I’ve always thought that the early explorers were fortunate to live on a planet that had a magnetic filed that aligned more or less with its axis of rotation. Now I wonder if that is just happenstance or whether it’s a result of planetary formation. Do we know whether any of the other planets have magnetic fields, and, if so, how closely they are aligned with axes of rotation?
A magnetic field is created by rotation. In the Earth and any other body, it would be aligned with the axis of rotation.
It was about ten years ago that I took an astronomy class, so what I’m about to say may be out of date.
Back then the best theory to explain why planets have magnetic fields was called the “dynamo model”. The magnetic field is apparently a consequence of rotating electric charges in the liquid metal core of the Earth. The strength of the magnetic field would depend in part on the rotational speed.
The Earth’s magnetic axis is inclined to the rotational axis by about 12º. Geological evidence shows that the irection of the magnetic field has switched several times over the Earth’s history, about once every 10,000 to 100,000 years.
Several other planets with liquid metal cores also have magnetic fields more-or-less as strong as predicted by this model: Mercury, Jupiter, and Saturn. In the case of Jupiter and Saturn, the metal in question is metallic hydrogen. The magnetic field of Jupiter is inclined to its rotational axis by about 10º.
But two planets, Mars and Venus, have much weaker magnetic fields than would be predicted by this model. (Venus is predicted to have a fairly weak magnetic field anyway because it rotates so slowly). It is possible one or both of them is in the process of switching the direction of its magnetic field. It seems unlikely they would both be undergoing this change at the same time.
The moon, does not appear to have a liquid metal core, which is probably why it does not have a strong magnetic field.
My understanding (in the case of Mars) is that its volcanism has largely subsided (at least as compared to earth), and its core cooled. So that molten core ain’t turning no more.
Okey-doke. I’ve got a bunch of Space Science series books in my office:
Mercury has a “stronger than expected” magnetic field (since at the time they expected none). It’s thought to be due to
a liquid outer core/dynamo.
Venus and Mars have rather weak magnetic fields.
Jupiter’s magnetic pole is offset from its rotation pole by an amount similar to Earth’s.
Saturn’s magnetic and rotational poles are nearly coincident.
Uranus and Neptune have magnetic axes that are offset from the rotation axis and not centered on the planet’s center (!), thought to be because the fields are generated in a salty water mantle rather than a liquid core.
I believe the asteroid Gaspra also has a magnetic field, and many others might, as well. This would be a remnant of a much earlier time in a larger body.
Hope this helps…
-Andy
additional info about Jupiter’s largest moons from The Nine Planets website…(also has some info about the magnetic fields of planets)…
http://www.seds.org/billa/tnp/
p.s. Hi asrivkin! Good to see you here again.
How about the Sun? Does the hydrogen plasma core generate a magnetic field?
Thanks Phobos! It’s been a crazy couple of months-- I switched jobs, moved cross-country… Hopefully I’ll be able to drop in more often!
-Andy
The sun does have a magnetic field, but it’s much more complicated than planetary magnetic fields. The problem is that the sun, which is not solid, does not rotate uniformly. (It rotates faster at the equator than at the poles). This causes the (imaginary) magnetic field lines to be twisted into bizarre shapes. The direction of the field switches about every 11 years, for a complete cycle of 22 years. This is believed to be what is behind the solar activity cycle, whereby the number of solar flares and sunspots waxes and wanes over an 11-year span.
From the same Nine Planets link I posted above…
Oh no! You’re not at the observatory anymore? Well, good luck in your new digs! But it seems like we only have small radio telescopes here on the East Coast. Unless you’re working for NASA now? 
Well, I was never really working at an observatory, it’s just that it seemed the only time I posted was when I was at the telescope observing. I’ll still get to do that, it’s just a longer plane flight. And I may get to observe on the 'scopes in Chile now!
Obligatory addition to the thread: The sun’s magnetic field is used by some to help delimit the solar system-- the heliopause is where the solar magnetic field meets the interstellar magnetic field and interstellar space begins. It was thought to be not far beyond Neptune, but the Voyagers haven’t gotten there yet (getting there is now the main Voyager mission objective), and estimates for its position are growing accordingly…
Get out! That’s huge! :envy:
Current status of the Voyagers…
http://vraptor.jpl.nasa.gov/flteam/weekly-rpts/current.html#RTLT
Come to think of it, early explorers would probably have had it easier were the magnetic poles not so close to the geographic ones. You can fairly easily determine both geographic north and your latitude by astronomical observations (ask sailor for more details), and if you also had the direction to the “West Pole”, you could, in principle, determine your longitude. I’m not sure how accurate this would be, but it would have to be better than dead reckoning, which is what sailors used before the invention of accurate shipborne timepieces.
I’m curious, Chronos - would such a point rotate with the earth or stay in one location? Because in the former case, it would be a single location, such as - say - Phoenix, Arizona, and thus would indeed be a perfect means for triangulation; however, in the second case it would seem to move in a perfect circle around the Earth, and so would simply be another line of longitude.