Roheluspindu

Roheluspindu is a term used in Estonian to describe a phenomenon related to the visual perception of the sky. It refers to a green flash, which is a brief optical illusion that can be observed shortly after sunset or before sunrise. The green flash occurs when the sun is very low on the horizon, and it appears as a green spot or ray above the upper limb of the sun.

This atmospheric optical phenomenon is caused by the refraction of sunlight in the Earth's atmosphere. When sunlight passes through the atmosphere, it is bent, or refracted. Different colors of light are refracted at slightly different angles. Blue and violet light are refracted more than red and orange light. As the sun sets, its light passes through a greater thickness of atmosphere. The lower, redder parts of the sun's disk disappear first, while the upper, bluer parts remain visible for a fraction of a second longer.

For a green flash to be observed, several conditions need to be met. A clear, unobstructed horizon is essential, typically over the sea or a large flat plain. The atmosphere should also be relatively free of pollution and haze, which can scatter the light and prevent the flash from being seen. While the green flash is the most common, other colors, such as blue or violet, can also be seen, though they are less frequent and often harder to distinguish. The duration of the green flash is typically only a second or two.