Uranus Retrograde Rotation

[YNM]

I wasn't implying you explanation was bad, all I'm saying is that this method can lead to confusion.

and when I say confusion, I mean in the general public, not for those who study this stuff

let me give you an example.

If you say earth axial tilt is 23°, this kinda gives me the idea of a "movement" of the axis from the vertical position to the 23° tilt

if earth was to stop rotating and then started rotating backwards (some say venus did just that) suddently you will say that earth axial tilt is 203°

which kinda makes me go "whoa, when did we turn upsidedown?"

same goes for uranus, let's say the axial tilt of uranus went from 83 to 97 overtime

this would have been the situation:

83<tilt<90 rotating prograde

tilti = 90 rotating, neither prograde nor retrograde (...?)

90<tilt<97 now we are rotating retrograde

Yup, that's right. Prograde and retrograde is precisely that... You don't see the angles between axes (which for uranus, 97, 83, 263, 277 degrees will result in the same condition), you look for the angle between the normal of the reference plane and the angular momentum vector (which is represented in that image as L). It's easier to tell the angle between two vectors than the angle between two axes, because in vectors, the directions are well-defined.

Edited June 27, 2015 by YNM

[Sigma88]

Yup, that's right. Prograde and retrograde is precisely that... You don't see the angles between axes (which for uranus, 97, 83, 263, 277 degrees will result in the same condition), you look for the angle between the normal of the reference plane and the angular momentum vector (which is represented in that image as L). It's easier to tell the angle between two vectors than the angle between two axes, because in vectors, the directions are well-defined.

my problem with this method is that it's redundant. 97° will always be retrograde. so why do you need 2 parameters if one is enough?

and with this method you lose the information about "how the planet got there"

the main theory about uranus tilt is that it's a result of a collision. this means that once upon a time uranus was rotating prograde and had a small axial tilt but then after the impact got so inclined that now it's 97° and thus it's now rotating retrograde (even tho it never inverted the direction of rotation relative to the body itself)

on the other hand, venus once rotated prograde. then gradually it stopped rotating and started rotating backwards. So the "north pole" jumped from over the ecliptic to under the ecliptic, even tho the planet never turned upside down.

[YNM]

my problem with this method is that it's redundant. 97° will always be retrograde. so why do you need 2 parameters if one is enough?

Hehe... yeah I agree. either you say "83 degrees and retrograde" or just "97 degrees".

and with this method you lose the information about "how the planet got there"

the main theory about uranus tilt is that it's a result of a collision. this means that once upon a time uranus was rotating prograde and had a small axial tilt but then after the impact got so inclined that now it's 97° and thus it's now rotating retrograde (even tho it never inverted the direction of rotation relative to the body itself)

on the other hand, venus once rotated prograde. then gradually it stopped rotating and started rotating backwards. So the "north pole" jumped from over the ecliptic to under the ecliptic, even tho the planet never turned upside down.

Venus : either "3 degrees and retrograde" or "177 degrees".

> 90 degrees axial tilt makes more sense mathematically, for example when you're trying to calculate the total angular momentum of Solar System. But if you're a planetarium guide then I guess it's better to say to the public that things are prograde/retrograde, and an axial tilt that max out at 90 degrees. Guess that most people are carried away to merge the two into one sentence (probably trying to give something that's mathematically correct and making things sensible for the public) - and there your confusion goes... Semantics really.

EDIT: Just realized you also asked about if a planet suddenly (or slowly) stop to spin then spin in the opposite direction. Actually you'd get the north and south being reversed - while the surface features don't, AFAIK north - south is determined by angular momentum vector, north is the site where the angular momentum points to. You can't really get an object to rise from the west of this definition. One result of this is that the galactic poles are actually reversed, the one located on the north hemisphere is the south revolutional pole. Silly indeed...

Edited June 28, 2015 by YNM
Clarifying a tad bit more

[Superfluous J]

At 2am in the morning sometimes you need redundancy, but I agree. "97 degree tilt" is more descriptive than "retrograde." The problem is, "retrograde" is easier to say, remember, and (in Venus' case) understand.

I personally never think of Uranus rotating retrograde. I think of it rotating "on its side." I've not enough experience cataloging planets' rotations to know where the divide between prograde, uhhh... side... grade... and retrograde are. But I figure maybe just split it into 3's and go 0-60, 60-120, 120-180.

Not that anybody in the scientific community is going to reference this thread in a proposal to change anything