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https://github.com/roe-dl/weewx-skyfield-almanac

Almanac extension to WeeWX using Skyfield module
https://github.com/roe-dl/weewx-skyfield-almanac

Last synced: 3 months ago
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Almanac extension to WeeWX using Skyfield module

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README 

        
# weewx-skyfield-almanac

Almanac extension to WeeWX using Skyfield module



## Why should I use this extension?



PyEphem is deprecated. Its astronomical database is outdated and won't get

updated any more. It ends in 2018. Dates after that year are calculated

by extrapolation.



Skyfield is the successor of PyEphem. It is from the same author, Brandon

Rhodes. It uses more modern and more precise formulae and actual ephemeris

provided by NASA's JPL.



There is no other requirement than installing this extension to replace

PyEphem calculated values by Skyfield calculated values in existing skins.



## Prerequisites



WeeWX from version 5.2 on is required.



Install Skyfield and NumPy



## Installation instructions



1) download



   ```shell

   wget -O weewx-skyfield-almanac.zip https://github.com/roe-dl/weewx-skyfield-almanac/archive/master.zip

   ```



2) run the installer



   WeeWX from version 5.2 on and WeeWX packet installation



   ```shell

   sudo weectl extension install weewx-skyfield-almanac.zip

   ```



   WeeWX from version 5.2 on and WeeWX pip installation into an virtual environment



   ```shell

   source ~/weewx-venv/bin/activate

   weectl extension install weewx-skyfield-almanac.zip

   ```

   

3) restart weewx



   for SysVinit systems:



   ```shell

   sudo /etc/init.d/weewx stop

   sudo /etc/init.d/weewx start

   ```



   for systemd systems:



   ```shell

   sudo systemctl stop weewx

   sudo systemctl start weewx

   ```



## Configuration instructions



There is no need to configure anything, but there are some tuning options

available if you have special requirements.



```

[Almanac]

    # which ephemeris file to use

    ephemeris = de440s.bsp  # or de440.bsp or de441.bsp

    # use builtin timescale data or download it from IERS

    use_builtin_timescale = true

    # URL(s) of the timescale file (optional)

    timescale_url = '...'

    # whether to log FTP responses (optional)

    log_ftp = false

    # update interval 1 year (set to 0 for no update)

    update_interval = 31557600

```



* `ephemeris`: Ephemeris file to use. Different files cover different

  scopes of heavenly bodies. Some of those files are huge. See

  [Planets and their moons: JPL ephemeris files](https://rhodesmill.org/skyfield/planets.html)

  for details.

* `use_builtin_timescale`: Use builtin timescale data or download them

  from IERS. See [UT1 and downloading IERS data](https://rhodesmill.org/skyfield/time.html#ut1-and-downloading-iers-data)

  for details.

* `timescale_url`: an URL or a list of URLs to download the timescale file from (optional). 

  There is a default URL hardcoded in Skyfield. Unfortunately the server

  is temporarily down. That's why you can specify an alternative

  source here.

* `log_ftp`: whether to log FTP responses of the server (optional).

  If you specified an alternative source for the timescale file in 

  `timescale_url` and that URL is at an FTP server, you can switch

  on logging of the server responses in case of trouble.

* `update_interval`: interval for updating ephemeris and timescale data

  (set to 0 to switch off updates)



## Attributes



See the WeeWX customization guide, section "The Cheetah generator",

sub-section

"[Almanac](http://weewx.com/docs/latest/custom/cheetah-generator/#almanac)",

for a detailed description how to use the almanac in WeeWX.



Once the weewx-skyfield-almanac extension is installed and initialized after

startup, `$almanac.hasExtras` becomes `True` and extended almanac

information is available. Initialization can take several archive

intervals to be completed at first run after installation, depending on 

configuration.



The general syntax is:



```

$almanac(almanac_time=time,            ## Unix epoch time

         lat=latitude, lon=longitude,  ## degrees

         altitude=altitude,            ## meters

         pressure=pressure,            ## mbars

         horizon=horizon,              ## degrees

         temperature=temperature_C     ## degrees C

       ).heavenly_body(use_center=[01]).attribute

```



### Date and time



Date and time values do not require a heavenly body. They refer to the

location on earth that `$almanac` is bound to. You can specify the

location by parameters to `$almanac`. If you do not set them, the

location of the station is used.



