Astronomically, sunrise occurs for only an instant: the moment at which the upper limb of the Sun appears tangent to the horizon. However, the term sunrise commonly refers to periods of time both before and after this point. Twilight, the period in the morning during which the sky is brightening, but the Sun is not yet visible. The beginning of morning twilight is called astronomical dawn. The period after the Sun rises during which striking colors and atmospheric effects are still seen.