# Installing Skyfield

Skyfield has only a single binary dependency, the NumPy vector library, and is designed to install cleanly with a single invocation of the standard Python package tool:

    pip install skyfield

This should install Skyfield, NumPy, and the small collection of pure-Python astronomy libraries that Skyfield depends on. If you lack `pip` and need to install each dependency by hand, consult Skyfield’s `setup.py` file for the full list.

If trying to install Skyfield gives you errors about NumPy, there are several other ways to get NumPy installed:

- It is best to simply use a science-ready version of Python that comes with NumPy built-in, like the Anaconda distribution.

- There are several approaches described in the SciPy install instructions.

- You can download and run an official NumPy installer.

- You can try to get the plain `pip`-powered install working by giving your system a functioning C compiler that matches the compiler used to build Python. On Windows with Python 2.7, try installing the free Visual Studio Express 2008. Mac users should install the “Xcode Command Line Tools” to give `pip` the superpower of being able to build and install binary Python dependencies like NumPy.

Read the Changelog below to learn about recent fixes, changes, and improvements to Skyfield. You can protect your project from any abrupt API changes by pinning a specific version of Skyfield in your `requirements.txt` or `setup.py` or install instructions:

    skyfield==1.34

By preventing Skyfield from getting accidentally upgraded until you are ready to advance the version number yourself, you can avoid even slight changes in behavior and output coordinates that might result from an upgrade. If you find any problems or would like to suggest an improvement, simply create an issue on the project’s GitHub page:

> https://github.com/brandon-rhodes/python-skyfield

Good luck!

## Checking your Skyfield version

The Skyfield package offers a `VERSION` tuple that your code can use to test which version of Skyfield is running. For example, this code checks whether Skyfield is at least at version 1.24:

    import skyfield

    if skyfield.VERSION < (1, 24):
        print('Too old')

A new featured added in Skyfield 1.36 (released January 2021) is that you can invoke the `skyfield` module from the command line to display its version:

    $ python -m skyfield

    Skyfield version: 1.37
    jplephem version: 2.14
    sgp4 version: 2.17
    Built-in leap seconds table ends with leap second at: 2016-12-31 23:59:60 UTC
    Built-in ∆T table from finals2000A.all covers: 1973-01-01 to 2022-01-29

Also displayed are the versions of the libraries Skyfield depends on, and the start and end dates of its built-in timescale tables.

## Citing Skyfield

If Skyfield has proven useful in your research, the project welcomes your citation to bring it to the attention of other academics who might benefit from it.

- ASCL: ascl:1907.024

- Bibcode: 2019ascl.soft07024R

- Current CITATION.cff file on GitHub: CITATION.cff

- Permanent URL, used by several papers to uniquely identify Skyfield (and which is by now a redirect to `https:` but remains `http:` in this URL to match papers already using it as an identifier): http://rhodesmill.org/skyfield/

## Changelog

### Released versions

#### v1.54 — 2026 January 18

- Skyfield’s internal table for the ∆T Earth orientation parameter has been updated, so observations extend to this month and predictions to January 2027. Compared to the previous version of Skyfield, this changes the Earth’s rotation angle by up to 0.2 arcseconds in 2025 and 1.1 arcseconds in 2026.

- The `apparent()` method now accepts an optional `deflectors=` argument that lets callers control the list of bodies (by default the Sun, Jupiter, and Saturn) for which the deflection of light is computed. Callers can provide an empty list to turn deflection off.

- The two routines `find_risings()` and `find_settings()` now skip deflection when computing the target’s apparent position, which increases their speed by around 30%. The routines aim for only millisecond precision, while deflection only makes a difference of microseconds.

- You can now subtract a NumPy array of floating point TT days from a Skyfield `Time` object of the same length; previously, only a plain int or float was supported as the subtrahend.

#### v1.53 — 2025 April 7

- Fix: calling `observe()` on a position generated by an ephemeris with multiple segments per target, like DE441, was raising an exception:

      File ".../skyfield/relativity.py", line 46, in add_deflection
        deflector = ephemeris[name]
      TypeError: 'NoneType' object is not subscriptable

  Users should no longer receive the exception. #1063

#### v1.52 — 2025 March 18

- Fix: on rare occasions the `find_risings()` and `find_settings()` routines could return an invalid result if no rising or setting was found, instead of returning the moment of transit as their fallback result. #1057

- Fix: if the `altaz()` method was given a position with a NaN coordinate, and was passed `temperature_C` and `pressure_mbar` parameters with which to compute refraction, then it would loop forever. #1046

- Fix: the new `mean_equator_and_equinox_of_date` reference frame was off by nearly 0.02 arcseconds because it applied precession but forgot to apply the frame tie between ICRS and J2000.

