Why is it so difficult to find any specific information about the "esoteric" commands like PrecessionRate, RotationOffset, etc ?
What is the formula for PrecessionRate ?
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Topic authorCham
- Posts: 4324
- Joined: 14.01.2004
- Age: 60
- With us: 21 years 1 month
- Location: Montreal
What is the formula for PrecessionRate ?
I need to know the exact formula used by Celestia related to the PrecessionRate command. It's clearly related to the RotationPeriod of a body, because if I set RotationPeriod equal to 0, then I get a precession rate of 0. I need to do some specific things with the PrecessionRate command.
Why is it so difficult to find any specific information about the "esoteric" commands like PrecessionRate, RotationOffset, etc ?
Why is it so difficult to find any specific information about the "esoteric" commands like PrecessionRate, RotationOffset, etc ?
"Well! I've often seen a cat without a grin", thought Alice; "but a grin without a cat! It's the most curious thing I ever saw in all my life!"
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Guest
Setting RotationPeriod to zero just breaks the model (because zero is a non-physical value for rotation period), so you shouldn't see anything at all.
There's no complex formula - the axis just precesses in the reference plane at the specified rate in degrees per day. Meanwhile, the object rotates around the axis in a period determined by RotationPeriod (in hours). So if you set RotationPeriod to 24 and PrecessionRate to 360, you'll see the object's axis precess once in 24 hours, while it rotates around that axis once in 24 hours - by the end of 24 hours, it has therefore rotated twice relative to the reference frame.
Just sum the angular velocities and you'll have the answer for any given situation.
Grant
There's no complex formula - the axis just precesses in the reference plane at the specified rate in degrees per day. Meanwhile, the object rotates around the axis in a period determined by RotationPeriod (in hours). So if you set RotationPeriod to 24 and PrecessionRate to 360, you'll see the object's axis precess once in 24 hours, while it rotates around that axis once in 24 hours - by the end of 24 hours, it has therefore rotated twice relative to the reference frame.
Just sum the angular velocities and you'll have the answer for any given situation.
Grant
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Topic authorCham
- Posts: 4324
- Joined: 14.01.2004
- Age: 60
- With us: 21 years 1 month
- Location: Montreal
Thanks Grant.
But my problem is this : I wanted to make two tidal bulges out of a star. That star is in orbit around another object (a black hole). The bulges should be permanently locked and oriented toward the black hole. So I made the star as an oblate shape (actually like a cigar), and gave to it a RotationPeriod = 1e12 and an Obliquity = 90. After that, I gave a PrecessionRate = 360 / (365*Period), so the star have its bulges rotating at the same frequency as the black hole. But there are TWO problems. First, I'm unable to get the bulges locked toward the black hole, and second, after a pretty long period of time, the bulges are getting farter away from the black hole. In other words, I'm unable to get the exact PrecessionRate so it rotate with EXACTLY the same frequency as the orbital Period. What should I do ?
I also tried another way : I made the oblate star (cigar shaped) and gave a RotationPeriod = 365*24*Period. I tried various value for RotationOffset. So apparently, the star is locked to the Black Hole. But after a pretty long period of time, the bulges are drifting and getting away from the Black Hole.
Please ?
But my problem is this : I wanted to make two tidal bulges out of a star. That star is in orbit around another object (a black hole). The bulges should be permanently locked and oriented toward the black hole. So I made the star as an oblate shape (actually like a cigar), and gave to it a RotationPeriod = 1e12 and an Obliquity = 90. After that, I gave a PrecessionRate = 360 / (365*Period), so the star have its bulges rotating at the same frequency as the black hole. But there are TWO problems. First, I'm unable to get the bulges locked toward the black hole, and second, after a pretty long period of time, the bulges are getting farter away from the black hole. In other words, I'm unable to get the exact PrecessionRate so it rotate with EXACTLY the same frequency as the orbital Period. What should I do ?
I also tried another way : I made the oblate star (cigar shaped) and gave a RotationPeriod = 365*24*Period. I tried various value for RotationOffset. So apparently, the star is locked to the Black Hole. But after a pretty long period of time, the bulges are drifting and getting away from the Black Hole.
Please ?
"Well! I've often seen a cat without a grin", thought Alice; "but a grin without a cat! It's the most curious thing I ever saw in all my life!"