This is a sensor that measures the magnetic field around your Android device. It is equivalent to the magnetic compass that is used for navigation.

If your device has this sensor, SkEye can autoscroll the map as you point the device to a location in the sky. It can also help guide a telescope via the feature called Virtual DSC.

Magnetometer Calibration

The magnetometer on some phones requires to be calibrated. This needs to be done every time you use any app that needs the magnetometer!

Calibration procedure

The procedure is very simple and takes just a few seconds. After starting the application, wave your arm holding the phone in a figure-8 motion. You may have to repeat this a couple of times.

This is best illustrated with a video.

If you can't click the link, try searching YouTube for “best compass calibration method”.

After doing so, check whether North as shown in the app, is approximately where you think it is. (If the sensors are not calibrated, you will see really wild errors that are easy to notice).

Sources of errors

Magnetometers sense the direction of Earth's magnetic field to provide orientation. There are various errors introduced at this stage:

  • The Earth's magnetic field itself is not consistent. It varies from place to place, and time to time, quite unpredictably. The major components of the variation are more predictable and taken care of but smaller components aren't.

  • There is often some magnetic interference from the environment, such as large metal structures, electric circuits, underground metal deposits, and so on.

  • Some phones induce a magnetic field around themselves, affecting their own sensors.

Substances that cause interference

Every substance interacts with ambient magnetic fields (such as the Earth's magnetic field) in a unique way. Their level of interaction is measured by the “relative magnetic permeability” of the substance. Higher the value, the more the substance distorts ambient magnetic fields. The relative permeability of some materials can be found in this wikipedia article.

Among the commonly found materials the following are relevant to us:

  • Iron
  • Nickel
  • Cobalt
  • Alloys of the above. For example, Steel.

All other materials don't pose a problem. Examples are:

  • Aluminum
  • Copper and it's alloys like Brass
  • Wood, Plastic and most of the other commonly found substances

The "Strange Magnetic Field" warning

SkEye shows a warning if abnormal magnetic fields are detected.

If you see this warning then, firstly, "Don't Panic"!

This message is shown when the difference between the detected field strength and the expected Earth's magnetic field strength is higher than usual at your location. The sensors are quite sensitive, so the difference is often negligible for day-to-day activities. But it could be substantial for navigation / sky map pointing.

There are couple of things you can do if you see this message:

  • Try calibrating the magnetometer as described above.
  • Try observing from a different location. You might have to go outdoors, far from interfering gadgets, transmission wires, etc.
  • Check if your device has a cover with a magnetic button. You might have to discard such a cover.
  • Check if you have specified your location correctly. The strength of the Earth's magnetic field is not constant; it varies by time and place.
  • Check the system date on your device.
  • If nothing else works, the magnetometer might be faulty. If you have access to another device, try running SkEye on it from the same location.