It is a month of strange moons as January draws to a close. For one thing, although it may well have totally escaped your notice, it contains two full moons; one occurred on Saturday, January 2nd, and there will be another tomorrow, January 31st.
Normally there is only one full moon a month. However, since the lunar cycle is a few days shorter than the calendar month, it happens now and then that 13 full moons are squeezed into the same year.
It used to be believed that this was a portent of extensive flooding; the worst of it would occur in the month in which the two full moons appeared. The second full moon in such a month is called a "blue moon".
As it happens, the event is not all that unusual; every third year or so there is a month with two full moons, but it does mean that tomorrow's is the last blue moon of the millennium.
Tomorrow's rising moon will have another feature: to a slight extent it will suffer an eclipse.
When the sun and the moon are aligned in such a way that Earth is directly in between it sometimes happens that Earth for a while blocks off the sunlight from the lunar surface, in an eclipse that may be total or just partial.
Occasionally in such circumstances, however, the moon may only skim through the outer fringes of the Earth's shadow, being for a time in such a position that it is illuminated by a portion of the sun, but not by all of it. Then the moon is described as being in Earth's penumbra, and it becomes just a little dimmer than normal.
As a rule, penumbral lunar eclipses are not noticed. Little change is evident to the casual observer of the moon's appearance, and so no one cares. However, tomorrow's lunar anaemia will be even less noticeable than usual because here in Ireland it will be at its maximum shortly after 4 p.m. in the afternoon; for much of the event the moon will lie unseen below the horizon.
If you are lucky, and the cloud allows, you may just notice a slight darkening of its upper limb as it begins to rise into the evening sky.
One fact alone might make you want to watch it. If you intend celebrating the arrival of a new millennium next New Year's Eve (some would say you ought to wait for 12 months more), then tomorrow's will be the last penumbral lunar eclipse of this millennium. Now surely that is worth a little glance or two?