what causes a mirage to appear?
961174
When the suns rays heat up the air above a surface and the heated air is appearing to shimmer (different index of refraction) then that can appear as a mirage (for water for example). I think some mirages (along the highway) are pure imagination and others (water in the desert) are also partly wishful thinking.
904924
I see the exact same thing.  And I thought it was only me that saw these !  You can see them outside in bright daylight if you look up at a bright sky for long enough.  They look like myriads of little white flashes moving around in all directions.  I don't think they are caused by an electrical overload in your retinas or optical nerves because I can also see them at normal light levels and even in a completely dark room.  If you stare at one spot for long enough (about 5 to 10 seconds) they will start to appear.   It even works when staring at a white computer screen (it doesn't have to be particularly bright), or even a blank sheet of paper !\n\nI have to say I don't see them unless I look for them, but when I do they are always there.  And, contrary to previous answers, they don't go away after a time, in fact they get stronger if anything.  If I don't blink and stay staring at the same spot for 1 to 2 minutes, eventually my entire field of vision is covered with these lights - they are all I can see until I move my eyes at which point they disappear completely !\n\nThey remind me of the 'static' noise that you see on a TV set that isn't tuned to any station, so they could be due to electrical noise in your optic nerves, or even in the brain itself.  I'm glad it's not only me that sees them, I thought I was starting to lose my eyesight !\n\nIncidentally, there is another phenomena that is similar to this, when you stare at a bright light in a dark room.  This effect looks like a myriad of bright streaks emanating out in straight lines in all directions away from the light source.  More like a halo than worms or static noise.  I think this is definitely something to do with the way your eye functions, but I have never seen any explanation for it.  Has anyone else experienced this ?
5896
A shadow (not always though).
160214
They also occur as a circle, if you're in a plane (i was in a microlight) above mist/cloud it's the most spectacular sight, I looked down and there was a circular rainbow. It has to do with the fact that you're viewing it from one point, all the rainbow is relative to you.
394647
That's not the earth's shadow.  That only happens during a lunar eclipse.  What you are seeing is the different phases of the moon.
570939
A mirage is something dehydrated people see, usually when they have heat stroke in the desert. Is it important in life? I'd say it's best to avoid it.
120878
http://www.grand-illusions.com/moon.htm\n\nThe Moon Illusion\n\nThe moon illusion is one of the most famous of all illusions. Stated simply, the full moon, when just above the horizon, appears much larger than when it is overhead. Yet the moon, a quarter of a million miles away from the earth, always subtends the same angle wherever it is in the sky, roughly 0.5 degrees.\n\nThe first problem is for photographers. A wonderful picture presents itself, with the full moon just rising above a spectacular horizon. Click, the picture is taken. Yet the result is disappointing. The moon seems much smaller in the photograph than it did when viewed with the naked eye. Even professional photographers fall for this one. Yet on a normal lens, 50mm on a 35mm camera, the field of view is around 50 degrees, and the width of the moon, subtending an angle of 0.5 degrees, will be 100th of the width of the photo! Many photographs that you see in magazines, containing both a moon and a landscape, will be composites. The landscape will be taken with a normal lens, the moon taken with a telephoto lens, to get a bigger image.\n\nHow does this illusion come about? Since the moon always subtends an angle of 0.5 degrees, the image on the retina must always be the same. Clearly the problem is one of interpretation. One simple experiment shows this to be so. A full moon just above the horizon will not appear so large to the human eye if a piece of paper is held up to that eye with a hole in it, so that only the moon can be seen through the hole and not the horizon. If the other eye is open at the same time, viewing both the moon and the horizon, the two eyes will each see different sized moons!\n\nThe explanation is believed to be as follows. We 'know' that a cloud that is overhead will be larger than when it moves towards the horizon. And an airplane that is a mere speck on the horizon becomes large when it is overhead. And we are all familiar with standing under a tree which seems enormous, yet at a couple of hundred paces seems insignificant. It would seem that so much of our world is interpreted this way that we are ill-equipped to cope with an object like the moon, that subtends the same angle at the eye, whatever position it occupies in the sky. And so our brain 'interprets' the image that it 'sees', and tells us that the moon is larger than it really is.