- To avoid deprecation warnings on recent Pythons, Skyfield now avoids Python’s `datetime.utcnow()` method, and avoids applying the `~` unary operator.

#### v1.51 — 2025 February 23

- Skyfield now supports ephemeris files like `de441.bsp` that have two or more segments per target. In earlier versions of Skyfield, only one segment per target would get used, which for `de441.bsp` was cutting the range of supported dates in half. #691

- The documentation now uses the newer name `.xyz`, which has been quietly supported for around three years, for the `ICRF` attribute that was originally named `.position` and holds its position vector. The old name (which will continue to be supported) could lead to code that looked a little redundant, asking for the `position` of a `position`:

      position = mars.at(t)
      x, y, z = position.position

  So Skyfield now encourages code to ask for `position.xyz` instead. An advantage is that the new name is self-documenting: the name reminds the user that it is a 3-vector of Cartesian components.

- Skyfield’s internal table for the ∆T Earth orientation parameter has been updated, so that its predictions now extend to 2026-02-28.

- Fix: the `find_events()` Earth satellite method was returning an empty list of events if the only event in the time period was a lone rising or setting. It should now detect and return them. #856 #996 #1017

- Fix: the `find_events()` Earth satellite method, faced with a single pass that was very close to the start time, was returning an inaccurate setting time. It should now return an accurate setting time. #1000

- Fix: the `find_events()` Earth satellite method would miss a rising that came a fraction of a second before the corresponding culmination. It should now find both. #559

- Fix: bodies with Kepler orbits (like comets and asteroids) were incorrectly returning positions with only a single dimension if given a `Time` that was an array but had only one element. This could cause the rising and setting almanac routines to raise a `ValueError` if they found only a single rising or setting. #959

- Fix: the position vectors for Kepler orbit bodies, like comets and asteroids, now have a useful `.target_name` like `'Ceres'` or `'1P/Halley'` instead of the less informative value `'str'`.

#### v1.50 — 2025 February 15

- A new `mean_equator_and_equinox_of_date` coordinate frame lets users generate the same coordinates that an almanac might give.

- Skyfield now offers a Solar System Barycenter object, so users don’t have to construct the position themselves: `SSB.at(t)` returns a position whose coordinates and velocity are both zero in the ICRS.

- The routines added last year, `find_risings()` and `find_settings()`, could occasionally miss a sunrise or moonrise at high latitudes like 70°N where the Sun or Moon barely crests the horizon. This has been fixed at a moderate cost to their runtime. #998

- Skyfield no longer tries to protect users by raising an exception if, contrary to the usual custom in astronomy, they ask for `ra.degrees` or `dec.hours`. So users no longer need to add an underscore prefix (`_degrees` or `_hours`) to bypass the exception, though both names will keep working to support legacy code.

- The time methods `utc_datetime()` and `utc_datetime_and_leap_second()` now intercept the `ValueError` that Python raises for a negative year or a Julian-only leap day, and replace the generic error message with a more specific one. #957 #992

- If you call `load.timescale(builtin=False)` to download an updated copy of the IERS `finals2000A.all` Earth orientation data file, Skyfield now fetches the file with HTTPS, since their old FTP server seems to have disappeared. #1019

#### v1.49 — 2024 June 13

- A new `from_omm()` Earth Satellite constructor has been added to load satellite elements from modern OMM data instead of from old TLE data. The Earth satellite documentation now describes two OMM formats and shows how to load satellites from each one. #763

- If you print an instance of the `PlanetaryConstants` class to the screen, it will list all of the segments that it has loaded from binary kernels. #952

- The Skyfield documentation is no longer installed alongside the Python code, reducing the size of Skyfield by around 25%. Users who need offline access to the documentation will now need to download it separately.

#### v1.48 — 2024 February 7

- Skyfield is now compatible with NumPy 2.0! (Previous versions of Skyfield would raise an `ImportError` trying to import the symbol `float_`, which is now named `float64`.)

- Times now support the `<` operator, so Python can sort them.

