and longitude beneath a celestial position.

* Added :func:`~skyfield.api.load_constellation_names()`.

* The :meth:`~skyfield.timelib.Time.utc_jpl()` method now correctly
designates its return value as ``UTC`` instead of the ambiguious ``UT``.
`515 <https://github.com/skyfielders/python-skyfield/issues/515>`_

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

* The position classes have gained methods
:func:`~skyfield.positionlib.ICRF.frame_xyz()`,
:func:`~skyfield.positionlib.ICRF.frame_xyz_and_velocity()`,
:func:`~skyfield.positionlib.ICRF.frame_latlon()`, and
:func:`~skyfield.positionlib.ICRF.from_time_and_frame_vectors()`
that work with a new library ``skyfield.framelib``
to offer a number of familiar reference frames.
These replace the existing ad-hoc position methods
for ecliptic and galactic coordinates,
which are now deprecated (but will continue to be supported).
See :ref:`reference_frames`.
`476 <https://github.com/skyfielders/python-skyfield/issues/476>`_

* Added an official :class:`~skyfield.framelib.itrs` reference frame.

* Added support for IERS :ref:`polar-motion` 𝑥 and 𝑦.

* Added a method :meth:`~skyfield.toposlib.GeographicPosition.lst_hours_at()`
that computes Local Sidereal Time.

* A new almanac routine :func:`~skyfield.almanac.moon_phase()` returns
the Moon phase as an angle where 0° is New Moon, 90° is First Quarter,
180° is Full, and 270° is Last Quarter.
`282 <https://github.com/skyfielders/python-skyfield/issues/282>`_

* Almanac search routines that previously returned a Boolean true/false
array now return an integer 0/1 array instead, to work around a new
deprecation warning in NumPy which, for example, would have outlawed
using the Boolean array from :func:`~skyfield.almanac.moon_nodes()` to
index into the ``MOON_NODES`` list that provides a name for each node.
`486 <https://github.com/skyfielders/python-skyfield/issues/486>`_

* The undocumented columns ``magnitude_H`` and ``magnitude_G`` in the
Minor Planet Center comets dataframe have been renamed ``magnitude_g``
and ``magnitude_k`` following further research on the file format
(which does not itself document which magnitude model is intended).
`416 <https://github.com/skyfielders/python-skyfield/issues/416>`_

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

* Fix: running ``load.timescale(builtin=False)`` was raising an
exception ``FileNotFoundError`` if the ``finals2000A.all`` file was
not already on disk, instead of downloading the file automatically.
`477 <https://github.com/skyfielders/python-skyfield/issues/477>`_