UDS astronomical simulation
UDS can position the sun, moon, and stars using real-world coordinates and dates instead of the simple yaw/pitch/offset settings. Three independent toggles + a location/date block + an OS-clock sync. All settings live in the Simulation category.
| Toggle | Overrides | Notes |
|---|---|---|
Simulate Real Sun | Sun Yaw / Pitch / Vertical Offset | Manually Position Sun Target still takes priority |
Simulate Real Moon | Moon Yaw / Pitch / Vertical Offset / Moon Phase | Phase is driven by simulated date |
Simulate Real Stars | Default tiling stars texture → 360° star map | Both textures exposed in Stars category |
When to use this skill
- Need sunrise/sunset times accurate for a real location and date.
- Setting the sky for a specific city — Tokyo at noon on Aug 3rd, etc.
- Star positions matching real constellations.
- Moon phases matching a real date.
- Time of Day should reflect the player's actual wall-clock time.
- Defining what "north" means in the level (level rotation vs world rotation).
Location setup
| Setting | Notes |
|---|---|
Latitude | Degrees N/S |
Longitude | Degrees E/W |
Time Zone | UTC offset. Critical — wrong time zone offsets sun position by hours, because Time of Day is interpreted as local time at this location. |
North Yaw | What world-space direction = north. Default: north is +X. Degrees, clockwise. |
City presets
A city preset applies latitude, longitude, time zone, and daylight savings in one step. The fastest path for "set the sky correctly for City X."
Date
| Setting | Effect |
|---|---|
Year, Month, Day | Simulation date |
| Date auto-increment | At runtime, increments forward every night at midnight |
For setting date from code, Set Date and Time and Get Date Time handle Date Time structs — see ue-uds-time.
Time speed with simulation on
When Simulate Real Sun is on, Day Length and Night Length are ignored. Instead:
| Variable | Meaning |
|---|---|
Simulation Speed | Scales runtime time progression. 1 = roughly real time (one real day per one real day) |
Daylight savings time
DST options in the Simulation category. When enabled, Time of Day is interpreted with DST applied within the DST start/end times set in the advanced dropdown of the Simulation category.
Use System Time
Use System Time (Simulation category) sets Time of Day and date values from the system clock's UTC time, offset by Time Zone.
This works without enabling the rest of the simulation features — you can use it solely to keep in-game time matching the player's wall clock, without simulating real sun positions.
Simulation accuracy
The complete astronomical algorithms are too expensive in blueprint math, so UDS uses approximations.
| Body | Typical accuracy | Caveat |
|---|---|---|
| Sun | Within fractions of a degree; worst case ~1° off | Leaves out Earth orbit eccentricity, Earth shape irregularity. Sufficient for almost all practical applications. |
| Moon | Calibrated using moon's position in 2017 | Generally accurate for dates within several decades. Moon appears in correct constellation with correct phase. |
If you find edge cases where deviation is worse, that's expected — it's an approximation.
Interaction with other features
| Other system | Interaction |
|---|---|
Manually Position Sun/Moon Target | Takes priority over simulation |
Dusk Time / Dawn Time (Basic Controls) | Ignored when Simulate Real Sun is on — simulation determines horizon-crossing times |
Animate Time of Day | Still works — uses Simulation Speed instead of Day Length / Night Length |
Runtime / Blueprint control
The simulation toggles, location, date, and DST are mostly static properties. After runtime changes, call Static Properties - Simulation (or Hard Reset Cache for full refresh) so they apply immediately instead of waiting for the next cache cycle.
Time of Day and the date variables remain settable directly per the standard runtime time-control functions in ue-uds-time; the simulation interprets them according to the configured location/DST.
Gotchas
- Sun in wrong position for the location — check Time Zone. Most common error is leaving Time Zone default while changing latitude/longitude.
- Sun position settings ignored — that's by design when
Simulate Real Sunis on. Disable it to use Yaw/Pitch/Vertical Offset again. Dusk Time/Dawn Timehaving no effect — the simulation overrides them.- Cycle speed feels wrong despite
Day Length/Night Length—Simulate Real SunusesSimulation Speedinstead. - Moon phase wrong —
Simulate Real Mooncontrols phase from the simulated date. To set phase manually, disableSimulate Real Moon. - Stars look like a low-res repeating texture —
Simulate Real Starsis off (default usesTiling Stars). Enable it for the 360 star map. - Wrong moon position for a 2050+ date — moon accuracy is calibrated for 2017; expect drift across many decades. Sun is fine.
- North isn't where you expect —
North Yawrotates the simulation. Default has north = +X.
References & source material
Docs (Ultra Dynamic Sky 9.5):
- Simulation of Real World Sun, Moon and Stars (incl. accuracy notes)
- Controlling Time of the Simulation
Related skills: ue-uds-time, ue-uds-sun-moon-stars, ue-uds-setup-and-modes.
