A compass set by the sun, not the magnetic field
Next to steel, speakers or a balcony railing, a phone's magnetic compass is easily off by 10–30°. None of that affects the sun: its direction can be computed for your place and time to a tenth of a degree. Sight it once and SunCompass shows true north – without the magnetic field, running on the orientation sensor.
Nothing to install: SunCompass runs in the browser. To keep it at hand, add it to your home screen – iPhone: Share › “Add to Home Screen”, Android: menu › “Install app”.
Guides
- How to find north with your phone – Finding north with a phone: why its compass is off by 10–30° next to steel, what declination and deviation mean, and how the sun gives you true north instead.
- Compass and magnetic declination – Why a compass misses by degrees and how to calibrate it on the sun – a SatFinder guide.
- Sun position and sun path – SunPosition: azimuth, altitude, sunrise, sunset and shadow length for any place, date and time.
- Align a satellite dish – SatFinder: calculate azimuth, elevation and skew and align the dish with AR camera and compass.
What SunCompass is good for
- Satellite dish on a steel mast: right at the mast the metal bends the magnetic field, but not the sun sighting. SatFinder computes the target azimuth for your location.
- Solar panels and balcony PV: check the orientation of a roof or module to within a few degrees, even between gutter, railing and mounting frame.
- Hiking and outdoors: find the cardinal directions when the phone compass is uncalibrated or jumps around.
- Motorhome and camping: in and around a steel body a magnetic compass is badly disturbed – the sun gives you the direction for satellite antenna and solar panel.
FAQ
Why is the phone's magnetic compass off?
Two errors add up. Declination is the angle between magnetic and true north; it is known (in Central Europe about 3 to 6° east, in the UK close to zero) and SunCompass corrects for it. Deviation is the pull of iron and magnets nearby – steel beams, speakers, balcony railings, a magnetic phone case. It cannot be predicted; next to steel, 10 to 30 degrees are not unusual.
How does the sun compass work?
Start the camera, put the crosshair on the sun and tap “Sun sighted”. For your place and time the sun's direction is known to a tenth of a degree – from that the compass knows true north and then runs on the orientation sensor, without the magnetic field. Look at the screen, never directly at the sun.
When does the sun compass not work?
At night and under complete cloud cover there is no sun to refer to; the magnetic compass, corrected for declination, remains. Below 5° altitude SunCompass does not let you sight, because buildings and trees hide the sun and its azimuth changes quickly near the horizon. When the sun is very high, as at midday in the tropics, sighting becomes imprecise: one degree off to the side means two degrees of direction error at 60° altitude and almost six at 80°.
What do true and magnetic mean?
True (geographic) means relative to the North Pole – that is how maps and the sun work. Magnetic means relative to the magnetic field – that is what a hand-held compass shows. The difference is the declination at your location; it is shown next to the switch.
Why should I re-sight the sun after a while?
The gyroscope in a phone drifts slowly. After about ten minutes a fresh sighting is worth it – SunCompass reminds you.
Why does the app need my location?
Only for the sun's position: azimuth and altitude depend on place and time. Your location to within about 10 km is enough; at 50° latitude, 100 km off shifts the sun's direction by 1 to 3°, most at noon in summer.
What SunCompass relies on
SunCompass computes the sun's direction with the equations in Jean Meeus, “Astronomical Algorithms”, which NOAA's solar calculator also uses – to better than 0.1°. How accurate the compass is afterwards depends on your sighting.
The magnetic declination comes from the World Magnetic Model by NOAA/NCEI and the British Geological Survey. The model describes only the Earth's main field. It knows neither natural anomalies of the crust (according to NOAA above 10° in places) nor the deviation caused by metal near you – which is exactly what the sun sighting gets around.