Negative rotational period?

[Heavy Johnson]

-2.1d rotational period... does this indicate rotation in the opposite direction to orbital direction?

If so, and those two are the same number (2.1 days).... that means the whole planet should get sunlight by the time the 2.1 orbit is up?

This was the first time I broke out a pen and paper in a while to figure out what was going on.

 

[Shadowdancer]

Yes, that's what a negative rotation period would indicate. This case looks a bit special because its orbital period and rotational period have the same absolute value (if the sign was the same too, the planet would be tidally locked, this way it just is not).

Assuming the "day" is 24 hours (86400 seconds to use a proper unit), the planet would complete an orbit around its parent body (which is not necessarily the star determining its day/night cycle) in 50.4 hours, and complete a rotation around its axis opposite its orbital motion in the same time. Assuming it is orbiting "its star", a local day, e.g., measured from noon to noon would only be half a rotation, so fun enough, that planet would have a ~25.2 hours long day cycle. If it wasn't a molten hellhole, it could be quite nice.

 

[varonica]

https://i.imgur.com/1SQ2zkc.png

-2.1d rotational period... does this indicate rotation in the opposite direction to orbital direction?

If so, and those two are the same number (2.1 days).... that means the whole planet should get sunlight by the time the 2.1 orbit is up?

This was the first time I broke out a pen and paper in a while to figure out what was going on.

Look at the axial tilt, 179.06%, the entire planet is upside down, hence the negative rotational period and the positive orbital period, it's spinning the opposite direction to all the other bodies in the system! No wonder the poor thing is confused.

 

[Heavy Johnson]

Look at the axial tilt, 179.06%, the entire planet is upside down, hence the negative rotational period and the positive orbital period, it's spinning the opposite direction to all the other bodies in the system! No wonder the poor thing is confused.

Nice catch! I didn’t notice.

In that case, the planet is almost close to tidal lock, if I have that reading correct.

Edit: no... wait... it’s upside down and rotating backwards. If it was right side up it would be rotating the opposite way, and so tidal lock. Wacky...

 

[Shadowdancer]

OK, I will now be leaning out the window of my comfort zone and into the unfamiliar territory of astronomical geometry quite a bit by stating this:

The planet has a negative rotation period because it has that >90° axial tilt. It is an artefact of how the direction of the planet's axis of rotation is defined (right-hand rule). To be tidally locked, the rotational period would need to be the same in sign and magnitude as the orbital period, so to have a planet with a retrograde rotation be locked, it would need a retrograde orbit too, at which point the responsible god drops the mic and tells you to "accrete this, muddyfunster!" (Or just late-capture it, you know, whatever.)

 

[Sapyx]

Another oddity of the stellar forge. Ideally, either a planet is reported as having an axial tilt varying to +/-180 degrees and an always-positive rotation period, or an axial tilt that varies +/- 90 degrees and a rotational period that might be either positive or negative.

To my mind, a planet with 0 degree tilt and +1.0 day rotation would be functionally identical to, and indistinguishable from, a planet with 180 degree tilt and -1.0 day rotation. The two negatives cancel out to equal a positive.

 

[Shadowdancer]

Another oddity of the stellar forge. Ideally, either a planet is reported as having an axial tilt varying to +/-180 degrees and an always-positive rotation period, or an axial tilt that varies +/- 90 degrees and a rotational period that might be either positive or negative.

Someone at NASA would disagree: https://nssdc.gsfc.nasa.gov/planetary/factsheet/venusfact.html lists Venus with an obliquity (axial tilt) of 177.36° and a sidereal rotational period of -5832.6 hours.

 

[marx]

Unless I'm mistaken, Elite actually lists the sidereal rotational period, and just leaves this first part out. In most cases, the distinction doesn't matter anyway, but on planet with retrograde rotation, it does. Imagine how the rotation of the night sky would look on such a planet when compared to the prograde ones in the same system.

 

[Major Klutz]

If it's upside down AND it's rotating backwards, doesn't that mean that it's rotating the same direction as all the other bodies?

How would you define upside down anyway? 10 degrees is indistinguishable from 170 degrees.
Maybe it shows a 180 degree angle because it's rotating backwards.

 

[Heavy Johnson]

Sorry. It was too easy... [haha]

 

[Heavy Johnson]

If it's upside down AND it's rotating backwards, doesn't that mean that it's rotating the same direction as all the other bodies?

How would you define upside down anyway? 10 degrees is indistinguishable from 170 degrees.
Maybe it shows a 180 degree angle because it's rotating backwards.

I think this is defined based on a given standard for the system. As in, we say venus is rotating "backwards" because it is of opposite sign to the majority of the planetary bodies in the solar system.

 

[Shadowdancer]

How would you define upside down anyway? 10 degrees is indistinguishable from 170 degrees.
Maybe it shows a 180 degree angle because it's rotating backwards.

Do a thumbs up with your right hand. The thumb is then, for the intents and purposes of our solar system, pointing "up", all planets orbit the sun in the direction of your fingers ("east"), and most planets including Earth are rotating that direction. For Venus, your thumb is pointing almost straight down, and for Uranus, it's slightly down from horizontal. If that rotational tilt/obliquity exceeds 90°, rotation is retrograde, the planet spins "westwards".

The rotational period is then the time it takes a planet to do one rotation eastwards, so if it is rotating retrograde, that time is obviously negative since you'd need to go back in time to make it spin that way.

And now excuse me, I have a slight headache and feel quite cross-eyed.

 

[varonica]

I think this is defined based on a given standard for the system. As in, we say venus is rotating "backwards" because it is of opposite sign to the majority of the planetary bodies in the solar system.

Of course Uranus, with an axial tilt of 98%, is pointing almost directly at the sun while it spins (cue crude joke about the sun looking straight up Uranus) so unusual axial tilts are probably not really that uncommon in the galaxy. I imagine the early solar system had a lot of impacts that had the necessary energy to send things pear shaped. I wonder if Uranus would ever have been named such if the discoverers had a time machine handy and could read the comments in the future, of course it may have been deliberate, some scientists have weird senses of humour.

 

[BaldEagle]

My head is hurting and I think I dislocated my elbow ...