- For convenience, geoids like `wgs84` have a new attribute `polar_radius`.

- You can no longer subtract two positions unless they have the same `.center`. Otherwise, a `ValueError` is raised. This check has always been performed when you subtract vector functions, but it was missing from the position subtraction routine.

- On days that the Sun fails to rise and set in the Arctic and Antarctic, the new rising and setting routines now correctly set the value `False` not only for sunrise but also for sunset.

- Fix: Skyfield no longer raises the following exception if you call `observe()` on a position whose coordinate and time arrays are empty.

      ValueError: zero-size array to reduction operation maximum which has no identity

  Instead, an empty apparent position is now returned. The exception was sometimes triggered by almanac routines if you searched for an event that didn’t occur between your start and end times. #991

#### v1.47 — 2024 January 13

- Added faster and more accurate rising and setting routines! See Risings and settings for documentation and examples of the new `find_risings()` and `find_settings()` and `find_transits()` functions. #662

- Skyfield’s internal table for the ∆T Earth orientation parameter has been updated, so that its predictions now extend to 2025-01-18.

- Constellation abbreviations are now consistent between the `load_constellation_map()` table and the `load_constellation_names()` list. Previously, `CVn` and `TrA` had been mis-capitalized in the list as `Cvn` and `Tra`. #906

#### v1.46 — 2023 April 9

- The `oppositions_conjunctions()` routine now measures ecliptic longitude using the ecliptic of each specific date, rather than always using the J2000 ecliptic, which should improve its accuracy by several seconds.

- Skyfield’s internal table for the ∆T Earth orientation parameter has been updated, so that its predictions now extend to 2024-04-13.

- Bugfix: Skyfield was giving values several kilometers off when computing the elevation above ground level of a target that was positioned directly above the Earth’s north or south pole.

- Bugfix: the `is_behind_earth()` method was incorrectly returning `True` if the Earth was on the line that joins the two satellites, but over on the far side of the other satellite where it wasn’t really in the way.

- Internals: the `altaz()` method now lives on the main position class instead of in two specific subclasses. If the user mistakenly tries to call `.altaz()` on an instance of the `Astrometric` position subclass — which previously lacked the method — then a friendly exception is raised explaining their error.

#### v1.45 — 2022 September 15

- Bugfix: minor planets and comets in Skyfield 1.44 would raise an exception if asked for a position in the half of their orbit where they are inbound towards their perihelion.

#### v1.44 — 2022 September 12

- Skyfield’s internal table for the ∆T Earth orientation parameter has been updated, so that instead of including measurements only through December 2021 it now knows Earth orientation through September 2022.

- Distance and velocity objects can now be created by calling their unit names as constructors, like `d = Distance.km(5.0)` and `v = Velocity.km_per_s(0.343)`.

- Updated the URL from which the Hipparcos database `hip_main.dat` is downloaded, following a change in the domain for the University of Strasbourg from `u-strasbg.fr` to `unistra.fr`.

#### v1.43.1 — 2022 July 6

- An attempt at overly clever scripting resulted in a Skyfield 1.43 release without a `setup.py` in its `.tar.gz`; within an hour, a Python 2.7 user had reported that Skyfield could no longer install. This release is identical to 1.43 but (hopefully) installs correctly for everyone!

#### v1.43 — 2022 July 6

- Fixed `planetary_magnitude()` so it works for Saturn even when the time is an array rather than a single time; also, improved its calculation slightly with respect to Uranus. #739

- Improved `load_comets_dataframe()` so that parsing `CometEls.txt` with the most recent version of Pandas doesn’t stumble over the commas in the final field of (for example) Halley’s Comet and give the error `ParserError: Error tokenizing data. C error: Expected 12 fields…saw 13`. #707

#### v1.42 — 2022 February 6

- Added two new position methods `phase_angle()` and `fraction_illuminated()` that, given an illuminator (usually the Sun) as their argument, compute whether the observer is looking at the bright side or the dark side of the target body. They replace a pair of old functions in the almanac module.

- The almanac routine `moon_nodes()` would sometimes skip nodes that were closer together than 14.0 days. It has been tightened down and should now detect all lunar nodes. #662

- Time objects now feature a `to_astropy()` method.

