Take a look at this page.
That was good, thanks. Stack Exchange is fantastic.
Have you had calculus yet?
LOL i admit my calculus is rubbish, after so many years since highschool. That's why i'm doing such basic math now, and also looking for peer support here. Thanks for it DVDdoug, i appreciate it! This is great exercise.
Now that the day is drawing to night, and it's getting darker, i can empirically see that the 0.76 seconds is too short a time for my LED's fadeout; it seems to be above 1 second.
A capacitor discharging through a resistor is an exponential function. (Since the current is not constant, you cannot use C = I x t except for "one instance" in-time.)
That's right, it makes sense; the LED is fading, and that is exactly the reason why it's getting less and less bright; the current is dropping... and thus it's bleeding the charge in the cap slower and slower.
My amended calculation, assuming an exponential decay of current, is -10000?0.0001F*ln(1.2V/5V)=1.43s. That seems to correlate better with sense-based observations. Next, maybe i should build a tool, using Arduino, a light-dependant-resistor and *millis(), that would measure the time it takes for the LED to fade 8)