\n\nHendrik Ball\n\n\n\nFrom: http://www.straightdope.com/classics/a2_110.html\n\n\nDear Cecil:\n\nI have noticed that the moon (and the sun, for that matter) appears very large just as it rises and sets. When it is overhead, it seems tiny by comparison. The color is also vastly different, but this can be easily explained by the fact that we view the moon through varying ''thicknesses'' of atmosphere as it makes its way across the sky. But this explanation seems insufficient to account for the change in size. So what's the straight dope? --Frank M., Los Angeles\n\nCecil replies:\n\nBelieve it or not, Frank, they've been arguing about this for two thousand years, and it's about time I got things straightened out. First of all, let me make it clear that the effect is an optical illusion. If you measure the moon with a ruler held at arm's length (a paper clip bent into the shape of a calipers will also work), you'll find it's always the same size no matter where it happens to be in the sky. If anything, the moon is slightly smaller at the horizon than it is at the zenith, mainly because it's 4,000 miles (the radius of the earth) farther away. (If you don't understand why this is so, draw yourself a picture.) Nonetheless, most people are convinced that, area-wise, the moon's at least twice as big when it's near the horizon as when it's overhead.\n\nNumerous theories have been advanced to explain the ""moon illusion.'' At one point people thought it had something to do with the angle at which you hold you
1051141
Atmospheric interference.
946890
I had the one similar "sighting" but it was a shadow, a shadow of a man that disappeared right before my eyes.  I was not asleep nor dreaming .  I have all my senses intact.  I was 9 years old then.  And i can never forget that incident.  Up to now I too can't explain what happened.  Perhaps children have more 6th sense than adults.  As the boy in one of a blockbuster movie said: "  I can see ghosts."
126277
As previously stated, starts twinkle due to the atmoshpere of the earth. If you look at a distant star from outside the atmosphere it will be a far less dynamic. I'm not sure of the origins of the pentagonal shape but it was probably due to religous/social explanations from the past (Pictures of the three wise men following the pentagonal shaped north star come to mind)
616574
mirage?  Like the desert hallucination?  A few moments.
239616
two factors, diffraction being one.  The other is that you rarely have a a small source.  The sun appears small, but is significantly larger than the Earth.  Therefore, on any edge shadow, you have light rays coming from both sides of the sun as well as the middle.  Since their trajectories are not parallel, the edge gets fuzzier farther away from the shadow because the rays are able to spread out more once they're past the actual edge casting the shadow.
17956
The most likely way is to have some sort of atmospheric distortion.  For example followin a volcanic eruption in Mexico, the sun appeared to be green due to the amount of dust and ash that was put up into the air.
76527
UFO???  I interpret this to be anything buzzing in the air that I have no clue what the heck it was,,, but this happens mostly just at night.. during tha daylight hours I can see things more clearly.\n\nDo I think aliens exhist??..The based on a true story movie "Fire in the sky" was pretty darn convincing
148010
There are many possible physical causes for the diffraction.  Eyes half shut: your eyelashes.  Driving: windshield wiper streaks (visible streaks are at right angles to wiping streaks).  Possibly cataracts or scarring from laser eye surgery.
960638
I've never experienced a phenomenon that could not be related to some outside perturbation\n\nAs for the human retina, it has about 125 million rod photoreceptor cells (plus about 6 million cone cells, used mostly at night).  On a surface roughly 1/4th of that of a full format digital SLR today, the retina has about 10x more "pixels", i.e. a density 40x higher (and our own "pixels" don't interfere w eachother, unlike those of cameras ;-)\n\nGiven that from even as low as 10 megapixels is is pretty much impossible to see any on a postcard size print, I'd be very surprised if anyone could identify individual megapixels in the case of our own 125 megapixel camera ;-)\n\nhope this helps\n\na
8264
It's because of the minor imperfections in the placement of the mirrors. Also, because the effect can't be seen without seeing it at an angle, it causes a curve based on your viewpoint.