January first marked the beginning of our new year, and the countdown to that momentous year, 2000. As if to celebrate with us, the night sky produced a full moon to brightly shine in the new year. Strange events can happen when there is a full moon on the first day of the month, events that only occur once in a blue moon.

The cycle of the moon provided our ancestors with one of the earliest ways of keeping time. The Sun's rising and setting made short periods of time easy enough to keep track of, but longer periods of time required another regular event. The moon provided this.

The cycle begins as a slender crescent in the western sky at sunset. Over the next week the moon rises about 55 minutes later each day and the crescent is waxing, growing larger each day. During this time, it is often possible to see the dark portion of the moon as well. This is called the  old moon in the new moon's arms  and is caused by earthshine, sunlight reflected from Earth and illuminating the night side of the moon.

One week after the cycle begins, the moon reaches first quarter, standing high in the southern sky at sunset. The term, "first quarter" can be a little confusing because in this phase, the moon is actually half full! The first quarter moon sets at about midnight. After that, during the second quarter, the moon is called a waxing gibbous. Each night the moon is farther east in the sky than it was at the same time the previous night.

One week after the waxing gibbous phase begins, the moon reaches full; it rises at sunset and is in the sky all night. After the full moon, during the third quarter, the lighted portion begins to shrink, first through a waning gibbous stage, then during the forth quarter, a waning crescent phase and finally back to an invisible new moon that rises and sets with the sun. In the waning stages, the moon rises later and later into the night. Just before a new moon, you will find the slender waning crescent on the eastern horizon at sunrise.

The entire cycle of the moon, from new moon to new moon takes 29.53 days. The cycles were regular and predictable and came to be used as a measure of time. The period from new moon to new moon was called a  moonth.  And this  moonth  was the ancestral form of our word month.

Our current calendar is not based on the cycles of the moon, because we have found more accurate ways of making calendars which also helps us keep track of even longer periods of time. Still, most calendars mark the phases of the moon, as if in some kind of tribute to its ancestral form.

Each month, though, we see the moon go through its phases. Each month there is a full moon. But wait, if the period of the moon is 29.53 days, then isn't it possible to have more than one full moon in a month? The answer is, of course, yes, and this month we are treated to a second full moon on January 31st.

We called the second full moon of the month a  blue moon.  Because it was not an event that happened often (and in fact, it can never happen in February, even in a leap year!) The term  once in a blue moon  came to refer to any event that did not happen often. We don't recommend watching the full moon from outside this month, however, or you may well become a blue watcher watching a blue moon!