Thursday, May 27, 2010

What would a compass on the moon point to?

Dear Dr. Stern,

I am student teaching in an 8th grade science class. We have been studying the earth's magnetic force, and the question was asked by one of the students, Lars, "What would a compass on the moon point to?" I know that the moon has no internal magnetic force, but does have some areas on the surface that are magnetized. Would a compass point to these? Thank you for your help with this question.


Your information about the moon is correct, but your students should bear in mind two points. One, we are used to regarding the compass needle as a sensitive instrument. It may be so, compared to other everyday indicators of magnetic forces, but space-borne electronic magnetometers, such as have mapped magnetism in space, are many thousands of times more sensitive. I would suspect that the weak magnetization of some areas on the moon is not nearly strong enough to move the needle of an ordinary compass, even in the weaker gravity.

You can find more about such magnetometers in

Even if the technical description of such instruments is beyond your students, they may be interested in the way they were used by Dr. David Cohen at MIT.

And secondly, the magnetic force, on Earth or on the moon, is usually not horizontal. That is discussed very briefly in
and in more detail at the end of

An ordinary compass only responds to the horizontal part of the force, and in a complicated magnetic field, this can be highly variable--towards magnetized patches, away from them, or neither. I do not have a magnetic map of the moon, but your students may look at one of Mars, which is also magnetized in patches, and appreciably more strongly than the Moon (though it lacks a core field like the Earth's). See map at the very end of
and the link given there (since then that map has been made more accurate).

21. Why do iron filings outline magnetic field lines

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