- The position method `to_skycoord()` now sets the `frame` attribute of the sky coordinate it returns, and for now only supports barycentric and geocentric positions. #577

#### v1.41 — 2021 December 16

- Times now support arithmetic: you can add or subtract from a time either a number representing days of Terrestrial Time (TT) or a Python `timedelta` which Skyfield interprets as TT days and seconds. #568

- Fixed the `.itrs_xyz` vector of the geographic position returned by the `subpoint_of()` method. #673

- Skyfield now uses HTTPS instead of FTP to download JPL ephemeris files like `de421.bsp`. This does risk raising an error for users whose machines have out-of-date root certificates. But it protects the connection from outside tampering, and will keep working if the `ssd.jpl.nasa.gov` FTP service is ever shut down — as happened earlier this year to FTP on NASA’s `cddis.nasa.gov` server. #666

#### v1.40 — 2021 November 14

- Extended the `planetary_magnitude()` routine to work with all the major planets, which upgrades it from a prototype feature to a production feature of Skyfield.

- The `subpoint()` method has been deprecated, because users reported that its name was a poor match for its behavior. Four new methods have replaced it: `latlon_of()`, `height_of()`, `geographic_position_of()`, and `subpoint_of()`. #644

- Added a timescale method `linspace()`. #617

- The `oppositions_conjunctions()` routine, which was originally designed only for planets, can now also handle the Moon (which moves from opposition to conjunction much faster).

#### v1.39 — 2021 April 14

- The `Angle.dstr()` and `Angle.hstr()` methods now accept a `format=` argument that lets callers override Skyfield’s default angle formatting and supply their own; see Formatting angles. #513

- The prototype `planetary_magnitude()` function now works not only when given a single position, but when given a vector of several positions.

#### v1.38 — 2021 April 3

- 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 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 `TEME` reference frame used by SGP4 Earth satellite elements.

#### v1.37 — 2021 February 15

- Added a `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

- The `Time` tuple `utc` and method `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

- Vector subtraction now returns the position subclass specific to the resulting vector’s center. #549

#### v1.36 — 2021 January 26

- 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

- Added an `hadec()` position method that returns hour angle and declination. #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

- Fixed `Angle.dstr()` and `Angle.hstr()` to return an array of strings when the angle itself is an array. #527

#### v1.35 — 2020 December 31

- 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 `latlon()` method of either the `wgs84` object or the `iers2010` object. Related discussion: #372

- The two new geoid objects `wgs84` and `iers2010` have also provided a happy new home for the `subpoint()` method — which was previously stranded over on the `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 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 `load_constellation_names()`.

- The `utc_jpl()` method now correctly designates its return value as `UTC` instead of the ambiguious `UT`. #515

#### v1.34 — 2020 December 10

- The position classes have gained methods `frame_xyz()`, `frame_xyz_and_velocity()`, `frame_latlon()`, and `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 Coordinates in other reference frames. #476

- Added an official `itrs` reference frame.

- Added support for IERS Polar Motion 𝑥 and 𝑦.

- Added a method `lst_hours_at()` that computes Local Sidereal Time.

- A new almanac routine `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

- 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 `moon_nodes()` to index into the `MOON_NODES` list that provides a name for each node. #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

#### v1.33 — 2020 November 18

- 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

#### v1.32 — 2020 November 16

- A new `lunar_eclipses()` routine finds lunar eclipses and determines their degree of totality. #445

- The almanac module’s new `meridian_transits()` routine can find the moments at which a body transits the meridian and antimeridian. #460

- Fix: the `find_minima()` function was ignoring its `epsilon` and `num` arguments and always using the default values instead. #475

- Fix: the `.epoch` attribute of Earth satellite objects that were built using `from_satrec()` was, alas, a half-day off. #466

- Fix: the `Topos` constructor arguments `x` and `y`, which never worked properly anyway, have been deprecated and are now ignored.

#### 1.31 — 2020 October 24

- Skyfield now uses the International Earth Rotation Service (IERS) file `finals2000A.all` for updated ∆T and leap seconds. The USNO is no longer updating the files `deltat.data` and `deltat.preds` that previous versions of Skyfield used, and the `cddis.nasa.gov` server from which they were fetched will discontinue anonymous FTP on 2020 October 31. See UT1 and downloading IERS data. #452 #464

- The comets dataframe built from the MPC file `CometEls.txt` now includes the `reference` column, so users can tell which orbit is most recent if there are several orbits for a single comet. (For example, the file currently lists two C/2020 F3 (NEOWISE) orbits.) The comet examples in the documentation now build a dataframe that only includes the most recent orbit for each comet. #463

- Two new methods `days_old()` and `download()` make it simple to download a fresh copy of a file if the copy on disk is older than you would like.

