(2) Significant reflected cultural lighting (such as the illumination caused by the reflection of a major metropolitan area’s lighting reflecting off a cloud ceiling), or (3) Limited cultural lighting combined with a high level of natural reflectivity of celestial illumination, such as that provided by a surface covered by snow or a desert surface. 2. Low lighting conditions are those that do not meet the high lighting conditions requirements. 3. Some areas may be considered a high lighting environment only in specific circumstances. For example, some surfaces, such as a forest with limited cultural lighting, normally have little reflectivity, requiring dependence on significant moonlight to achieve a high lighting condition. However, when that same forest is covered with snow, its reflectivity may support a high lighting condition based only on starlight. Similarly, a desolate area, with little cultural lighting, such as a desert, may have such inherent natural reflectivity that it may be considered a high lighting conditions area regardless of season, provided the cloud cover does not prevent starlight from being reflected from the surface. Other surfaces, such as areas of open water, may never have enough reflectivity or cultural lighting to ever be character- ized as a high lighting area. 4. Through the accumulation of night flying experience in a particular area, the operator will develop the ability to determine, prior to departure, which areas can be considered supporting high or low lighting conditions. Without that operational experi- ence, low lighting considerations should be applied by operators for both pre − flight planning and operations until high lighting conditions are observed or determined to be regularly available. (a) Horizon. Wherever one is located on or near the Earth’s surface, the Earth is perceived as essentially flat and, therefore, as a plane. If there are no visual obstructions, the apparent intersection of the sky with the Earth’s (plane) surface is the horizon, which appears as a circle centered at the observer. For rise/set computations, the observer’s eye is consid- ered to be on the surface of the Earth, so that the 2. Title 14 of the Code of Federal Regulations applies these concepts and definitions in addressing the definition of night (Section 1.1), the requirement for aircraft lighting (Section 91.209) and pilot recency of night experience (Section 61.67). (g) Civil twilight is defined to begin in the morning, and to end in the evening when the center of the Sun is geometrically 6 degrees below the horizon. This  is the limit at which twilight illumination is sufficient, under good weather conditions, for terrestrial objects to be clearly distinguished. (f) Twilight. Before sunrise and again after sunset there are intervals of time, known as “twilight,” during which there is natural light provided by the upper atmosphere, which does receive direct sunlight and reflects part of it toward the Earth’s surface. (e) Transit. The transit time of a celestial body refers to the instant that its center crosses an imaginary line in the sky  −  the observer’s meridian  − running from north to south. (d) Moonrise and moonset times are com- puted for exactly the same circumstances as for sunrise and sunset. However, moonrise and moonset may occur at any time during a 24 hour period and, consequently, it is often possible for the Moon to be seen during daylight, and to have moonless nights. It is also possible that a moonrise or moonset does not occur relative to a specific place on a given date. (c) Sunrise and sunset refer to the times when the upper edge of the disk of the Sun is on the horizon, considered unobstructed relative to the location of interest. Atmospheric conditions are assumed to be average, and the location is in a level region on the Earth’s surface. (b) Rise, Set. During the course of a day the Earth rotates once on its axis causing the phenomena of rising and setting. All celestial bodies, the Sun, Moon, stars and planets, seem to appear in the sky at the horizon to the East of any particular place, then to cross the sky and again disappear at the horizon to the West. Because the Sun and Moon appear as circular disks and not as points of light, a definition of rise or set must be very specific, because not all of either body is seen to rise or set at once. horizon is geometrically exactly 90 degrees from the local vertical direction. 8/15/19 AIM  Special Operations 10 − 2 − 8 b. Astronomical Definitions and Background Information for Night Operations 1. Definitions