SunCast Accuracy Guide
How accurate are sun position apps?
Sun position apps are usually accurate for the astronomical position of the sun when location, date, and time are correct. The parts that vary more are AR alignment, shadow geometry, terrain or building data, and weather forecasts.
By Peter Szucs - Last updated: July 14, 2026
Short answer
A good sun position app can reliably answer where the sun should be in the sky for a selected place and time. SunCast uses Meeus-based solar position calculations for solar position, then combines those calculations with device sensors, maps, terrain context, shadow geometry, and WeatherKit forecast data where relevant.
The practical rule is simple: trust the sun angle calculation for planning, then treat AR alignment, shadows, and cloud cover as visual planning layers that depend on sensor, geometry, map, terrain, and forecast quality.
What sun position apps calculate
Sun position apps calculate the apparent position of the sun for a specific latitude, longitude, date, and time. The most useful outputs are solar azimuth, solar elevation, sunrise, sunset, solar noon, golden hour, blue hour, and twilight phases.
| Output | Meaning |
|---|---|
| Azimuth | The compass direction of the sun along the horizon. |
| Elevation | The angle of the sun above or below the horizon. |
| Sunrise and sunset | The times when the sun crosses the visible horizon. |
| Golden and blue hour | Light windows near sunrise and sunset used for photography and outdoor planning. |
| Shadow direction | The direction a shadow should fall from a known object and sun angle. |
Calculation methodology
SunCast calculates solar position using astronomical formulas derived from Jean Meeus solar position algorithms, the same class of calculations commonly used for sunrise, sunset, solar azimuth, solar elevation, and twilight estimates.
These calculations produce the sun's apparent position for a selected place and time, including solar azimuth, solar elevation, sunrise, sunset, solar noon, golden hour, blue hour, and civil, nautical, and astronomical twilight estimates.
Shadows
SunCast calculates shadows from solar position and the available scene, building, terrain, or user-placed geometry.
Weather
SunCast uses Apple WeatherKit for weather and cloud-cover forecast data.
Maps and terrain
SunCast uses map and terrain context from various map providers, including Apple MapKit and Google Maps.
What affects real-world accuracy?
When a sun app seems wrong, the cause is often outside the astronomical calculation. The most common accuracy factors are location, time, device calibration, scene geometry, weather forecast quality, and the visible horizon around the user.
| Factor | Why it matters |
|---|---|
| Location | Solar position depends on latitude and longitude. A wrong GPS location, manually selected address, or map pin changes the calculated sun angle. |
| Date and time zone | Sunrise, sunset, golden hour, and shadow direction all change with date and local time. Planning for the wrong day or time zone is a common source of mismatch. |
| Compass and device sensors | AR views rely on device orientation. Magnetic interference, poor calibration, or holding the phone near metal can make the overlay look offset even when the solar calculation is correct. |
| Scene geometry | Shadow previews depend on the geometry available to the app: buildings, terrain, map data, or user-placed objects. Missing or simplified geometry changes the visual shadow result. |
| Weather forecasts | Cloud cover is a forecast, not an astronomical calculation. A sun path can be correct while the actual sky is cloudier or clearer than predicted. |
| Atmosphere and horizon | Refraction, mountains, trees, nearby buildings, and a raised or blocked horizon can shift what a person sees at sunrise, sunset, and low solar elevations. |
Good uses
- Planning golden hour, blue hour, backlight, and side light for photography.
- Checking apartment, balcony, window, and garden sunlight before visiting.
- Previewing seasonal shadow direction for homes, streets, parks, and rooftops.
- Doing preliminary sunlight review for solar, architecture, and construction ideas.
Do not use it for
- Legal surveying, boundary disputes, or planning-permission evidence.
- Engineering certification, structural design, or final solar yield analysis.
- Safety-critical navigation, aviation, maritime, or emergency decisions.
- Any situation where an expert site survey or certified instrument is required.
Accuracy checklist
- 1. Confirm the selected location or map pin before trusting a result.
- 2. Check the date, local time, and time zone for future planning.
- 3. Calibrate the phone compass and move away from cars, railings, and large metal objects.
- 4. Use the map and AR views together when planning a real location.
- 5. Treat cloud cover as forecast guidance, not a guarantee of visible sunlight.
- 6. For professional decisions, verify the site in person or with domain-specific tools.
Frequently asked questions
Are sun position apps accurate?
Sun position apps can be highly accurate for planning solar azimuth, solar elevation, sunrise, sunset, twilight, golden hour, and blue hour when they use established astronomical calculations and correct location/time inputs.
Why can an AR sun path look slightly wrong?
An AR sun path can look offset because the phone compass, gyroscope, location, or calibration is off. The astronomical sun position can still be correct while the device orientation layer is imperfect.
Are shadow maps as accurate as the sun calculation?
Shadow maps depend on both the solar calculation and the geometry used to cast shadows. The sun angle can be correct, but shadows may differ if buildings, terrain, trees, or user-placed objects are missing or simplified.
Can SunCast replace engineering or surveying tools?
SunCast is designed for planning light, shadows, photography, gardening, property sunlight, and preliminary solar or construction review. It is not a substitute for safety-critical navigation, legal surveying, engineering certification, or permitting analysis.