#### 1.30 — 2020 October 11

- The various `strftime()` Skyfield methods now support the `%j` day-of-year format code.

- Fix: the new Julian calendar support broke support for out-of-range month numbers, wrapping them into the current year instead of letting them overflow into subsequent years. #461

- Fix: a stray debugging `print()` statement was stranded in `t.dut1`. #455

- The `Time` object, if manually instantiated without a Julian date fraction, now provides a fraction array with dimensions that match the Julian date argument. #458

#### 1.29 — 2020 September 25

- Fix: the new Julian calendar feature was raising an exception in the calendar methods like `tt_calendar()` if the time object was in fact an array of times. #450

- Fix: trying to iterate over a time object would raise an exception if the time was created through `ut1()`.

#### 1.28 — 2020 September 24

- Broken URL: Because the VizieR archive apparently decided to uncompress their copy of the `hip_main.dat.gz` Hipparcos catalog file, the old URL now returns a 404 error. As an emergency fix, this version of Skyfield switches to their uncompressed `hip_main.dat`. Hopefully they don’t compress it again and break the new URL! A more permanent solution is discussed at: #454

- To unblock this release, removed a few deprecated pre-1.0 experiments from April 2015 in `skyfield.hipparcos` and `skyfield.named_stars` that broke because the Hipparcos catalog is no longer compressed; hopefully no one was using them.

- In a sweeping internal change, the `Timescale()` and `Time()` objects now offer support for the Julian calendar that’s used by historians for dates preceding the adoption of the Gregorian calendar in 1582. See Ancient and modern dates if you want to turn on Julian dates in your application. #450

#### 1.27 — 2020 September 15

- The printed appearance of both vectors and of vector functions like Earth locations and Earth satellites have been rewritten to be more informative and consistent.

- Added `compute_calendar_date()` which lets the caller choose the Julian calendar for ancient dates instead of always using the proleptic Gregorian calendar. This should be particularly useful for historians.

- Added `J()` that builds a time array from an array of floating point years. #436

- Added four new `strftime` methods for the non-UTC timescales (#443). All four of them support `%f` for microseconds, and provide a reasonable default format string for callers who don’t wish to concoct their own:

  - `tai_strftime()`

  - `tt_strftime()`

  - `tdb_strftime()`

  - `ut1_strftime()`

- Thanks to several fixes, comets and asteroids with parabolic and hyperbolic orbits should now raise fewer errors.

- The prototype `planetary_magnitude()` can now return magnitudes for Uranus without raising an exception. The routine does not yet take into account whether the observer is facing the equator or poles of Uranus, so the magnitude predicted for the planet will only be accurate to within about 0.1 magnitudes.

#### 1.26 — 2020 August 1

- The official ∆T files on NASA’s FTP server have stopped receiving updates — they have no new data beyond February, the start of the global pandemic. Unless they are updated by next February, older versions of Skyfield will unfortunately download the files all over again every time `timescale()` is called (unless the `builtin=True` parameter is provided). To make Skyfield less fragile going forward:

  1. The loader’s `timescale()` method now defaults to `builtin=True`, telling it to use the ∆T and leap second files that ship with Skyfield internally. To download new ∆T files from NASA and the leap second file from the International Earth Rotation Service, specify `builtin=False`.

  2. The concept of an “expired” file has been removed from `load()`. Skyfield is now much simpler: if a file with the correct name exists, Skyfield uses it. See UT1 and downloading IERS data if you still want your application to check the age of your timescale files and automatically download new ones.

- The `ICRF.separation_from()` method now officially supports the combination of an array of positions with a single reference position! Its previous support for that combination was, alas, accidental, and was broken with the 1.23 release. #414 #424

- A prototype `planetary_magnitude()` routine has been added with support for several planets. #210

- The `utc` timezone that Skyfield returns in Python datetimes is now either the Python Standard Library’s own UTC object, if it supplies one, or else is defined by Skyfield itself. Skyfield no longer silently tries importing the whole `pytz` package merely to use its UTC object — which also means that the timezone returned by Skyfield longer offers the non-standard `localize()` method. #413

#### 1.25 — 2020 July 24

- Added `parse_constellations()` and `parse_star_names()` to load Stellarium star names and constellation lines. Constellation lines are featured in a new example script Drawing a finder chart for comet NEOWISE that produces a finder chart for comet C/2020 F3 NEOWISE.

