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Thread: Appearance of sun on Mars as compared to its appearance on Earth.

  1. #1
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    Appearance of sun on Mars as compared to its appearance on Earth.

    Hello,
    I'm new to this forum, just joined, but old to this world.
    On Mars, how would the sun appear, in size, as compared to its size appearance on Earth?

    Thank you,
    Patrick

  2. #2
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    Hey, Patrick!

    Mars is about 1.5 times as far from the Sun as Earth is,
    so the Sun's angular diameter on Mars is 1/1.5, or 2/3
    what it is on Earth. 2/3rds the angular diameter means
    4/9ths (a bit less than half) the angular area, so the Sun
    is 4/9ths as bright on Mars as it is on Earth.

    Next question.

    -- Jeff, in Minneapolis
    http://www.FreeMars.org/jeff/

    "I find astronomy very interesting, but I wouldn't if I thought we
    were just going to sit here and look." -- "Van Rijn"

    "The other planets? Well, they just happen to be there, but the
    point of rockets is to explore them!" -- Kai Yeves

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    There is a photo on wikipedia on a sunset on Mars:

    http://en.wikipedia.org/wiki/File:MarsSunset.jpg

    Unfortunately the scale is very hard to determine, but at least it gives you an idea of the colors that would be expected.

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    A note of thanks.

    Thank you for the information.

    Patrick

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    Quote Originally Posted by Jeff Root View Post
    Hey, Patrick!

    Mars is about 1.5 times as far from the Sun as Earth is,
    so the Sun's angular diameter on Mars is 1/1.5, or 2/3
    what it is on Earth. 2/3rds the angular diameter means
    4/9ths (a bit less than half) the angular area, so the Sun
    is 4/9ths as bright on Mars as it is on Earth.

    Next question.

    -- Jeff, in Minneapolis
    Thank you. Isn't 2/3 equal to 4/6 ?

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    Quote Originally Posted by potoole View Post
    Thank you. Isn't 2/3 equal to 4/6 ?
    Yes, but 2/3 squared is 4/9.

  7. #7
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    The area of a figure is proportional to the square of its
    linear dimensions. In this case the angular diameter of
    the Sun as seen from Mars is 2/3 of what is seen from
    Earth, and 2/3 squared = 2 squared / 3 squared = 4/9.

    The diameter is 2/3, the area is less than 1/2.

    -- Jeff, in Minneapolis
    http://www.FreeMars.org/jeff/

    "I find astronomy very interesting, but I wouldn't if I thought we
    were just going to sit here and look." -- "Van Rijn"

    "The other planets? Well, they just happen to be there, but the
    point of rockets is to explore them!" -- Kai Yeves

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    Bearing in mind that the Sun covers only 4/9ths as much of Mars' sky as it does on Earth, it is only a little bit bigger than Phobos in the sky - as could be seen when Phobos transits in front of the Sun's disk.

    See this interesting page showing transits of both Phobos and Deimos.
    http://astrobob.areavoices.com/2011/...-dime-a-dozen/

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    Quote Originally Posted by eburacum45 View Post
    Bearing in mind that the Sun covers only 4/9ths as much of Mars' sky as it does on Earth, it is only a little bit bigger than Phobos in the sky - as could be seen when Phobos transits in front of the Sun's disk.

    See this interesting page showing transits of both Phobos and Deimos.
    http://astrobob.areavoices.com/2011/...-dime-a-dozen/
    Nice!

    But looking at those photos, and using those figures, I'd say that Phobos appears less than half the angular area of the sun, from the surface of Mars. The picture looks even smaller.

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    One oddity is very noticeable in the image of the links above: a blue halo around the Sun. This is quite different than what we see here in our atmosphere. The rest of the Martian sky is not blue but a brownish color, contrary to what was originally expexcted.

    The reason for this somewhat odd color reversal seems to be found in the particle sizes in the atmosphere of Mars. There seems to be enough particles of a size that allows for red light to scatter away, and sized too large to cause blue light to scatter -- blue scattering (Rayleigh) is what takes place in our atmosphere. This kind of scattering is called selective scattering, though it may have a fancier name.

