The period of time in the event the Solar is not more than six degrees under the horizon at both sunrise or sunset. The horizon needs to be clearly described as well as brightest stars need to be seen beneath excellent atmospheric situations (i.e. no moonlight, or other lights). 1 https://jejuweather11110.thekatyblog.com/26437393/helping-the-others-realize-the-advantages-of-jeju-weather