A compass, and nothing else

The heading

Your phone contains a magnetometer, a three-axis sensor that measures the strength of the magnetic field around it. The operating system combines that with the accelerometer, which knows which way is down, and works out which way the device is pointing. The browser exposes the result to this page through the Device Orientation API.

On iOS the browser reports a compass heading directly, already corrected for how the device is held. Everywhere else the page receives a rotation angle and inverts it, because that angle runs anticlockwise while a bearing runs clockwise. It also adds the screen rotation, which the sensor does not account for, so the reading stays right in landscape.

The reading is smoothed slightly on its way to the screen. A magnetometer is noisy at the last degree, and an unsmoothed needle vibrates in a way that makes it harder to read, not more honest. When you turn the card by hand there is no smoothing at all, because a control that lags your own finger feels broken.

What happens if there is no sensor

Almost no desktop or laptop has a magnetometer. Rather than show a needle that never moves, the page waits about a second for a reading and then says plainly that none arrived, and hands you the card to turn yourself. That mode is a first-class path: drag the rose, or focus it and use the arrow keys.

The status line under the compass always names which mode you are in. A compass that does not tell you whether it is reading a sensor or echoing your own drag is worse than no compass at all.

The declination this site does not calculate

Magnetic north and true north are different directions, and the angle between them depends on where you are standing. Offering true north means knowing that angle.

A centred-dipole estimate was implemented here, tested, and removed. It treated the field as one tilted bar magnet and returned the great-circle bearing to the geomagnetic pole, which for a centred dipole is the declination. Measured against published values it was 15 degrees out in London, 13 in New York, 14 in Cape Town and 17 in São Paulo. The non-dipole part of the earth's field is too large to approximate away.

Doing it properly means the World Magnetic Model, 168 coefficients revised every five years, which is not something to reproduce from memory. So the compass asks for the figure and links to NOAA for it. An empty field means true north is unavailable, which is an honest state rather than a degraded one.

Location is still used, for your coordinates and for the qibla. Both are exact spherical trigonometry, both are computed in your browser, and neither sends anything anywhere. The qibla bearing was checked against published figures: 119.0 degrees from London against a published 118.9.