(1) From New Scientist The Last Word Science Questions and Answers:

The moonies

Question
On 23 December 1995, around eight o'clock on a clear frosty evening, we were looking out at a gibbous Moon and noticed that the full disc of the Moon could be seen faintly. Most of it looked like a coin, dark grey, with one brightly lit edge. Usually, only the illuminated part of a gibbous Moon is visible. What made the shadowed side visible on this night, but not on others with similar sky conditions and with the same amount of background light from the countryside?

EDWARD RZEMPLUCK , Simon Frazer University British Columbia Canada

Answers
I am afraid that the questioner has his astronomical terms mixed up. A gibbous Moon is one that is between half and full. What he saw was a very new crescent Moon--one between new and half (the new Moon itself having occurred only the day before).

The explanation for the faint illumination of the rest of the Moon's disc is very simple. If one thinks about the relative positions of the Earth and the Moon when the Moon is new, one can work out that the Earth is full (when viewed from the Moon) when the Moon is new (when viewed from the Earth). The full Earth in the Moon's sky is a very bright object, being both larger and of a higher albedo than the full Moon, and the faint illumination of the rest of the crescent Moon's disc is simply due to the earthlight being reflected back to Earthbound observers.

This phenomenon is most easily viewed when the Moon is a very thin crescent because the Earth is then at its brightest in the Moon's sky and because the relatively faint illumination of the dark part of the disc is easily drowned by moonlight when the Moon is older.

Stewart Lloyd , Brigg South Humberside   

This unusual appearance of the part of the Moon which is not illuminated by the Sun can only be caused by an unusually high albedo of the part of Earth which is still in daylight, while its reflected sunlight strikes the Moon.

This implies dense layers of cloud not far beyond the observer's western horizon. For sailors, this could well be a sign of an approaching cyclone.

This phenomenon has, from early times, forecast disaster (a safe bet, because disasters always occur, even without the aid of omens). A particular example appears in the Ballad of Sir Patrick Spens:

I saw the new moon late yestere'en With the auld moon in her arm And if we put to sea this night I fear we'll come to harm

John Woodgate , Rayleigh Essex   

The eastern hemisphere has much more land surface than the western hemisphere, so the effect is actually more noticeable in the days before the new Moon. Seawater has a much lower reflection index than land does.

Knowing this and measuring the intensity of the earthshine, ancient astronomers had figured out that somewhere in the southern seas a great landmass ought to be present. This land was, in due course, discovered and named Australia.

Richard Keijzer , Hilversum The Netherlands

(2) From Astronomy 2:

Man and the Moon (1997)

Astronomers have long been puzzled by the formation of the Moon. It has now (1999) thought to have a very small iron core and has a correspondingly lower density than the Earth. It is also a large body relative to the size of its parent. These unusual characteristics have led to speculation that a large wayward body hit the Earth off-centre soon after its formation, resulting in vast quantities of Earth material to vaporise and settle in a surrounding orbit. Recently a team of American and Japanese astronomers, using simulations with the help of a powerful computer, have concluded that a body three times the size of Mars impacting on the early Earth could have produced the right amount of material. Most of the debris fell back to Earth but the remainder very quickly accreted into the Moon. The Moon has had an important influence on human beings over scores of centuries and is also the main cause of ocean tides. Its romance is expressed in so much poetry and it is a beautiful object in the sky. Its mountains, craters and plains can easily be seen through quite small binoculars. It always displays the same side towards Earth (59% can be seen at one time or another) and it was only 30 years ago when photographs of the other side became available to human kind. It is sometimes possible to see the dark part of the near-new moon. What actually happens is that the Sun's reflection from the daytime Earth bounces back to the Moon and it is called earthshine. It is a very strange sight and can normally be seen in winter time only just before dawn on a waning Moon a day or so before new Moon or just after dusk on a waxing Moon a day or two after new Moon. This effect was sometimes known by country people as the 'stork' (first quoted in 1750). Also known as 'the old moon in the new moon's arms' (Sir Patrick Spens 17??), and 'I see the old Moon in her lap' (Coleridge 1802).