WeeWX datatype   | Pure float result | Meaning

-----------------|-------------------|----------------

`sidereal_angle` | `sidereal_time`   | Local Apparent Sidereal Time

`solar_angle`    | `solar_time`      | Local Apparent Solar Time

`solar_datetime` | —           | true local solar date and time



Note: The tags `$almanac.sidereal_angle`, `$almanac.sidereal_time`,

`$almanac.solar_angle`, and `$almanac.solar_time` return a value

in decimal degrees rather than the more customary value from 0 to

24 hours.



Solar time is the time a sundial would show.



If you do not install this extension together with Skyfield but use PyEphem,

which is supported by core WeeWX, `sidereal_angle` and `sidereal_time` are

available only.



### Calendar events



Calendar events do not require a heavenly body. They refer to earth.

This extension provides the events described in the WeeWX customization

guide, but calculated using Skyfield. They happen independent of your

location on earth at the same instant all over the world. The local time

differs only. Here is a list of available events:



Previous event | Next event |

---------------|------------|

`previous_equinox` | `next_equinox`

`previous_solstice` | `next_solstice`

`previous_autumnal_equinox` | `next_autumnal_equinox`

`previous_vernal_equinox` | `next_vernal_equinox`

`previous_winter_solstice` | `next_winter_solstice`

`previous_summer_solstice` | `next_summer_solstice`

`previous_new_moon` | `next_new_moon`

`previous_first_quarter_moon` | `next_first_quarter_moon`

`previous_full_moon` | `next_full_moon`

`previous_last_quarter_moon` | `next_last_quarter_moon`



Example:

```

$almanac.next_solstice

```



### Heavenly bodies



This extension provides the attributes described in the WeeWX customization 

guide, but calculated using Skyfield. Additionally it provides some

extra attributes, that are not available with PyEphem.



Here is the list of attributes provided by this extension but not by

core WeeWX using PyEphem:



WeeWX datatype | Pure float result | Meaning

---------------|-------------------|----------------

`astro_dist`   | `a_dist`          | astrometric geocentric distance

`geo_dist`     | `g_dist`          | apparent astrometric geocentric distance

`topo_dist`    | `dist`            | apparent topocentric distance 

`alt_distance` | `alt_dist`        | distance in reference to the coordinate system of altitude and azimuth

`hour angle`   | `ha`              | topocentric hour angle

`ha_declination` | `ha_dec`        | declination in reference to the coordinate system of the hour angle

`ha_distance`  | `ha_dist`         | distance in referenc to the coordinate system of the hour angle

`max_altitude` | `max_alt`         | maximum altitude

`max_alt_time` | —           | timestamp of the maximum altitude



Depending on the ephemeris you chose you may be required to add

`_barycenter` to the name of a heavenly body to get results

(for example `jupiter_barycenter`).



## PyEphem and Skyfield



If you install both PyEphem and Skyfield, Skyfield is preferred. If the

given heavenly body is available with Skyfield, the attribute is calculated

using Skyfield. Otherwise PyEphem is tried. If neither Skyfield nor 

PyEphem know about the body, an exception is raised.



## FAQ



Q: I installed the Skyfield module, but no extended almanac values

   are displayed.



A: Depending on the ephemeris file you chose initialization takes more or

   less time. While it is not finished, the skin displays some core

   values only. Wait a moment, and the extended values will show up.

   If the ephemeris file is huge and it is the first run after

   installation, it can take several archive intervals to be completed.



Q: Which ephemeris file should I use?



A: The ephemeris files differ in size and coverage of heavenly bodies and

   time. The default, if you do not choose anything, is `de440s.bsp`. It

   covers sun, earth, moon, and some data of the long known planets. If

   you need more, try `de440.bsp` or even `de441.bsp`.

   From time to time the NASA releases new sets of ephemeris files. Then you

   can try the new edition.

   See [Planets and their moons: JPL ephemeris files](https://rhodesmill.org/skyfield/planets.html)

   for more details.



Q: Are there disadvantages of Skyfield?



A: Yes. Always are. Skyfield depends on NumPy, while PyEphem does not.



Q: How do I have to adapt my skin to use this extension?



A: There is nothing to do. Installing this extension is enough. But you

   could add the additional attributes supported by this extension

   to your skin to display them there.



## Links



* [WeeWX](https://weewx.com)

* [Skyfield](https://rhodesmill.org/skyfield/)

* [International Earth Rotation and Reference Service IERS](https://iers.org)

  (provides the timescale file `finals2000A.all`)

* [Jet Propulsion Laboratory JPL](https://www.jpl.nasa.gov)

  (provides the ephemeris files)

* [Issue #981: PyEphem is deprecated](https://github.com/weewx/weewx/issues/981)