The Moon at a phase angle of zero degrees
The Moon at a phase angle of zero degrees
Today, 12 December 2019, is the last full moon of the year. On such occasions, the Moon is perceived to be much brighter. This corresponds with reality, because sunlight illuminates the Moon’s surface perpendicularly, and is then reflected towards Earth. This means that the Sun's rays 'lose' less of their luminosity than if they were to illuminate the lunar surface at an oblique angle, which would cause more scattered light and absorption effects. In this case, the 'phase angle' – that is, the angle between the light source and the observer/camera – is zero degrees. At full moon, the Sun, Earth and Moon are in a relatively straight line. Due to the different orbit inclinations of the Earth and Moon, an exactly linear arrangement only occurs rarely and results in a lunar eclipse. In fact, the lunar disc, covered approximately three-quarters in pale highland regions and one quarter in dark, volcanic lowlands, reflects only about 12 percent of the incident sunlight. At a phase angle of zero degrees there are no visible shadows. However, it is particularly easy to make out the rays of ejecta material from two large and (geologically speaking) recent impact craters on the near side of the Moon: Copernicus (centre of image) and Tycho (lower half of image). If the Sun illuminates the Moon at an oblique angle, these rays are not visible.
