weewx-skyfield

weewx-skyfield

The drop-in upgrade for WeeWX’s almanac — Skyfield-powered $almanac tags, stars, eclipses, and a planetarium-style Sky page.

Installation · Almanac tags · The Sky page · Sky panels in your skin · Translating (i18n) · Known Skyfield issues · GitHub project

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weewx-skyfield replaces WeeWX’s built-in almanac (PyEphem or weeutil) for report generation. Report tags such as $almanac.sunrise, $almanac.moon.transit, $almanac(horizon=-6).sun(use_center=1).rise and $almanac.next_full_moon (as used, for example, in the Seasons skin’s Celestial page) are computed with Skyfield and JPL’s DE421 ephemeris, which is installed with the extension — no downloads at runtime, and much more accurate values than PyEphem, which is deprecated by its own author in favor of Skyfield.

No skin changes are needed: the extension answers the same $almanac tags as the built-in almanac. Install it, restart WeeWX, and every report generated from then on uses Skyfield values.

Highlights

• Every standard tag, computed natively. Rise/set/transit with custom horizons and use_center, positions, magnitudes, phases, equinoxes, solstices and moon phases for the sun, the moon and all planets (plus Pluto) — PyEphem is not required for any tag used by WeeWX’s standard skins. See the almanac tag reference.
• 420 named stars, or all 118,218. $almanac.rigel.mag, $almanac.polaris.circumpolar — the IAU Catalog of Star Names plus PyEphem’s names for backward compatibility, served from a bundled Hipparcos excerpt; drop in the full catalog and address any star as $almanac.hip_57939.
• Eclipses and constellations — tag families with no PyEphem counterpart at all: $almanac.next_eclipse finds the next eclipse visible from your station, and every body reports the constellation it stands in ($almanac.saturn.constellation).
• The Sky page — a bundled, self-contained, planetarium-style showcase page computed for your station’s location: a sky dome, rise & set ribbons, the sun’s path, the solar year, the lunar month, an orrery, an analemma and more. Tour the page, or embed its panels in your own skin.
• Speaks your language — new in 1.12: the Sky page, its panels and the almanac’s body names ($almanac.moon.label) are fully translatable through WeeWX’s own lang files, with per-string English fallback; complete German and French (both native-speaker reviewed) and Dutch and Spanish (Beta) translations ship with the skin. 1.13 adds the constellations ($almanac.saturn.constellation.label), all 88 in each. How to translate.
• Safe to upgrade over a running WeeWX. The ephemeris is read fully into memory at startup (about 16 MB), so replacing the extension’s files cannot disturb — or crash — the running almanac; the new files take effect on the restart that follows the install.
• Fast. A transparent result cache reuses expensive rise/set searches within and across report cycles — on a heavy eight-page site, template generation dropped from ~17.7 s per report cycle to ~4.6 s. Details under the result cache.

Requirements

• Python 3.9 or later
• WeeWX 5.2 or later
• Skyfield 1.47 or later, and NumPy
• PyEphem is not required

Skyfield 1.47 is the minimum supported version; development and the test suite track the current Skyfield release, so upgrading is recommended — see Upgrading Skyfield.

Quick start

weectl extension install weewx-skyfield.zip

then restart WeeWX — the installation page has the full steps, including installing the Skyfield prerequisite.

weewx-skyfield in action

The celestial pages of www.paloaltoweather.com — Today, Sun, Moon, Planets and Stars — demonstrate what can be accomplished with this extension, live.

Live-updating pages

weewx-skyfield computes at report time: its tags — and the Sky page — refresh once per report cycle. For pages that update continuously in the browser, add weewx-loopdata (same author, 5.0 or later): its almanac fields — report almanac tags with the $ removed — are evaluated against the registered almanac (this extension’s, once installed) on every loop packet and published in loop-data.txt for the page’s JavaScript to pick up. One computation engine serves the report tags and the live values, so they always agree. weewx-celestial is a complete worked example — a live Geocentric panel built entirely from loopdata almanac fields — and the paloaltoweather.com pages above update the same way.

Relationship to other extensions

• weewx-celestial (same author) ships a live celestial page driven by weewx-loopdata almanac fields (see Live-updating pages above). Since celestial 6.0 it runs no service and computes nothing itself, so the two extensions coexist with no configuration — weewx-skyfield is the atlas, weewx-celestial the live instrument. (Only the historical celestial 3.x, which embedded this same almanac engine, needs replace_builtin_almanac = false in the [Celestial] section of weewx.conf when run alongside weewx-skyfield.)
• weewx-skyfield-almanac (by a different author) is an independent Skyfield almanac extension with a different design (it downloads its ephemerides and catalogs at runtime). Choose one or the other; installing both would leave reports using whichever registered last.

Credits

weewx-skyfield stands on the work of others: Brandon Rhodes, author of the Skyfield astronomy library that performs every computation in this extension; NASA’s Jet Propulsion Laboratory (JPL), whose DE421 planetary ephemeris provides the positions of the sun, moon, and planets; the European Space Agency (ESA), whose Hipparcos mission produced the star catalog, and the CDS (Strasbourg astronomical Data Centre), which distributes it via VizieR; the International Astronomical Union (IAU) Working Group on Star Names, whose Catalog of Star Names supplies the named-star tags; the U.S. Naval Observatory (USNO) and Jean Meeus, whose published definitions and algorithms are the reference for rise/set, twilight, and other almanac conventions; and Tom Keffer, Matthew Wall, and the WeeWX project, whose almanac framework this extension plugs into.

Licensing

weewx-skyfield is Copyright (C)2022-2026 by John A Kline and licensed under the GNU Public License v3. The bundled star catalog excerpt contains data from the Hipparcos and Tycho Catalogues, which ESA distributes under the CC BY-NC 3.0 IGO licence. Credit: ESA.

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