MoonWatch
July 1998
The north pole region of the Moon. The bright rim of the crater Peary can be seen on the extreme limb.
CCD image 9 March 1998 Peter Grego
NASA's announcement in March of the discovery of vast quantities of lunar ice mixed with the rubble and soil on the floors of perpetually shadow-filled craters at the Moon's poles, continues centuries of speculation as to whether our satellite possesses water.
It has long been known the highest peaks and the rims of some prominent craters at the Moon's poles are continually illuminated bythe Sun, and also that any deep craters at the Moon's poles never receive direct sunlight. Of the peaks at the Moon's south pole, the British lunar observer Percy Wilkins (1896-1960) wrote "so lofty are these mountains that their tops know no night, and they may be called the Mountains of Eternal Light". Equally, those deep shadow-filled craters that are thought to harbour lunar ice may be called "Polar Pits of Perpetual Darkness".
The slow rocking phenomenon of the Moon known as libration regularly tilts the Moon's poles slightly towards the Earth, so that some of the craters containing the ice discovered by Lunar Prospector may be seen near the Moon's limb. Though these craters are very foreshortened, it is a fascinating, even thrilling exercise to identify them through the telescope eyepiece, for there is no doubt these regions will attain a tremendous importance when humans revisit the Moon in the decades to come.
During the first week of July, as the Moon waxes from first quarter to gibbous, its south pole wili be tilted very slightly towards the observer. From July 15 to 19, as the Moon wanes from gibbous to crescent phase, the north polar region will be presented very well - an ideal opportunity to glimpse some of these vast natural lunar deep freezers. A 60 mm refractor will reveal all of the main north polar craters including Peary, Byrd and Nansen, each named after a famous terrestrial polar explorer (the south pole contains the large craters Scott and Amundsen). Close scrutiny at high powers will take you right up to the rims of these "Polar Pits of Perpetual Darkness", and the view through a 150 mm reflector on a night of good seeing can be quite breathtaking.
