Phase of the Moon and Phases on the Earth

October 03, 2014

Moon phases

: Till Credner; Till's Web site; Progymnasium Rosenfeld
Summary Author: Till Credner; Progymnasium Rosenfeld

Shown above (left) is the young crescent Moon, as photographed on May 10, 2008, hovering above a rock formation in southern Germany. Do you notice the similar "phase" of the rocks in the foreground? What can we learn from this? If you look at the Moon during the day and observe sunlit objects on the Earth in the same viewing direction, you might notice that the direction of illumination is the same. Furthermore, the angle of incidence seems to be similar: So the lunar phase is nearly the same as the "phase" of the rocks in the above image. This is because the light on the Moon and on the rock comes from the same direction -- the Sun.

Just take a white ball, hold it next to the Moon and compare the sunlit areas. This little experiment is a strong hint that the Moon and Earth are illuminated by the same source. It also more or less explains the phase itself. In addition, it shows that Earth and Moon have the same large distance to the Sun. The Moon's motion around the Earth is quite small compared to the Sun's distance. If the Moon were much further away from Earth, its phase would be fuller.

By the way, which of the items that the students are holding best fits the Moon in terms of its shape, color and albedo? Considering the shape, it's probably the spherical ball, but considering its color and albedo, it most closely resembles the item on the stick just right of center -- a piece of cheese (mozzarella) of course. The albedo of the Moon, including even non-visible wavelengths, is approximately 0.11 -- about the same as that of Mars. So, it's brightness is closer to the gray wall above the students than it is to the whitish rim atop the wall.

Photo details: Top - Camera Model: NIKON D200; Focal Length: 135.0mm (35mm equivalent: 202mm); Aperture: f/8.0; Exposure Time: 0.0031 s (1/320); ISO equiv: 100. Bottom - same except: Aperture: f/32.0; Exposure Time: 0.025 s (1/40); ISO equiv: 200. Photos © Copyright T. Credner & S. Kohle,