This weekend will be a good time to look for the earthshine on the moon’s unilluminated face. On 21 May, look to the west about 21.00 BST, near the time of sunset (but of course never look at the sun directly). The moon will be a thin crescent with less than 5% of its visible surface illuminated by the sun.
As the sky darkens, the moon’s unilluminated portion will also glow faintly. This is the earthshine, caused by sunlight reflecting off our planet on to the moon. Seen from the moon, Earth is 50 times brighter than the full moon seen from Earth, meaning the lunar nearside never experiences a truly dark night. Earthshine was first explained by Leonardo da Vinci in the 16th century. The phenomenon is also called the ashen glow, or the poetic “old moon in the arms of the new”.
Some years ago, it was thought earthshine was at its brightest in northern spring but subsequent research has not supported this conclusion. Nevertheless, studying the brightness of earthshine, which depends on Earth’s cloud cover, is being used as a way of searching for changes in the atmosphere.
