Horizons What is a negative rotational period?
- Thread starter Crunchy_to_the_MAX
- Start date
Note for the inattentive: this is a thread necro from 2017. 
Just adding my two cents, about some illogicality in the Stellar Forge.
The Stellar Forge generates planets with negative rotation periods. Fair enough; those planets are "rotating backwards" relative to the direction of motion of the planet's orbit, and/or the direction of motion of rotation of the primary star.
The Stellar Forge also generates planets with an axial tilt more than 90 degrees. You can have axial tilts all the way up to 180 degrees. Also fair enough; the planet has been knocked "upside-down".
However, these two effects are functionally identical - they both make a planet "rotate backwards". And a planet that has both a negative rotation rate and 180 degree axial tilt would look exactly the same as a planet with a normal rotation rate and axial tilt. The "two negatives" cancel each other out.
You can have a universe where planets are described as having negative rotation, or a universe where planets are described as having axial tilts more than 90 degrees. But you cannot logically have both.
In my database I personally record rotation periods as an absolute value (since for calculating day length, the rotation being "backwards" or not is irrelevant) and I convert all axial tilts to a 0-90 degree basis. Thus if a planet is reported as haing an axial tilt of "135.7 degrees", then I report it as "180-135.7 = 44.3 degrees", which is functionally what the planet would have: a 44.3 degree axial tilt and backwards rotation.
Just adding my two cents, about some illogicality in the Stellar Forge.
The Stellar Forge generates planets with negative rotation periods. Fair enough; those planets are "rotating backwards" relative to the direction of motion of the planet's orbit, and/or the direction of motion of rotation of the primary star.
The Stellar Forge also generates planets with an axial tilt more than 90 degrees. You can have axial tilts all the way up to 180 degrees. Also fair enough; the planet has been knocked "upside-down".
However, these two effects are functionally identical - they both make a planet "rotate backwards". And a planet that has both a negative rotation rate and 180 degree axial tilt would look exactly the same as a planet with a normal rotation rate and axial tilt. The "two negatives" cancel each other out.
You can have a universe where planets are described as having negative rotation, or a universe where planets are described as having axial tilts more than 90 degrees. But you cannot logically have both.
In my database I personally record rotation periods as an absolute value (since for calculating day length, the rotation being "backwards" or not is irrelevant) and I convert all axial tilts to a 0-90 degree basis. Thus if a planet is reported as haing an axial tilt of "135.7 degrees", then I report it as "180-135.7 = 44.3 degrees", which is functionally what the planet would have: a 44.3 degree axial tilt and backwards rotation.
Which end of a standard surveying bar magnet points at the point where the axis of rotation meets the planets surface.
Actually, that's a terrible definition. It works well for Earth today, but also works on only one other solid planet in the solar system: Mercury. Ganymede is the only other world in Sol system with a worldwide magnetic field, and on Ganymede, the magnetic South pole is at or near the rotational North pole, so a magnet will point the "wrong way". All other solid planets don't have a magnetic field to speak of, just localized magnetic anomalies that appear to be the result of ancient impacts. Magnetic compasses on those other worlds (Venus, Mars, the Moon) barely move at all and, if they do move, will point in random directions, and in different directions at different places on the surface, not always towards one of the planet's poles.
That definition also wouldn't be true on Earth itself for about half of Earth's history: In what are called geomagnetic reversals, Earth's magnetic field flip-flops back and forth every few hundred thousands years. The geomagnetic record shows no statistical preference that a magnetic compass would point North at any randomly selected time period in Earth's history; it's just as likely to point South. And of course for the few thousand years a reversal is actually happening, the magnetic field goes chaotic, with multiple localized "poles" scattered all over the surface and a compass would point in a random direction, just like it would on Mars or Venus.
Note for the inattentive: this is a thread necro from 2017.
No wonder I couldn't remember writing my comment from two years ago, at least it's still attracting likes!
I always thought magnetic north was the magnetic pole aligned with the geographical North pole. A matter of convention. If the magnetic field would reverse, the Magn. South would just be named as new North. Dunno if the magnetic field itself has attributes by which you can identify its orientation - I always assumed the field is uniform with no labels stuck on its "wings".
The magnetic pole isn't fixed like the pole of rotation, it moves around, currently at about 50-60 kilometers per year. It's also not straight through, a line straight through the globe at the north magnetic pole won't come out at the south magnetic pole. This is because the interior of the earth is liquid with convection currents and magma currents which change the fields as it moves. The North Pole is fixed because you can't just change the point of rotations of a mass of 5.972 × 10^24 kg easily.
So the answer to this;
Is neither end, unless the magnetic pole just happens by chance to wander across the same point as the pole of rotations, but it's close enough for sailing ships and slower surface tarvel to use for navigation and get where to want to be. It doesn't move fast enough for triangulation to be quickly wrong or indeed far enough to be wrong enough to easily notice without very accurate isntruments.
Well, not to be pedantic but you have to take into account declination which varies across the earth (and with time due to pole migration) and locally due to geo-magnetic variance, then again there is deviation which is important in vessels and aircraft (the effect of the craft on compass accuracy).
Nice gif showing effect of polar wandering on declination over time:
Last edited:
I know that. And the magnetic fields sometimes reverse. I mean, when that happens, could you tell the Magnetic South is now at Geo North? Maybe the Magnetic South smells differently? Has blue particles instead of red? Its quanticles turn right and not left? Or does the magnetic field just look identical and if you pin an N over the S on that needle noone will notice
The convention is of course that the north pole (of any magnet including a planet) is where the lines of the magnetic field ENTER perpendicular to the surface.
How was this thread even found?!
Don't they enter surface perpendicular at south and north?
No - at the south pole they EMERGE perpendicular.
Maybe you could read-up on magnetic fields - after all, much of modern life relies upon the interaction of electrons and electric fields with magnetic lines of force.
Aha! So there is a direction to them sos you can identify which whay is what.
It's educational thread.So, how do magnets work?
Must be a slow news week. That's the 2nd time I've not checked the dates on a thread 
If the rotation is the same as the orbit, it's not tidal locked. It only appears to be because like the moon with a few degrees of difference see the same portion of the surface every night and or day it's visible.