- The Hipparcos star catalog should now load faster, having switched behind the scenes to a higher performance Pandas import routine.

- Fixed the ability of `utc()` to accept a Python `datetime.date` object as its argument. #409

- Slightly lowered the precision of two tests when they detect that Python is compiled for a 32-bit processor, so the test suite can succeed when contributors package Skyfield for 32-bit Linux. #411

#### 1.24 — 2020 July 20

- Added methods `from_datetime()` and `from_datetimes()` to the `Timescale` class, to better advertise the ability to build a Skyfield time from a Python `datetime` — an ability that was previously overloaded into the `year` parameter of the `utc()` method (where it is still supported for backwards compatibility, but no longer documented).

- Fix: improved the accuracy with which velocity is converted between the Earth-fixed ITRF frame that rotates with the Earth and the inertial GCRS frame that does not. In particular, this should make Earth satellite velocities more accurate.

#### 1.23 — 2020 July 9

- Added Kepler Orbits support for generating the positions of comets and asteroids from Minor Planet Center data files.

- Added `is_behind_earth()` to determine whether a celestial object is blocked from an Earth satellite’s view by the Earth itself.

- Replaced the awkward and hard-to-explain `rough_period` search parameter with the conceptually simpler `step_days` parameter, and updated the instructions in Searching for the dates of astronomical events to match.

- Made the `tle_file()` import method less strict about Earth satellite names: any text on the line before two lines of TLE data is now saved as the satellite name. A parameter `skip_names=True` turns this off if, for particular TLE files, this leads to unwanted text being saved.

#### 1.22 — 2020 Jun 8

- Skyfield’s improved time precision (stored internally as two floats) is now used in computing ephemeris positions, Earth orientation, and light-travel time, producing position angles which change much more smoothly over time on a sub-milliarcsecond scale.

- Searching for the dates of astronomical events is now documented for custom events that users define themselves, instead of only being documented for the official pre-written Almanac Computation functions. Not only discrete events but also maxima and minima are now officially supported and documented, thanks to a rewrite of the underlying code.

- Time objects no longer cache the nutation and precession matrices, since they are never used again after being multiplied together to create the equinox-of-date rotation matrix. This should save 144 bytes for each time in a `Time` array.

- It is now possible to Build a satellite with a specific gravity model thanks to a new Earth satellite constructor method. #384

- Added `build_url()` that returns the URL from which Skyfield will download a file. #382

- Added `close()` to support applications that need to do fine-grained resource management or whose testing framework check for dangling open files. #374

- Skyfield’s dependency list now asks for “jplephem” version 2.13 or later. Skyfield 1.21, alas, could incur a `Module not found` error when importing `jplephem.exceptions` if a user had an old “jplephem” version already installed. #386

#### 1.21 — 2020 May 29

- Added `is_sunlit()` to determine whether Earth satellites in orbit are in Earth’s shadow or not, thanks to a pull request from Jesse Coffey.

- Added `position_of_radec()` to replace the poorly designed `position_from_radec()`.

- Skyfield `Time` objects now have microsecond internal accuracy, so round trips to and from Python datetimes should now preserve all the microsecond digits.

- The `utc_strftime()` method now rounds to the nearest minute or second if it sees that either minutes or seconds are the smallest unit of time in the format string.

- The 6 numbers in the sequence `t.utc` can now be accessed by the attribute names `year`, `month`, `day`, `hour`, `minute`, and `second`.

- Nutation routines should now be faster and have a smaller memory footprint, thanks to a rewrite that uses more optimized NumPy calls. #373

- Thanks to Jérôme Deuchnord, the exception raised when asking for a position out-of-range of a JPL ephemeris now shows the calendar dates for which the ephemeris is valid and carries several useful attributes. #356

#### 1.20 — 2020 April 24

- Erik Tollerud contributed a fix for a deprecation warning about SSL from the most recent versions of Python (“cafile, cpath and cadefault are deprecated, use a custom context instead”). The file download routine now auto-detects which mechanism your Python supports. #363

- Added an `elevation_m` argument to `build_latlon_degrees()`.

