For the same reason that the same thing happens everywhere else on Earth . . .
sometimes the sun is up, and the rest of the time, the sun is down.
That phenomenon is believed to arise as a consequence of the fact that sometimes the sun is up in the sky, whereas during the rest of the time, the sun is down below the horizon. The observed statistical correlation with the periods of light and dark respectively is uncanny.
There is no sunrise at the circle about June 21 and no sunset at the circle about December 21.
blus or black
sometimes
the bone
in the light, sometimes. it's horrible at night or in the dark
Chiaroscuro
Seeds and bulbs sometimes requires to be kept in the dark, but plants always need sunlight
Because the dark reactions are CO2 assimilation's and do not need light energy. They use the ATP and NADPH created from the light reactions to convert CO2 to carbohydrates, so this can happen in the "dark" as long as the light reactions already happened.
perfectly normal. Mine are doing the same thing. but one nipple stays dark while the other keeps changing from dark to light.
Because it depends on the season. For example, it's lighter in the summer that in the winter. ;)
brown sometimes it could be light,dark,and even bright and colorful