Celestial Watchface — Garmin Venu 3

A celestial watchface that lets you pretend you're standing on any of the 9 classical planets in the solar system. Pick the planet in the phone settings; the watchface then shows local sol time on that world, the day/night cycle at the observer's spot, real stars projected through that planet's pole, and the other planets of the solar system as they would appear in that sky.

The watch's wrist time (24-hour Earth clock you use day to day) is always shown as analog hands in the center, colored to match the selected planet.

What's on the face

• Analog clock hands — minute (white) + hour (planet-colored). No second hand. Shows your wrist time regardless of which planet is selected.
• Site label at the top — e.g. MARS · GALE, JUPITER · RED SPOT, EARTH · SAN FRANCISCO. Clearly identifies the world and the spot on it.
• Day/night ring around the edge — 24-hour dial showing the selected planet's day at the observer's location. See the table below for what the marker speed and arc length mean per planet.
• Heart-rate row at the top (❤ + bpm).
• Compass markers (N/E/S/W) at night, anchoring the sky to local cardinal directions.
• Real stars at night, projected through that planet's pole. Up to 8 of the brightest visible stars and planets get name labels (Sirius, Canopus, Vega, Antares, Jupiter, Saturn, …).
• Other planets drawn in their characteristic colors. The catalog rotates appropriately — Mars is visible from Jupiter, Earth is visible from Pluto, etc.
• Phobos & Deimos only in Mars mode (Mars has dedicated moon tracking).
• Daytime scene when the sun is above the horizon: sky tint, horizon glow, low sun, foreground hill silhouette. Tones shift per planet (Mars butterscotch, Earth blue, Jupiter tan-glow, etc.).

The 9 worlds

Mode is set via Garmin Connect → watchface settings → Observer planet.

#	World	Observer site	Notes on the location
0	Mercury	Caloris Basin (30°N, 195°E)	One of the largest impact basins in the solar system
1	Venus	Maxwell Montes (65.2°N, 3.3°E)	Tallest mountain on Venus, named after James Clerk Maxwell
2	Earth	Your last-known GPS position	Nearest of ~210 bundled cities shown in label
3	Mars	Gale Crater (−4.59°N, 137.44°E)	Curiosity rover's landing site
4	Jupiter	Great Red Spot (−22°N, 240°E)	The iconic centuries-old storm
5	Saturn	North Polar Hexagon (78°N, 0°E)	Saturn's bizarre persistent hexagonal jet stream
6	Uranus	Cloud Bands (30°N, 0°E)	Featureless atmosphere has no real landmarks — generic spot
7	Neptune	Great Dark Spot (−22°N, 0°E)	Voyager 2 spotted it in 1989; has come and gone since
8	Pluto	Tombaugh Regio (25°N, 155°E)	The famous nitrogen-ice "heart" from New Horizons

The lat/lon use each planet's IAU-defined coordinate system (planetographic for solid bodies, System II/III for the gas giants). Gas-giant "sites" are at cloud-top altitude — there's no solid surface to stand on.

What the day/night ring means per planet

The ring is always a 24-hour dial — noon at the top, midnight at the bottom. The marker shows the current local solar hour at the observer's site. The amber arc spans sunrise → sunset at that latitude. What varies between planets is how fast the marker moves and how long the arc is.

Planet	Solar day	Ring marker behavior	Day-arc length
Mercury	~176 Earth days	Visually static — moves ~2° per Earth day. The ring is more a thermometer than a clock.	~12 ring-hours (tilt is essentially 0°)
Venus	~117 Earth days, retrograde	Static AND moving the opposite direction.	~12 ring-hours (tilt 2.6°)
Earth	24 Earth hours	1:1 with your wrist clock — one ring rotation per Earth day.	Varies with lat + season; e.g. ~14.6h at SF in June
Mars (Gale)	24.66 Earth hours	One full rotation per Martian sol — slightly slower than Earth.	~12.2h today; varies with Mars's seasons
Jupiter	9.93 Earth hours	Visibly sweeps — one rotation every ~10 Earth hours.	~12 ring-hours (tilt 3.1°)
Saturn	10.66 Earth hours	Visibly sweeps. At North Hex (78°N) the sun is up or down for years at a time — arc can be all-day or all-night.	Highly seasonal (tilt 26.7°); at high lat varies 0–24h
Uranus	17.24 Earth hours, retrograde	Sweeps once per ~17 Earth hours, backwards.	Extreme — tilt is 98° (on its side). Equator gets near-polar lighting.
Neptune	16.11 Earth hours	Sweeps once per ~16 Earth hours.	Seasonal (tilt 28.3°); at -22°N varies through the year
Pluto	~6.4 Earth days	Marker moves only ~4° per Earth day.	Tilt 122° → extreme seasonal swings

