Recently, while returning to Hanford from San Jose, I observed an interesting optical illusion. It was twilight, with only scattered thin cirrus in the sky. The Moon was in the waxing crescent phase, three days shy of first quarter. And as the Moon skirted a cirrus patch, it looked as if the edge of the cirrus had slipped behind the Moon.

While this could not have occurred in reality, the illusion demonstrates how the mind can sometimes "fill in the blanks" when observing something. What had happened was that there was a fairly pronounced "earthshine" effect. This occurs when the Moon is located in its orbit to allow sunlight reflected off the Earth to reflect back off the Moon, faintly illuminating the normally dark portion of the Moon's disk. This occurs during the waxing and waning crescent phases, and is known as "the old Moon in the new Moon's arms." At the same time, the diffuse edge of the cirrus patch was losing its illumination from the already set Sun, and the brightness of the cirrus edge was the same as that of the earthshine-lit part of the Moon. My mind followed the edge of the Moon's disk from the bright crescent around the faintly lit portion, and where the edge of the Moon was overlapped by the cirrus of the same brightness, inserted the missing piece of the Moon.

While interesting in its own right, this illusion demonstrates a potential problem when observing short-lived meteorological phenomena under less than ideal conditions. Observers should be careful to consider all possibilities when observing uncertain phenomena. For example, a narrow band of virga may appear similar to a high funnel cloud.

Also, don't forget to, on occasion, just watch the sky for the pleasure of doing so. It's an inexpensive form of relaxation, and, who knows, you may have your own interesting optical effects to write about.