antarctic circle
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wassock
moderator
As the planet wobbles I assume that the position of the polar circles will vary? Anyone have an idea what the min and Max lattitudes would be?
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Kitharode
moderator
I'm assuming you mean the precessional cycle of Mars? Also that you want min/max declinations (latitudes on the sky)?
Very tough question. This from an abstract "A major result of the present analysis is the observation that, within the plausible range of present axial precession rates, a very wide range of obliquity histories are possible, including some which involve resonance passages within the relatively recent past. As a result, obliquities as high as 51.4°, or as low as 0.2°, may have occurred within the last 10-m.y". Full extract here http://onlinelibrary.wiley.com/doi/10.1029/JB095iB09p14137/abstract
Obliquity = axial tilt, currently near 25deg. The precession cycle is 93,000 Martian years (175,000 Earth years). Resonance is gravitational influence of other bodies in certain scenarios. Hope that's useful.
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Kitharode
moderator
No, I realise now that's not what you want. You want to know the limits of permanent day/night latitudes on mars throughout a precession cycle, yes? If so I've not answered your question, but at least you have the info to work it out (I think). I'll try and get an answer together so we can compare results, unless someone has the answer to hand and can post it.
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wassock
moderator
What I meant was where the line of latitude that marks the arctic circles lies - this being the lowest latitude at which the sun never sets/rises according to season at the appropriate solstist (I think). Not a measure of the annual ice cap but an indicator of how big/small it may get over (lots of) time
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Kitharode
moderator
OK. In round figures. 90deg minus 25deg (axial tilt) = 65deg. This is the latitude that can just get 24hrs day/night (for one day/night at least). This is your arctic / antartic circle on Mars. On Earth with tilt of 24deg, arctic circle is 66deg.
At tilt = 51deg (proposed maximum) then 90 - 51 = 39 so polar day/night can reach latitude 39deg. etc, etc.
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wassock
moderator
in response to Kitharode's comment.
And which way are we going at the moment, smaller or bigger tilt?
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Kitharode
moderator
Sorry, don't know. Difficult to find the answer too. Will post as and when...
Found this; "Earth's rotational axis and the normal to the plane of its orbit (obliquity) oscillates between 22.1 and 24.5 degrees on a 41,000-year cycle. It is currently 23.44 degrees and decreasing". (Wikipedia)
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Kitharode
moderator
I've found just about everything there is to find about Mars' axial tilt - except whether its getting more or less. Maybe later...
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mschwamb
scientist, translator
in response to wassock's comment.
Mostly people have been interested in current and past Mars climate, so that's been looked at in terms of the past to compare to sedimentary layers like the polar layered deposits. This may be of interest.
Cheers,
~MegPosted
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Kitharode
moderator
Thanks Meg. Interesting indeed, without actually answering the question. 😦
But the graph at the bottom is useful. The obliquity graph, at time zero, appears to run out below the 25 degree mark, but we know that the present day tilt is near 25.2 deg so the end of the line (if seen clearly) is surely on the up. Given the hourglass shape of preceding epochs I would venture to suggest that the axial tilt of Mars is increasing and will approach (or even reach) around 30 degrees.
What do you think?
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Kitharode
moderator
Although this is for Earth, if you ignore any dates this is virtually the martian situation for permant day/night. Option 1 puts you at the south pole, Option 6 puts you on the antarctic circle. All latitudes between this circle and the pole will experience some time in 'permanent' day or night. http://astro.unl.edu/classaction/questions/coordsmotion/ca_coordsmotion_sunpaths1.html
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