'Mak ready, mak ready, my merry men a',
Our guid ship sails the morn,'
'0 say na sae, my maister dear,
For I fear a deidly storm.

I saw the new moon late yestreen,
Wi' the auld moon in her arm,
And I fear, I fear, my maister dear,
That we will come to harm.

--Sir Partick Spens, Child Ballad #58, 18th century

Earthshine is an astronomy term, which is the dim light highlighting the shadowed portions of the moon during her slim crescent phase. It is called "earthshine" because, as Leonardo da Vinci recognized, the ghostly, faint light is caused by sunlight that's first reflected off the sunlit side of the earth and then bounced off the moon. Earthshine faintly illuminates the surface "between the horns" of the crescent moon, highlighting the shadowed side. In other words, earthshine causes the new moon to appear with the "auld moon in her arm," as described above in the 18th century balad.

This pale illumination of the darkened side of the lunar surface only occurs sometimes during a crescent moon. It was viewed as an ill omen for travelers, particularly those traveling on the sea. The moon was believed to not only rule the tides, but also govern storms.

Climate scientists, studying whatever factors may influence global temperatures, use earthshine to measure the reflectivity of the earth's surface and cloud cover. Solar physicists use earthshine to study how much solar energy reaches the cloud cover.

There is no clear correlation between the presence of earthshine, visible on the moon, and forthcoming storms.

--copyright 2006 Myth Woodling

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