by dpiddy » Wed Sep 27, 2006 11:56 pm
"How come you can only see earth? Where are the rest of the stars?"
A. "rest of the stars?" Earth is not a star so "where are the stars?" would be more appropriate.
B. There's at least one other point source of light in that image. Not sure what it is. Might be a moon of Saturn.
C. Stand near a bright light in the middle of the night and look up at the sky. Note how many stars you see. It won't be many. Now move away from the light so that you're in darkness for a couple minutes and look up at the sky again. You'll see many more stars. This explains the lack of stars in the Saturn/Earth image as well as the lack of stars in images from the moon. When you have a very bright object in the same field as a very dim object, your iris will shrink so the bright object doesn't temporarily blind you. This prevents you from noticing the faint object. A camera does the same thing (but with its shutter and/or f stop). Cassini could've been commanded to take a longer exposure so that stars were visible but the light from Saturn would likely overwhelm the photosensors of the camera causing either a washed-out image and/or "blooming" (white streaks caused by the camera's photosites being forced to bleed electrons into neighboring photosites).
D. Images like these are processed by humans to reach a desired effect. It's possible that stars were visible in the images but that the image was processed such that the specific objects of interest are highlighted (Saturn's rings and the dot of Earth), thereby leaving other captured objects no longer visible. But, with such a bright object nearby (Saturn, along with its main rings), the exposure was likely very short and few, if any, stars were captured in the first place.
Best.
"How come you can only see earth? Where are the rest of the stars?"
A. "rest of the stars?" Earth is not a star so "where are the stars?" would be more appropriate. :)
B. There's at least one other point source of light in that image. Not sure what it is. Might be a moon of Saturn.
C. Stand near a bright light in the middle of the night and look up at the sky. Note how many stars you see. It won't be many. Now move away from the light so that you're in darkness for a couple minutes and look up at the sky again. You'll see many more stars. This explains the lack of stars in the Saturn/Earth image as well as the lack of stars in images from the moon. When you have a very bright object in the same field as a very dim object, your iris will shrink so the bright object doesn't temporarily blind you. This prevents you from noticing the faint object. A camera does the same thing (but with its shutter and/or f stop). Cassini could've been commanded to take a longer exposure so that stars were visible but the light from Saturn would likely overwhelm the photosensors of the camera causing either a washed-out image and/or "blooming" (white streaks caused by the camera's photosites being forced to bleed electrons into neighboring photosites).
D. Images like these are processed by humans to reach a desired effect. It's possible that stars were visible in the images but that the image was processed such that the specific objects of interest are highlighted (Saturn's rings and the dot of Earth), thereby leaving other captured objects no longer visible. But, with such a bright object nearby (Saturn, along with its main rings), the exposure was likely very short and few, if any, stars were captured in the first place.
Best.