At either sunrise or sunset, when the sun is no more than 6 degrees below the horizon. Under good atmospheric conditions, the horizon should be clearly defined, and the brightest stars should be seen (i.e. no moonlight, or other lights). It is still necessary to be able to engage in normal outdoor activities. The time between sunrise and sunset when the sun is between 6 and 12 degrees below the horizon. Without artificial light, the horizon is strongly defined, and the outline of things may be discernible. Ordinary outdoor activities are not viable without additional lighting at this time.
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