Lighting : Lighting June 2014 - Vol 34 Issue 3
46 LIGHTING MAGAZINE | June/July 2014 June/July 2014 | LIGHTING MAGAZINE 47 DISCUSSION OF THE MEASUREMENTS Illuminance The moonlight illuminances shown in Figures 6 to 8 support the statement “Moonlight illuminance varies according to phase, altitude and atmospheric extinction”6 . The moonlight horizontal illuminance at full moon, when the moon is at the zenith and the moon is at mean orbital distance, is stated6 to be 0.27 lux (marked on Figure 6). Figure 9 shows curves for horizontal illuminance from moonlight6. E refers to Elongation where Elongation, with respect to the moon, is the angle from an axis drawn between the centre of the sun and passing through the centre of Earth and an axis from the centre of Earth and the centre of the moon. E=0̊ when the moon is between the sun and Earth and directly aligned, that is, new moon. The opposite is E=180̊ which is full moon, when the Sun and Earth are in direct alignment and the moon is on the opposite side of Earth (refer Figure 4). The moon is in quadrature when E=90̊, that is, first and third Quarters. Bunning and Moser7 claim that “with the altitude of the moon at 60̊, the intensity may reach 0.7 lux. With the moon at still higher altitudes in tropical or subtropical regions, the maximum intensity may reach 1 lux.” This has been cited in significant environmental assessments for projects in Western Australia. Boyce11 also states that moonlight illuminance is in the range 0.5 to 1.0 lux. Measurements taken at Darwin and Woy Woy (Table 2) indicate no support for these values, however, there would seem to be strong evidence to support the 0.27 lux value6. Taking the mean distance of the moon to be 385,000 km and applying the inverse square law would indicate that, based on distance only, the illuminance from the full moon at the zenith on a clear night may vary between 0.32 and 0.24 lux. The precession movements of the lunar orbit referred to earlier could further marginally increase or decrease those values. Seidelmann6 states that at half moon, the horizontal illuminance is 0.11 of that of the full moon. Our measurements indicate at half moon the fraction is between 0.17 and 0.2 of that of the full moon for a particular lunar cycle, however, with margin for error at the second decimal that fraction may be smaller. Lunar spectral power distribution Referring to Figure 8, the effects on the solar curves are due diffraction (more bluish) at twilight and to atmospheric path length at sunrise (more reddish) and atmospheric conditions at the time. Since moonlight is reflected sunlight, its spectral power distribution is similar to that of the high altitude sun (more bluish) together with the atmospheric conditions at the time. PROBABILITY OF RECEIVING MOONLIGHT The probability of uninterrupted moonlight at a particular location varies with location and time of the year. Assuming the average probabilities are similar both night and day, then annually Sydney has clear conditions 28.6 percent of the time and of the 71.4 percent duration cloud cover, half of that time the cloud cover ≥75 percent. The most cloud cover occurs in February (80.4 percent of the time) and the least in August (57.5 percent) with approximately half of those durations providing ≥75 percent cloud cover10. November to March is an important period in north-west Western Australia for marine turtle nesting. Broome (WA) has an annual average probability of a clear day of 49.9 percent with 50.1 percent duration cloud in the sky and of that approximately 40 percent of that time there is ≥75 percent cloud cover. The most cloud cover occurs Figure 8. Relative spectral characteristics of natural light at at Woy Woy, NSW, on 29th July 2012. Figure 9. Ground illumination from various sources. The position of the moon relative to Earth changes constantly in a cyclical manner with a number of perturbations influencing the regular elliptical orbit of the moon. Since the same surface of the moon is always oriented towards Earth, the albedo remains relatively constant.
Lighting April 2014 - Vol 34 Issue 2
Lighting August 2014 - Vol34 Issue 4