#### 1.19 — 2020 April 23

- To hopefully fix the `SSL: CERTIFICATE_VERIFY_FAILED` errors that some users encounter when downloading timescale files, Skyfield has taken the risk of switching away from your system’s SSL certificates to the certificate bundle from the `certifi` package. #317

- Added a new almanac routine for finding Lunar Nodes. #361

- Gave geographic location objects a new `itrf_xyz()` method that returns their raw ITRF coordinates. #354

- Fixed the sign of the velocity vector when two vectors are directly geometrically subtracted. #355

#### 1.18 — 2020 March 26

- Deprecated the old hybrid-key satellite dictionary returned by `load.tle()` in favor of a simple list returned by the new `tle_file()` routine. #345

- The almanac `find_discrete()` routine no longer returns extraneous values in its second return value if no changes of state were found. #339 #351

- Added documentation and support for computing lunar libration. #80

#### 1.17 — 2020 February 2

- Upgraded to a new version of the `sgp4` Python library that, when possible, uses the fast official C++ implementation of SGP4.

- Added a `find_events()` Earth satellite method that finds the times at which a satellite rises, culminates, and sets.

- Improved the logic behind the Almanac Computation routines to avoid rare situations in which a cluster of nearly identical times would be produced for what should really be considered a single event. #333

- Fixed the `utc_strftime()` method so it does not report that every day in all of recorded history is a Monday. #335

#### 1.16 — 2019 December 20

- Added basic Planetary Reference Frames support, enough to compute the position of a given latitude and longitude on the surface of the Moon. #79 #124 #258

- Added `oppositions_conjunctions()` for finding the dates when a planet is at opposition and conjunction with the sun.

- Added `position_angle_of()` for computing astronomical position angles.

#### 1.15 — 2019 November 20

- Changed the URL for the Hipparcos catalog, because the VizieR archives FTP server is no longer responding. #301

- Added a `dark_twilight_day()` function that not only handles sunrise and sunset but also all three kinds of twilight. #225

#### 1.14 — 2019 November 1

- Changed the URL from which leap second files are downloaded; the server that previously provided them is no longer responding. Thanks to Richard Shaw for the pull request. #296 #297

- Added a `risings_and_settings()` function for computing rising and setting times. #271

#### 1.13 — 2019 October 10

- Provided a constellation lookup routine through `load_constellation_map()`.

- Added a `position_from_radec()` function.

- Fixed the `apparent()` method in the case where a single observer position is observing an entire vector of target positions. #229

#### 1.12 — 2019 September 2

- Fix: an exception was being thrown when creating a `Loader` pointed at a Windows directory for which Python’s `os.makedirs()` function returned a spurious error. #283

- The internal `reverse_terra()` routine can now be given an `iterations=0` argument if the caller wants geocentric latitude and longitude.

#### 1.11 — 2019 July 22

- You can now call `load.timescale(builtin=True)` to use time scale files that Skyfield carries internally, instead of downloading them. Note that the time scale files distributed with any given version of Skyfield will gradually fall out of date.

- Fix: indexing a position now returns a position with an actual velocity. #241

- Fix: the `Star` method `from_dataframe()` now correctly pulls stellar parallax data from the dataframe if available. #266

- Fix: `find_discrete()` was generating empty arrays of search dates, upsetting the astronomy code, if the start and end dates were very close together. #240

#### 1.10 — 2019 February 2

- Fix: teach Skyfield the new format of the Naval Observatory ∆T data file `deltat.preds`, whose change in format caused Skyfield to start throwing an exception for new users. #236

#### 1.9 — 2018 September 23

- Added `seasons()` to the Almanac Computation module that can be used to predict solstices and equinoxes.

- Fix: the ecliptic coordinate routines no longer raise `ValueError: too many values to unpack` if they are passed a time array. #207 #208

#### 1.8 — 2018 September 12

- There is now an Almanac Computation module that can compute the times of sunrise, sunset, and the phases of the moon, based on the search algorithms announced at my recent PyBay talk “An Import Loop and a Fiery Reentry.”

- Two new methods `cirs_xyz()` and `cirs_radec()` have been contributed which provide support for rotating a position into the Celestial Intermediate Reference System (CIRS). #192

#### 1.7 — 2018 September 3

- Skyfield now supports loading the Hipparcos star catalog as a Pandas dataframe, providing the user with convenient mechanisms for looking up a single star by HIP number or filtering the entire catalog by magnitude. See Stars and Distant Objects for details.

