A practical guide
How to read the Spacetime Watch and understand the astronomy behind timekeeping
Counterclockwise revolution#
The Earth rotates around its axis once per day, so if we put a rotating North Pole-centered map on a 24 hour clock, we can see what time it is everywhere on Earth at once!
The Earth rotates counterclockwise, so our clock will have to do the same.
Here, the "hour hand" is simply the central longitude of your timezone, as the Earth map makes its daily rotations.
In a bit, we'll add the sky map as an overlay, and at that point, the direction of the hour hand will also serve as a "compass needle" pointing roughly south. This means you will be able to find the cardinal directions by aligning your Spacetime Watch to any celestial objects you can identify in the sky.
Note: This compass direction may be a little off because your actual longitude differs from your timezone's central longitude – especially during daylight saving time, when timezones shift one step counterclockwise for long obsolete historical reasons.
Map projection#
The watch uses a North Pole-centered azimuthal equal-area projection, which flattens the Earth into a circle in a way that lets you see every part of Earth without changing the sizes of areas – continents maintain accurate relative areas even though things get increasingly flattened out and distorted toward the circumference of the circle.
This projection works fairly well because most of Earth's land mass is in the northern hemisphere. Its biggest weakness is that whatever is at the South Pole becomes a thin smudge covering the entire circumference. Luckily, Antarctica can be ignored, at least for watchface purposes, and on the Spacetime Watch, it has been entirely removed, for aesthetic reasons.
The visualization shows Earth in two projections: orthographic (as seen from space—here from the Sun's perspective at vernal equinox, side-on to Earth's tilt) and azimuthal equal-area (a flat map centered on the North Pole). The arrow indicates Earth's counterclockwise rotation.
The sky map#
The night sky can be mapped onto the same projection. Stars, galaxies, and the Milky Way all have fixed positions on the celestial sphere. Click the buttons to highlight a selection of skymarks (landmarks of the sky).
Note: East and west (that is, clockwise and counterclockwise) are flipped for this sky map, because we project it as if it is a sphere viewed from outside, in order to make it comparable with the Earth map. That's why the constellations looks mirrored.
Sagittarius A* is the supermassive black hole at the center of our Milky Way galaxy, about 26,000 light-years away. While invisible itself, it's surrounded by glowing matter and represents the rotational center of our galaxy. It is marked by a black dot in the middle of the smudge representing the central side of the Milky Way.
Andromeda, more famously, is our nearest major galaxy – a spiral galaxy about 2.5 million light-years away. It's the most distant object visible to the naked eye and is slowly approaching us at 110 km/s. In about 4.5 billion years, Andromeda and the Milky Way will collide and merge.
The Sun, Moon and planets#
The stars are fixed on this map, but the Sun, Moon, and planets move against them. To understand their motion, we need to understand the plane they travel on, which is essentially the same for most orbits in our Solar System, including our own. This plane is called the ecliptic.
Earth's axis is tilted 23.5° from perpendicular to this plane. This tilt stays fixed in space as Earth orbits, causing the seasons, as the hemispheres get more or less sunlight if pointed toward or away from the Sun.
Winter in the Northern Hemisphere, summer in the Southern. The North Pole tilts away from the Sun – shortest day in the north.
Viewed from Earth, the ecliptic looks like a line, fixed against the stars. When we observe their locations against the stars over time, all the objects of the Solar System appear to move along this path, as they and we travel around the Sun on our orbits.
This is not the same as the path each celestial object takes across the sky over the course of the day (or night), which is determined by the Earth's axial rotation, not orbits. The above visualization is at noon, and so the Sun is at the highest point in the sky it will be on that day. The date is set to the vernal equinox, to make the ecliptic line maximally tilted from our noon point of view.
Putting the ecliptic line on our map projection, it becomes a circle again (actually slightly elliptical because of the equal-area azimuthal projection). This allows us to see the positions of all the celestial objects of our Solar System, whether they are above or below our horizon. The side of the ecliptic that is closer to the center will be higher in the sky on the northern hemisphere and lower in the southern hemisphere. In other words, radial distance relates to declination.
Angular positions around the clock corresponds to "right ascension", the celestial equivalent to longitude. If you turn toward the Sun, and see on your Spacetime Watch that the Moon is, let's say, 90 degrees to its right, you can turn 90 degrees to your right and expect to see the Moon – if it is above the horizon. This is the case for all the planets and constellations as well. If you can identify one celestial object, the Spacetime Watch can clue you in to where you can find the rest.
Celestial motion#
Since we have defined the top of the clock circle to be noon, and the Sun appears to make one full rotation around the ecliptic circle as the Earth travels along on its orbit, we need to give the whole star map one clockwise rotation per year to make sure the Sun stays roughly at the noon position.
The Spacetime Watch then has two rotating layers: The Earth map making one counterclockwise rotation per day, while the sky map making one clockwise rotation per year. Play around with the time speed settings below to see how things move relative to each other.
At 1 day/sec, you see the Moon traveling around the ecliptic, changing its phase accordingly. You also begin to see the sky map slowly rotate.
At 1 week/sec, we hide the Earth map, as it becomes a dizzying swirl. The Moon now races around the ecliptic, roughly once every four seconds. We also clearly see the Sun moving against the stars, and the inner planets, Mercury and Venus, swinging back and forth near the Sun, going behind it when moving from right to left, and in front when moving from left to right.
At 1 month/sec, you might notice the "retrograde motion" of planets around opposition (that is, when they are furthest from the Sun on the ecliptic). This is particularly noticeable with Mars, which clearly moves counterclockwise relative to the stars for most of the year, but then turns around and moves clockwise relative to the stars for a few months when it is near the bottom of the clock. This happens as a result of the Earth overtaking the outer planets at roughly the same orbital angle.
If you wait long enough (about a minute at 1 month/sec), you can also see the rings of Saturn slowly opening. The angle we see Saturn from changes over the course of its orbit. In 2025, we saw it edge-on, so the rings were almost invisible, but in 2032, it will be the Saturnian summer solstice, meaning that its axis will point toward the Sun (at its 26.7° tilt). This will also be roughly the angle we see it from, as Saturn is close to ten times further from the Sun than we are (so our orbital position does not have a big impact on our viewing angle of it).
The complete watch#
The final watchface adds the hour hand and text display to create a complete astronomical timepiece. As mentioned at the start, the hour hand represents the longitude of your timezone and doubles as a rough compass needle, with the red side pointing toward the north (at the center of our map projection), and the white pointing south.
Note: The colored segments of the hour hand compass needle are meant to meet at your actual latitude. On this website, this is approximated by the latitude of the city named in the timezone name given by your browser (like "Europe/Oslo"). This doesn't always work, but in the Wear OS watchface, you can set your latitude in the watchface settings (it is unfortunately not possible to determine automatically).
If you see the Moon (or any celestial object) in the sky, find it on the watchface. Rotate until its position on the watch matches where you see it in the sky. Now the compass needle hour hand will be pointing roughly south, and all other celestial object on the watchface will be roughly in the directions suggested.
Half of the sky is above the horizon at any one time, and unless you are near the poles, that means the celestial objects you see on roughly the right half of the Spacetime Watch is visible in the morning. The top half in the day. The left in the evening, and the bottom at night.
In-depth essays#
- Astronomy: See constellations melt as stars drift over hundreds of thousands of years. Learn why photographing the Sun at the same time each day traces a figure-8 in the sky, why eclipses repeat in an 18-year cycle, and how Polaris will lose its place as the North Star over millennia as the Earth's axis slowly wobbles.
- History: How our timekeeping devices used to connect us to the sky, until clockwork made time abstract.