Optional features#
These features can be enabled in the watchface settings via the Wear OS app on your phone. Toggle them below to preview how they look.
Analemma: The figure-8 path the Sun traces over a year if observed at the same time each day. It reveals the equation of time – the difference between clock time and solar time.
Meridians: Longitude lines on the Earth map, with each line representing a timezone boundary. The highlighted line shows your current timezone.
Horizon: A circle showing which celestial objects are above your local horizon. Objects inside the circle are visible in the sky; objects outside are below the horizon.
Lunar nodes: The points where the Moon's tilted orbit crosses the ecliptic plane. They regress westward over ~18.6 years. Eclipses only occur when the Sun is near a node.
Meteor showers: Markers showing when major showers peak. When a marker aligns with the Sun, that shower is at maximum activity.
Zodiac signs: The 12 traditional divisions of the ecliptic, each spanning 30°. The tropical zodiac begins at the vernal equinox (0° = Aries).
Seasonal markers: Inner notches mark solstices and equinoxes (larger) and cross-quarter days (smaller). Cross-quarters are the Celtic calendar markers midway between solstices and equinoxes.
Latitude: Your geographic latitude affects which stars are visible and where the horizon circle appears. The hour hand also adjusts to show your approximate position on Earth.
Frequently asked questions#
What devices are supported?
Spacetime requires Wear OS 4 or later. It has been tested on Pixel Watch 4 and should work on other Wear OS 4+ devices including Samsung Galaxy Watch 7 and newer.
How accurate are the planetary positions?
The watchface uses hand-rolled trigonometric approximations optimized for the small screen. All bodies are within 5° of their true geocentric positions: Sun (0.15°), outer planets (0.05-1.2°), Venus (2°), Moon (3.8°), and Mercury (4.4°). This is accurate enough for practical sky observation and navigation on a watch. The web version uses astronomy-engine with JPL-verified accuracy (0.01-0.03°) if you need reference-quality precision.
Is the Moon really on the ecliptic?
No – the Moon's orbit is inclined 5° to the ecliptic. But we project all celestial bodies onto the ecliptic ring for simplicity, ignoring their ecliptic latitude. This introduces a small visual error for the Moon (typically a few degrees), but the position is still directionally correct and useful for finding the Moon in the sky.
How can I tell if a planet is visible tonight?
Look at the planet's position relative to the Sun. Left of the Sun (counterclockwise) = evening sky, visible after sunset. Right of the Sun (clockwise) = morning sky, visible before sunrise. Opposite the Sun = visible all night, at its brightest. Close to the Sun = lost in the glare.
Why do the constellations look mirrored?
The star map shows the sky as if looking from outside the celestial sphere, the same convention used by celestial globes and traditional astrolabes. This matches the Earth map, which also looks from above (outside). When you hold a celestial globe, this is how the stars appear on its surface.
Why is the Sun always at the top?
The watchface is a Sun-centered astrolabe – everything rotates to keep the Sun at 12 o'clock (solar noon). This makes it intuitive to judge when celestial objects are visible. The star map drifts slowly relative to the Earth map as we orbit the Sun, which is why different constellations are visible in different seasons.
What does the hour hand represent?
The hour hand marks your timezone's central meridian on the Earth map. The red tip points north, white points south, and they meet at your approximate latitude. It doubles as a rough compass – if you identify a celestial object in the sky, you can use it to orient yourself.
Why does the Earth rotate counter-clockwise?
Because that's how Earth actually rotates when viewed from above the North Pole. Counter-clockwise is also the direction all planets orbit the Sun and the Moon orbits Earth. Traditional analog clocks got it backwards.
What's shown in star map mode?
About 1,600 stars (magnitude ≤5.0) from the HYG database with proper motion applied to current year positions, plus the Milky Way band, Andromeda galaxy, and Sagittarius A* (the supermassive black hole at our galaxy's center). The celestial background rotates with sidereal time.
Why are Saturn's rings so thin right now?
Saturn's rings are currently near edge-on as seen from Earth (they were exactly edge-on in March 2025). The ring tilt varies over Saturn's ~29.5 year orbit. The watchface calculates the current tilt angle and orientation in real-time, so you'll see the rings gradually open up over the next several years.
How do I toggle between Earth and star map?
Tap the watchface to cycle through display modes. You can also configure the default mode in the watchface settings on your phone.
Can I see the horizon for my latitude?
Yes. The horizon circle and celestial coordinate grid are optional features, disabled by default for a cleaner look. Enable them in the watchface settings via the Wear OS app on your phone.
Does this work without an internet connection?
Yes, completely. All celestial calculations are done on-device using mathematical models. No internet connection or GPS is required.
Have any measures been taken to preserve battery life?
The watchface is optimized to only recalculate what's necessary: the Earth map rotates every minute, Moon and Mercury update hourly, and other planets update daily (since they move slowly). It uses ambient mode (simplified display) when not actively viewing. Optional features like grid lines can be disabled if desired.
Can I get a refund?
Google Play handles all purchases and refunds. You can request a refund through the Play Store within their refund window.
Technical notes#
Data sources
Celestial positions are calculated using the astronomy-engine library, which implements the VSOP87 planetary theory and ELP/MPP02 lunar theory. These calculations are verified against NASA JPL Horizons ephemeris data, typically agreeing within 0.01-0.03° for planetary positions.
- Star positions: HYG database (compiled from Hipparcos, Yale BSC, and Gliese catalogs)
- Proper motion: Individual stellar velocities from catalog data, applied to 2025 positions
- Precession model: IAU 2006 precession theory
- Galactic parameters: IAU 1958 galactic coordinate system
What's simplified
The watchface prioritizes clarity over completeness. Several phenomena are simplified or omitted:
- All bodies shown on a single ecliptic plane (orbital inclinations ignored)
- Nutation (short-term axis wobble) not shown in precession visualization
- Proper motion calculation ignores perspective acceleration (significant only over millions of years)
- Sun lifecycle shows representative stellar evolution, not precise solar modeling
- Light travel time not accounted for (positions are "instantaneous")
Watch face approximations
The Wear OS watch face uses hand-rolled trigonometric formulas rather than full ephemeris calculations, due to platform constraints. These approximations typically achieve 1-2° accuracy for planetary positions – sufficient for the small screen, where one degree corresponds to less than one pixel. The web version uses full astronomy-engine calculations and serves as the ground truth reference.