Sunrise and Sunset
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Kitharode
moderator
I'm sure we were told (but I can't find where) that in the HiRise images we have South at the top. Let's hope so. That being the case, would anybody like to tell me which side of the images will be the sunrise side? I've got my own answer, but I'm not convinced I've worked it out right. Cheers.
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p.titchin
in response to Kitharode's comment.
Please, someone tell us, I've made my bets for a year, but, you boffins should tell us!
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michaelaye
scientist
I'd say about 90 - 95 % of all images are taken during the downward path of the orbiter and as the images have been uploaded 'unprojected' to ground coordinates, they come out the way the camera records them on the respective path. This means that these 90-95 % of the images have south-ish on their top. We say south-ish, because the orbit does not cross the pole exactly but is offset, but that error is negligible in approximate statements (but we of course take it into account in solar irradiation calculations). 5-10 % of the images are taken at the upward path when the orbiter has crossed the pole, but that's only rarely possible due to lightning constraints, therefore only 5-10%. Hope that helps.
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michaelaye
scientist
Oh, one important part of this explanations is of course that so far we only have uploaded south pole images. The argument would reverse for the north pole images that we plan to upload once we have finished the south. All you major submitters amaze me, some of you guys have more than 10,000 submissions. Even if you would have used that only to look at pretty pictures, that shows an amazing effort on your side! 😉
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wassock
moderator
Michaelaye, at risk of appearing picky, you haven't actually answered Kitharodes question
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Kitharode
moderator
Thanks wassock. 😉
The question is; does the sun rise on the left of the image, or does it rise on the right of the image?
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michaelaye
scientist
Oh sorry, somehow thought the south orientation was the question. Answer: If South is on top-ish, the sun came up left-ish, as that's where East is then. (As I said, because MRO is not crossing the pole perfectly, i.e. there are remaining angles involved, hence the '-ish' reduction of precision in my statement).
See also here: http://curious.astro.cornell.edu/question.php?number=736 (I.e. Also on Mars, the sun always rises from East).
Have you guys ever played with the free program Celestia? I believe it contains MRO flight data as well, so one could zoom in and examine exactly how MRO is orbiting Mars).
Why are you interested in the sunrise side, if I may ask?Posted
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michaelaye
scientist
Looks like Celestia website is down. Also a very cool web application comes directly from JPL.
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Kitharode
moderator
in response to michaelaye's comment.
Many thanks. Lots of interesting and useful info there. And an answer to the question too. 😉
Yes, we've talked about the MRO orbit and it's 'tilt'. There's a video somewhere in the 'Science of the HiRise Camera' discussion showing us how progression/recession works. Cool stuff. It also explains 'sun-synchronous orbit' really well.
No specific reason for my 'sunny side up' enquiry. Just wanted to confirm that I'd worked it out correctly for myself, with my plastic martian and football Mars globe. Talk about getting in a spin. 😃
At the moment I'm still building my 'big picture' of the south polar regions, in case I get asked questions by some school children and such. Also (in more scientific mood) I'm unable to grasp the details of a situation unless I can 'see' the whole scenario. So I've been collecting things like extent and size of icelayer, maps of SPLD's, Crytic Region, spider latitude zone, etc. I want to get a feel for what it's like 'to be there'. What does the Sun look like? How long is it above the horizon at Ls = x degrees? How would it feel to be inside 'Inca City'? That sort of stuff. Bit silly perhaps, but it works for me.
Like I said, anything that helps with the bigger picture. Also means I don't have to be a 'proper' scientist. Hahaha ... 😛
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Kitharode
moderator
I've just discovered a reason why I might need to know the sunrise side, but before all that (elsewhere) I need help with this:
Hirise image, south at the top. We're at, say, 80 deg S. The Sun has just begun to return to this latitude after winter. The Sun rises in the East (left of image) travels along a small arc and reaches its greatest elevation at midday, before setting in the West. My question is;
If I am facing South does the Sun rise and shine on my back, or does it rise and shine in my face? Put another way, when the sun rises in the left of an image, is the Sun at greatest elevation at the top of the image (south) or at the bottom (north). HELP !! 😛
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p.titchin
in response to Kitharode's comment.
