Lighting : Lighting June 2014 - Vol 34 Issue 3
40 LIGHTING MAGAZINE | June/July 2014 June/July 2014 | LIGHTING MAGAZINE 41 TECHNICAL FEATURE Moonlight as a reference for environmental assessment By Tim Shotbolt MBdgSc and Ian Cowling PhD INTRODUCTION The moon and the lunar cycle has formed the basis of ancient calendars and featured in many mythologies across the world, for example, in determining Easter and Chinese New Year. The word lunatic implies that the moon causes insanity. The words moon and month have common origins. Moonlight is important for animals at night, allowing useful vision for some diurnal animals as well as good vision for the nocturnal ones. This paper reports on research on the illuminances that can be expected from moonlight. Measurements were taken and compared with calculated values and those published in the literature. The research suggests that some sources overstate the value moonlight illuminance. SOME FACTS AND FIGURES All of the planets orbit the sun in approximately the same plane, the ecliptic. The Earth orbits at approximately 29.8 km/s, taking a year to complete one, slightly elliptical Figure 1. The Earth’s and the moon’s orbits, phase and elongation and inclination of moon’s orbital plane to ecliptic plane1, 6 . Figure 2. Lunar phases (as seen in the southern hemisphere). orbit. The axis of rotation of the Earth is at 23.4̊ to the ecliptic, giving rise to the seasons. The moon orbits the earth at approximately 1 km/s and is elliptical: its greatest distance away (apogee) is 406,700 km, whereas, the shortest (perigee) is 356,400 km1 (see Figure 1). The plane of the moon’s orbit is inclined 5.1̊ to the ecliptic. The moon rotates anti-clockwise on its axis, with one rotation being equal to the time one orbit (~28 days) of the Earth; therefore, the same side of the moon is always seen from Earth. The rotation of the Earth about its inclined axis results in the observer’s view of the moon changing during the course of an evening. As can be seen from Figure 1, the moon travels a sinusoidal path with respect to the path of Earth’s orbit. Moonlight is reflected sunlight and shape of the illuminated area (phase) has a regular cycle from new moon waxing through to full moon and waning to new moon (see Figure 2). The waxing crescent moon will set in the west early in the evening, whereas the waning crescent moon only rises in the east before dawn. The moon appears larger near the horizon and smaller when closer to the zenith. This is an illusion, since its angular size is the same at all elevations. The lunar cycle (the time for one orbit of Earth by the moon) depends on the reference point defining the beginning and end of one orbit. A sidereal month is the time taken for the moon to cross the same international celestial reference frame and is 27d 7h 43m 1.5s. The more common experience is the synodic The waxing crescent moon will set in the west early in the evening, whereas the waning crescent moon only rises in the east before dawn. The moon appears larger near the horizon and smaller when closer to the zenith. This is an illusion, since its angular size is the same at all elevations.
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