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Once in a blue moon

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They were talking about this on the news recently, because the month of August this year (2012) will have two full moons – one at the beginning, and one at the end. They say that every nineteen years there are seven cases of this – of two full moons in a month, giving the year thirteen moons rather than just twelve. And by one definition anyway, that’s what makes a “blue moon”.

A blue moon? Why blue? Well, wouldn’t you be too if you’d been unemployed as long as she has? The moon used to be an essential element of the world’s calendars, but not anymore. You may have noticed the similarity between the words “month” and “moon”. That’s not an accident. Many languages use exactly the same word for month and moon, because at one time they were essentially the same thing. The moon defined the month. The moon marked out a period of 29-30 days which, logically, was called a “moon”, or in English, a “month”. By that system of counting days, it wouldn’t have made any sense to talk about two moons in a month. The lengths of months were more regular than they are now, but a little bit shorter, such that each year would lose about 11 days compared to the solar year. To keep the seasons in the right place – summer in the summertime and winter in the winter, roughly one out of every three years was given an extra month. Or to be more precise, 7 years out of every 19 – exactly what was reported about the blue moons.

I said unemployed, but maybe the correct term would be underemployed. At least one lunar calendar is still in use. But most of the world has abandoned it in favor of a solar calendar. Many centuries ago it was the norm for calendars to be based on the moon. But somewhere along the way, maybe about the time of one of the Roman emperors, the system was reworked to make the year a more consistent length. An adjustment is still needed to keep the seasons in the right place, but now it’s an extra day every several years rather than an extra month.

Not a bad idea really, but it kind of cut the moon out of the system. We still notice the moon up there from time to time, maybe when it’s particularly big and full, but we’ve lost our awareness of its significance. We’ve lost the skill of using the moon to help chart our course through the month. Late in the afternoon I see a half moon floating with the clouds up high in the sky. It looks cool, but what does it actually tell me about the time of the month? We’ve heard about “waxing” and “waning” moons, one closer to the beginning of a lunar month and one closer to the end. Can we tell which it is? People used to know this sort of thing.

To help us regain some of these lost skills, here are some observations:

  1. A lunar month is just over 4 weeks long, alternating between 29 and 30 days from the beginning of one month to the beginning of the next.
  2. From Earth’s perspective both the Sun and moon travel around it about once a day, with the Sun leading and the moon not quite keeping up, lagging by about an hour each day, and lagging by a full revolution, 24 hours, after 29 1/2 days.
  3. Each lunar month begins and ends when the Sun has once more passed up the moon in its travel, at the new moon plus a little bit, when all but a sliver of the moon is still hidden in the earth’s shadow.
  4. In about the middle of each month is a full moon. Not necessarily exactly the middle. It can fall on the 12th-15th day of the month, depending on the time of year.
  5. In the first half of the month, before the full moon, the best time to look for the moon is in the evening.
  6. In the second half of the month, after the full moon, the best time to look for the moon is in the morning.
  7. The phase of the moon tells us for how much of the night the moon should be visible. A full moon is up the whole night. A half moon for just half the night. A quarter moon for just a quarter of the night. A sliver moon for just a small sliver of the night.

Now stepping through the month from beginning to end:

  • At the beginning of the month this first sliver is seen for just a few moments at sunset, and each day the moon stays up about an hour later into the night as it “waxes” or grows in size.
  • About a week into the month (5-8 days) it’s become a half moon and is staying up half the night, or about 6 hours. You’ll find that the half moon can also be readily seen for about 6 hours of the daytime, rising about noon and setting about midnight.
  • About 2 weeks into the month (12-15 days) when the moon is full, it stays up the whole night, rising at sunset and setting at sunrise.
  • About 3 weeks into the month (20-22 days) we get another half moon, again staying up half the night. But now it’s the other half of the night, from midnight to sunrise, with the moon rising at midnight and setting about noon, as it “wanes” or shrinks in size.
  • About 4 weeks into the month (26-27 days) we see the last of the moon for the month, becoming a sliver that rises and sets with the sun and is only visible just before sunrise. Give it about 3 days and it will again be visible at sunset, marking the start of the next month.

So with this picture in mind of how the system works, when I see that half moon floating with the clouds high in the sky in the late afternoon, it tells me something about where we’re at in the calendar. Only a waxing moon, in the first half of the lunar month, can be visible in the afternoon. Being a half moon places it about one week into the month, about a week after the new moon, and about a week before the full moon. We were at this point in the cycle on July 25-26 (2012), and will be again on August 23-24. Note that adding a week to each of these dates does indeed place the full moons at the beginning and end of August.

Now, having figured out that I’m one week into the lunar month clues me in about what to expect from the moon, and when to expect it to be full. But how can I relate this to our present-day calendar? The best answer I’ve found to this is at torahcalendar.com/MOON.asp. For any given date this will tell you when the current lunar month began, and “View Calendar” will put the info into a nice calendar format, showing the days of the week, and showing the Gregorian date (today’s common calendar) which corresponds to each Lunar calendar date.

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