The Daytime Moon

October 20, 2009


Photographer: Mark Daly
Summary Author: Mark Daly, Jim Foster

The two photos above show the Moon during its waning crescent and waning gibbous phases as taken on September, 13 and October, 8 respectively from Galway, Ireland. Though the brightness of the daytime Moon is feeble in comparison to its nighttime counterpart, it is, nonetheless, present during daylight hours over much of its 27.3 day cycle. Under cloudless conditions and skies having relatively few aerosols, the optical thickness of the atmosphere is small, so little light is scattered (most is transmitted) and the Moon appears white. As the Moon nears the horizon, the optical thickness increases since more particles are available to scatter the shorter wavelengths of light, and thus the Moon appears yellow or orange. Note on both photos the angle of the shadows on the house roofs compared to a line perpendicular to the terminator on the Moon – they’re almost the same. Of course, the Sun is illuminating both the buildings and the Moon.