"Everyone is a moon," said Mark Twain, "and has a dark side which he never shows to anybody."
His metaphor recalls the fact that the moon rotates on its axis at a rate that exactly corresponds to its period of orbit around the earth, and in the same direction; consequently, the moon always turns the same face towards the earth, and we never see the other side of it.
As it happens, we see a little more than half. Variations in the moon's orbit, and in the tilt of its axis relative to the Earth, make the lunar sphere appear to oscillate in a period which nearly matches that of its revolution.
This apparent oscillation, or libration, , amounts to six or seven degrees on either side of the mean position, so three-fifths or thereabouts of the lunar surface comes into our field of view at one time or another; the remaining two-fifths are forever hidden from our sight.
Indeed one of the highlights of the early lunar missions was our first glimpse by photograph of the moon's dark side.
But last evening, if the sky in your vicinity was clear enough, a casual glance at where the moon ought to have been would have revealed no moon at all.
Shortly before 7 p.m. the full moon began to pass into the shadow of the Earth, and from 8 p.m. to 9 p.m. it was almost totally invisible. Its front side was a dark side too, so to speak.
Eclipses of the moon are very easy to observe. They are visible from anywhere on the dark side of the world if the moon happens to be above the horizon at the time, unlike a solar eclipse which can be observed only along a relatively short and well-defined track. There is no simple way of predicting the next eclipse of the sun or moon from the one immediately before, but there is a cycle lasting 18 years and 10 days over which the pattern of eclipses repeats itself exactly.
There are two periods each year when eclipses are possible, and these "eclipse seasons" slip backwards by about three weeks per calendar year.
In 1995, for example, the eclipse seasons were in April and October; in 1998 they were in February and August; and by now they have edged backwards to the time around midsummer and New Year.
A second eclipse - this time of the sun but not visible in Ireland - will occur on June 21st this year; a second lunar eclipse will take place on July 5th; and the last eclipse of 2001, again of the sun, will occur on December 14th.