Looking for understanding of mars tilt and its direction.

[ncc1701d]

I was looking at the Mars.cfg file and noticed the obliquity = .4397415938 and was not remarked out.

Later in the cfg file the more typically known ;obliquity(deg) = 25.1953374 is mentioned in degrees but remarked so its not used.

so 1st question is ..what is .4397415938 refering to and doing inside orbiter?


----2nd

look at this pic
http://newsimg.bbc.co.uk/media/images/45591000/gif/_45591063_mars_obliquity2_inf226.gif
it shows a side view, if you will, of the axial tilt or obliquity for mars.
Where in the Mars orbit does this tilt happen tilting directly twords the sun? Is it always in Mars summer solctice season?
or does it happen at the periapsis point (perihelion) where Mars is closest to to the sun? Iam confused because its not so simple like earth is.

---3rd

At Julian date 0.00000,in what direction is Mars tilted tword?

I would think you would have to know what direction it was tilted tword at the very start of orbiter and then just add mars years from their and spin Mars 360 for each Mars sol so it eands up where it should befor much later dates but iam kind of guessing?

thanks

 

[Andy44]

To answer question 1: 25.195 degrees is 0.4397 radians.

 

[Keithth G]

----2nd

look at this pic
http://newsimg.bbc.co.uk/media/image...ty2_inf226.gif
it shows a side view, if you will, of the axial tilt or obliquity for mars.
Where in the Mars orbit does this tilt happen tilting directly twords the sun? Is it always in Mars summer solctice season?
or does it happen at the periapsis point (perihelion) where Mars is closest to to the sun? Iam confused because its not so simple like earth is.

Dealing with planetary rotations properly is always a mathematically messy thing. But one can make some headway if we assume that Mars orbits the Sun more or less in the ecliptic plane.

According to NASA (http://nssdc.gsfc.nasa.gov/planetary/factsheet/marsfact.html), Mars' Longitude of Periapsis is around 336 degrees. We also know from the Mars.cfg file that the LAN of the rotational axis is 0.621 radians (35.6 degrees).

So, if you take a 'top down view' of Mars' orbit, and measuring angles in a anti-clockwise direction from the x-axis, a vector pointing towards Mars' perihelion is 336 degrees (i.e., 24 degrees 'below' the x-axis); and the LAN is a vector pointing at about 36 degrees 'above' the x-axis. In this top-down view Mars' rotation axis is a vector pointing at an angle 36 - 90 degrees = -54 degrees (i.e., a vector pointing 54 degrees below the 'x-axis). Roughly speaking, then, the north pole of Mars' rotation axis points towards perihelion (to within 30 degrees or so, anyway.)

So, at perihelion, Mars' north pole is tilted away from the Sun, exposing the southern hemisphere to the Sun. Conversely, when Mars is at aphelion, its northern hemisphere is exposed to the Sun.

But then again, I could be entirely mistaken.

 

[ncc1701d]

"to within 30 degrees" seems like a big range

Does the image i found in link below reinforce what your saying even though i think in this pic the x axis is facing upward?
http://www.nakedeyeplanets.com/mars-seasons.htm

Your comment
"But then again, I could be entirely mistaken"
makes me worry. I dont want a guess.

 

[asbjos]

The axial tilt tilts directly towards the Sun on summer solstice. That's the defenition of summer solstice.

A planet in general always rotates around an axis that is fixed in reference to the stars. Of course there is a certain precession, but this effect is incredibly slow (a couple of 10 000 years), so I don't think this is calculated by Orbiter.
So no, the direction of the tilt does not change in reference to the global frame.

 

[Keithth G]

"to within 30 degrees" seems like a big range

The point is that the rotation does not point exactly towards Mars' periapsis. According to my simple calculation, it points at angle that is about 30 degrees off pointing directly towards periapsis. If you want an 'exact' number, you are most welcome to work through the relevant coordinate transformations and Euler angle rotations at your leisure.

Does the image i found in link below reinforce what your saying even though i think in this pic the x axis is facing upward?
http://www.nakedeyeplanets.com/mars-seasons.htm

The diagram is a bit hard to decipher - but broadly, yes: Mars' northern hemisphere winter occurs when the planet is close to perihelion. This makes the northern hemisphere relatively short in comparison with the southern hemisphere winter.

Your comment
"But then again, I could be entirely mistaken"
makes me worry. I don't want a guess.

I gave this a couple of minutes thought. My logic could be faulty. Currently working and not much inclined at the moment to give it much more thought, however.