Hi Kith! It depends on the sun' s declination (celestial latitude) and your martian latitude. Thinking simply, the equator of a celestial body is the line of latitude where the sun is overhead at noon. (noon being the time at which the sun reaches it's highest point over the horizon), so, standing at any point to the north ( Northerly latitudes) the sun will be to the south of you,and standing to the south (southerly latitudes), the sun will be to the north. thus at 80,south, if you face the pole, the sun will be on your back. The exact 'noon' declination of the sun at a given latitude varies with the precession of the celestial body you are standing on, and this will give you the 'seasons' . I hope this makes sense! 😃
Seriously, (well, almost!) have a quick look at the Wikepedia entry on 'declination' . It gives quite a good simple presentation on the celestial sphere and 'precession' that may answer your questions.Posted
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Kitharode
moderator
Thanks Pete. That'll do nicely. Yes, I had the sun on my back too when facing south, but second opinions are always good. 😉
Right Ascension and Declination (RA & Dec) play their part to be sure, as does latitude, but I don't see how precession comes into it for my scenario. Obliquity causes seasons, precession is the slow rotation of the tilted axis (26.000yrs for Earth). Whatever, your logic is good and we've got the right answer I'm sure. Which means I can go investigate walls !!?!! Cheers. 😃
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Kitharode
moderator
Ok then, how does this sound? Same scenarios as before, but instead of me being at latitude -85 deg we've got a wall. It's like a wall from Inca City with sloping sides and a ridge. It stands alone and is aligned directly East to West. Therefore it presents a north-facing slope and a south-facing slope.
The Sun has just returned to this latitude and as it makes its small arc over the northern horizon the sunlight falls on the north-face slope of the wall. As time passes the Sun rises more easterly and rises higher in the sky over the northern horizon so that the south-face receives some sunlight in the early morning and late evening, but again most if not all of the solar radiation falls on the north-face slope.
When we reach high summer the Sun rises so high that it does not set below the southern horizon, so both faces are in sunlight for the same amount of time. But once again the north-face slope receives more sunlight because of the higher elevation of the Sun to the north.
Which all adds up, on the back of my envelope, to this; Over the course of a martian year, a north-facing slope will receive significantly more solar radiation than a south-facing slope. What say you? 😉
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p.titchin
in response to Kitharode's comment.
Your musings seem ok to me, but unless the the ridges are fairly acute ( perhaps toblerone shaped), the effect would not be large, as the downslope would be illuminated not long after sunrise. I mused on, and thought you were maybe thinking that North facing slopes might therefore warm more than the south facing slopes. I think this could be so, as viniculturists on Earth can't be wrong! 😃 May be the larger effect will be from the fact that the sunlight will be striking the North faces at an angle nearer to the normal than on the south facing slopes and reflect less. I don't know how big the effect might be, but expect to find south polar vineyards on the North facing slopes! 😃
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Kitharode
moderator
Vineyards on Mars - Now there's an image!! No good to me though, I don't drink. 😦
Yes, sunlight at angle nearer normal on north face. Also, I think, north face in sunlight for longer over the year. I'm not sure yet just where I'm going with this, but 'warmer/cooler' sides may be involved. Also, 'heat' being transmitted through the wall from north side through to south side is in my mind.
It all comes about from my observation (gut-feeling) that some areas of fans/spiders seem to occur in 'shaded' places, hence my need to clarify which side is what when I look at these objects. Some of these shaded areas are in the concave side of wave-like structures, or alcoves, or shaded sides of craters, etc. Now that I'm happy with the sunrise/sunset situation I can look more closly at these areas to see if there is any merit in the idea of 'shaded fans/spiders'.
Thanks for helping out. As my mate used to say; Two heads are better than one, even if ones a sheep! (That'd be me) 😃
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wassock
moderator
Kith have a look at the grey scale jpg image for "the heart" - toward bottom end of that theres a big obvious crater. One side of it is clearly in fairly deep shade but boasts fans and spiders. Lots of images so you can see it in all seasons if you shop around, you may need to use the hirise viewer to see the detail. Think there may be some discussion of this in the first heart thread
http://hirise.lpl.arizona.edu/ESP_011460_0980
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Kitharode
moderator
Nice one. Thanks wassock. Which is the 'first' heart thread?
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