- Ecliptic coordinates can now be produced for epochs other than J2000 thanks to a new optional parameter specifying the desired epoch for the `ecliptic_latlon()` method.

- A position that gives a position, velocity, and time can now be converted into full osculating orbital elements through the routine `osculating_elements_of()`.

- A couple of bugs in the `load()` routine have been fixed. #193 #194

#### 1.6 — 2018 July 25

- Both of the loader methods `open()` and `tle()` now accept not just URLs but also plain local file paths; they correctly re-download a remote file if “reload=True” is specified; and they allow specifying a different local “filename=” than the one at the end of the URL.

- Earth satellite objects no longer try to instantiate a timescale object of their own, which often kicked off an unexpected download of the three files needed to build a timescale.

- Satellite names are now correctly loaded from Space-Track TLE files.

- The ability to create times using Julian Dates is now better advertised, thanks to dedicated timescale methods whose names end in `…_jd()`.

#### 1.5 — 2018 July 4

- The `subpoint()` method now normalizes the longitude values it returns into the range −180° to 180° #182 and returns an actual elevation instead of zero. #185

- Earth satellites now return a real velocity vector instead of zero. #187

- Earth satellites now offer an `ITRF_position_velocity_error()` method that returns raw ITRF coordinates for users interested in them. #85

#### 1.4 — 2018 May 20

- You can now specify the distance to an object when generating a position from altitude and azimuth coordinates. #158

- The dictionary of satellites returned when you read a TLE file now supports lookup by integer satellite ID, not just by name, and now knows how to parse TLE files from Space-Track. #163 #167

- Star coordinates can now be offered for any epoch, not just J2000. #166

- You can now create a time object given the UT1 date. #91

- Fractional Julian years are now available on `Time` objects as `.J`.

- The parameter DUT1 is now available on `Time` objects as `.dut1`. #176

#### 1.3 — 2018 April 15

- Geocentric coordinates now have a `subpoint()` method that computes the latitude and longitude of the point beneath that body.

- All of the `Timescale` time constructor methods now accept arrays.

- Emergency fix to stop Skyfield from endlessly downloading new copies of `deltat.preds`, since the file has gone out of date at the USNO site.

- Fixed ability of a `Star` to be initialized with a tuple that breaks units into minutes and seconds (broke in version 1.2).

- Issues fixed: #170 #172

#### 1.2 — 2018 March 29

- The documentation now describes how to create an excerpt of a large JPL ephemeris without downloading the entire file. Several Skyfield tests now run much faster because they use an ephemeris excerpt instead of waiting for a download.

- For `load_file()` a leading `~` now means “your home directory”.

- You can now initialize a velocity from kilometers per second with `Velocity(km_per_s=...)`.

- Empty time and angle objects no longer raise an exception when printed. (Thanks, JoshPaterson!)

- Issues fixed: #160 #161 #162

#### 1.1 — 2018 January 14

- Positions can now be converted to AstroPy with `to_skycoord()`.

- You can now provide a timescale of your own to an `EarthSatellite()` instead of having it trying to load one itself.

- Downloaded files are no longer marked as executable on Windows.

- A friendly error message, rather than an obscure traceback, is now returned if you try converting a position to alt/az coordinates but the position was not measured from a position on the Earth’s surface.

#### 1.0 — 2017 March 15

- Brought the core API to maturity: replaced the narrow concept of building a “body” from several ephemeris segments with the general concept of a vector function that is the sum of several simpler vector functions.

- Added support for adding and subtracting vector functions.

- Deprecated the Earth `topos()` method in favor of vector addition.

- Deprecated the Earth `satellite()` method in favor of vector addition.

- Deprecated the body `geometry_of()` method in favor of vector subtraction.

- Celestrak satellite files can now be opened with `load.tle(url_or_filename)`.

#### 0.9.1 — 2016 December 10

- 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.

#### 0.9

- 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.

#### 0.8

- Added an API Reference 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.

#### 0.7

- 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')

#### 0.6.1

- 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.

#### 0.6

- 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.

#### 0.5

- 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`.

#### 0.4

- To prevent confusion, the `astimezone()` and `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 `astimezone_and_leap_second()` and `utc_datetime_and_leap_second()`.

#### 0.3

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