2.8 C
Saturday, November 26, 2022
Dr. Brown: Navigation using the Earths magnetic fields

Every airplane I’ve flown had a magnetic compass located in plain view well above the panel.

With it, I could determine my magnetic heading should vacuum-driven devices such as the directional gyro or horizontal situation indicator fail, or perhaps their electrically driven equivalents fail for want of electrical power.

There are errors associated with magnetic compasses, such as lagging or leading on turns off northerly or southerly headings or speeding up or slowing down, especially on a westward or eastward headings, but understood, those are momentary errors and easily compensated for.

So important is the magnetic compass, that just before takeoff, I always check to see whether the heading shown on the directional gyro and horizontal situation indicator corresponds with the runway heading and the magnetic compass. It’s good to have agreement all round, then, throughout the flight and before making approaches.

Of course these days, with ubiquitous GPS devices, full panel flight instruments and moving map simulations available on iPads, backup has become so multilayered that it poses an unwelcome distraction in emergencies when dissonate information may be not only confusing, but dangerous.

But what if I told you that fish, including great white sharks, can navigate thousands of miles using the Earth’s magnetic fields to guide them?

And what if I told you that some types of bacterial cells orient themselves according to the surrounding magnetic field or that mammals such as mole rats and wood mice apparently find their way to their nests using magnetic field lines.

That’s very impressive and all the more so when we realize that the strength of the Earth’s magnetic field is so tiny (25 to 60 micro-tesla).

Given the relative weakness of the magnetic field, it's no surprise that radio interference, especially in the AM band, can disrupt some migratory birds who depend on the Earth’s magnetic fields to find their way over long distances out of sight of land.

If so many different species possess a magnetic sense, it’s fair to ask whether humans or for that matter our ape relatives, also possess the same sixth sense. The answer is probably yes – but proving so is tricky.

One way to test for a human magnetic sense is to see whether changing the magnetic field surrounding test subjects alters the electrical activity generated by the brain recorded using electroencephalography (EEG).

This approach, developed by Joe Kirschvink, a geophysicist at Cal Tech in California, showed that altering the magnetic field surrounding humans changes the EEG – but how and whether the changes provide meaningful information are two questions that remain unanswered.

Given that so many remotely related species appear to possess magnetic field sensing, suggests that the acquisition of magnetic sensing occurred very early in evolution, possibly as early as some of the first bacteria.

Some of the latter magnetic-field sensitive bacteria may have been adopted by other cells roughly two billion years ago, to become the host cell’s resident energy machines – the mitochondria – which confer sensitivity to magnetic fields on the host cell. Or at least that’s what Kirschvink speculates.

There is evidence that some nerve cells in the inner ears of pigeons fire in response to changing magnetic fields. Maybe so, but where are the postulated magnetic-field sensitive receptors in the inner ear that would be equivalent to photoreceptors in the case of vision, cochlear cells in the case of hearing or skin receptors in the case of touch? So far, none have been found.

What’s known is that a wide variety of lifeforms from bacteria to worms, frogs, lobsters, fish, birds and some mammals exist, which have been shown to be sensitive to the Earth’s magnetic fields, and some use those fields to navigate great distances. The missing link is figuring out how those fields are translated into meaningful information.

To that end the search is on to find the receptors. Are they tiny intracellular magnetic crystals – perhaps magnetite or as some suggest, are cytochrome molecules found in mitochondria the missing link? We don’t know.

To return to the magnetic compass and flying – on two occasions I was in cloud and lost my directional gyro because of instrument failure but with the magnetic compass on hand and the full suite of other navigational instruments on board, flying the remaining course and approach was easy.

For that happy, safe outcome, I was grateful for the periodic flight checks and many instructors who from time to time “failed” the odd instrument or two by covering them to find out whether I was up to doing with less.

Doing with less is not a bad lesson for living, too, and I’m not talking about flying.

Dr. William Brown is a professor of neurology at McMaster University and co-founder of the Infohealth series at the Niagara-on-the-Lake Public Library.