    Another quirk is that the surface brightness -- this is the amount of brightness per unit area -- of the Sun as seen from Mars is a little brighter than what we Earthlings see, oddly enough. Overall brightness is about half as much as Earth's view, but the apparent size is also about half, so the unit area brightness is still horribly bright. Also, there is much less atmosphere on Mars so sunlight is not diminished quite as much, though the difference between horribly bright and severely bright don't matter much, I suppose.
    We know time flies, we just can't see its wings.

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    I'm wondering, since Mars is smaller than Earth, its horizon, to an Earthling, would appear to be closer than Earth's horizon. Due to the smaller appearance of the sun on Mars, would that (the closer appearance of the Mars Horison) add to the 'illusion' of the sun being even smaller looking on Mars? Or, would it be the other way around?

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    Illusions of that sort, which are not caused by physical processes like reflection or refraction, are sometimes specific to the individual.

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    Would shadows look a bit different due to the smaller apparent size of the sun? I think that shadows on Mars might appear to spread out a bit more than on earth.

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    Quote Originally Posted by Clive Tester View Post
    Would shadows look a bit different due to the smaller apparent size of the sun? I think that shadows on Mars might appear to spread out a bit more than on earth.
    I would expect just the opposite - sharper edges.

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    Quote Originally Posted by potoole View Post
    I'm wondering, since Mars is smaller than Earth, its horizon, to an Earthling, would appear to be closer than Earth's horizon. Due to the smaller appearance of the sun on Mars, would that (the closer appearance of the Mars Horison) add to the 'illusion' of the sun being even smaller looking on Mars? Or, would it be the other way around?
    I would not expect any significant visual difference. True, there is somewhat more horizon dip and departure from a great circle, but it still is very slight from eye level above the ground. I don't think it would be noticed if one was not looking for it. In my case, my glasses have so much pincushion distortion that all bets are off concerning the appearance of the horizon.

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    Quote Originally Posted by Hornblower View Post
    I would expect just the opposite - sharper edges.
    That would be aesthetically very different from a desert of earth. I guess a thinner atmosphere would mean less scattered light from the sky. Thus sharper(from a smaller sun) and deeper shadows (from less back lighting): it might look like the stark and eerie lunar shadows.

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    Quote Originally Posted by Clive Tester View Post
    That would be aesthetically very different from a desert of earth. I guess a thinner atmosphere would mean less scattered light from the sky. Thus sharper(from a smaller sun) and deeper shadows (from less back lighting): it might look like the stark and eerie lunar shadows.
    That thin atmosphere is very dusty and appears to make about as bright a sky as we have on the ground here, at least in the lander images I have seen. My guess is something similar to what I saw on a sunny day in Los Angeles 50 years ago, when the smog was really heavy.

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    Quote Originally Posted by Hornblower View Post
    That thin atmosphere is very dusty and appears to make about as bright a sky as we have on the ground here, at least in the lander images I have seen. My guess is something similar to what I saw on a sunny day in Los Angeles 50 years ago, when the smog was really heavy.
    I remember some very dense fogs here on the Thames estuary 40 years ago. Luckily we donít get those any more.

    I recall seeing the images from Mars back in 1997. There was something subtly different about the vista, which struck me only once: when I saw the image for the first time. Couldnít put my finger on it, but it had to be something to do with the lighting. Maybe it was an effect of the photo, but maybe something of the true conditions on the surface that was different from a terrestrial scene.

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    Quote Originally Posted by Clive Tester View Post
    I remember some very dense fogs here on the Thames estuary 40 years ago. Luckily we don’t get those any more.
    What happened to the fog??
    We know time flies, we just can't see its wings.

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    Quote Originally Posted by George View Post
    What happened to the fog??
    The Clean Air Act removed most of the pollutants that accompanied the fogs in London and elsewhere in the UK.
    http://en.wikipedia.org/wiki/Clean_Air_Act_1956
    London remains on a river estuary, so (relatively) clean fogs still occur in that region, but thankfully no more pea-soupers.

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    Thanks for clearing-up the....fog (in my mind). I can't say how many times I've watched visitors look surprised when they see that the Alamo is not out in the cactus countryside, but in downtown San Antonio. Movies do make lasting impressions.
    We know time flies, we just can't see its wings.

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