Why some marker behavior looks "wrong": It's accurate, not broken. Mercury genuinely takes ~176 Earth days to complete one sunrise-to-sunrise cycle, so the ring marker really should barely move. Uranus genuinely rotates retrograde on its side. The face is reflecting physics, not artifact.

Build & install

You need Garmin's free Connect IQ SDK (Java 18+ is a prerequisite).

1. Install the SDK Manager from https://developer.garmin.com/connect-iq/sdk/, sign in with a free Garmin account, download the latest SDK and the Venu 3 device files.
2. Build:

   cd ~/Dropbox/Private/Projects/CelestialWatchface
   ./build.sh            # creates bin/CelestialWatchface.prg
   ./build.sh sim        # also opens the simulator with the watchface loaded

   The signing key is read from ~/.garmin/celestial/developer_key.der (kept outside this synced folder so the private key never reaches the cloud). Override the path with CIQ_KEY=/path/to/key.der. The master .pem is backed up in 1Password — if the key is ever missing, restore it from there rather than generating a new one (a new key would break store updates).
3. Sideload to the watch:
   • Connect the watch by USB.
   • Copy bin/CelestialWatchface.prg to the watch's GARMIN/Apps/ folder.
   • Eject, then pick "Celestial Watchface" from the watchface list.

Phone settings

Setting	Default	What it does
Observer planet	Mars (Gale)	Dropdown of all 9 worlds
Show Mars (Gale) time	off	Toggles a digital sol-time line (e.g. MARS 18:42 or JUPITER 04:23) below the analog hands. Hidden in Earth mode.
Phobos phase calibration (deg)	0	Tweak ±180° if Phobos's modeled up/down times drift from a trusted ephemeris. Mars mode only.
Deimos phase calibration (deg)	0	Same for Deimos.

Project layout

manifest.xml           app id, targets venu3, asks for Position permission
monkey.jungle          build config
build.sh               builds a signed .prg (and can launch the sim)
source/
  CelestialApp.mc     application entry point
  CelestialView.mc    all drawing — analog hands, sky, day/night ring, site label
  Bodies.mc           orbital elements + IAU pole + rotation + landmark site
                      for the 9 worlds
  Site.mc             generic observer-on-planet astronomy: sun, sunrise/sunset,
                      LST, star catalog projection, other planets in the sky
  Stars.mc            bundled bright-star catalog (RA/Dec, name, magnitude)
  Kepler.mc           Keplerian orbital propagation (helioEcliptic helper)
  MarsMoons.mc        Phobos & Deimos topocentric positions (Mars-only feature)
  Cities.mc           ~210 world cities for Earth's nearest-city lookup
resources/
  strings/strings.xml
  drawables/launcher_icon.png
  properties/properties.xml   planetIndex, showMarsTime, phobosCal, deimosCal
  settings/settings.xml       phone-side settings UI

Accuracy notes

• Star positions: ~1° accuracy (mean orbital elements; no proper motion, no precession since J2000). Fine at watch-screen resolution.
• Planet positions: same — Keplerian propagation from J2000 mean elements.
• Mars solar position uses the higher-precision Mars24 algorithm (Allison & McEwen) for the Gale-specific MarsMoons module; the generic Site module uses a simpler model that's good to ~1°.
• The IAU pole + prime-meridian values bundled in Bodies are accurate as of the 2009 IAU Report on Cartographic Coordinates and Rotational Elements.