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Thursday, April 30, 2009


Half a Blue Moon is better than no Blue Moon.

Idly looking at my lunar tables recently, I noted that May has two First Quarter Moons (2nd and 31st respectively in Australia). This shouldn’t be a surprise. It takes approximately 29.4 days for the Moon to go from any given phase to return to that phase, so if a given phase falls on the 1st or 2nd of a month, it will turn up again on the 30th or 31st (of course, 28 day long February messes things up).

Yet, we are (sort of) surprised when we get two Full Moons in one month. Because the time from Phase to Phase (eg Full Moon to Full Moon) is 29.4 days, and the average length of a month is 30.4 days[1], any lunar phase will be roughly a day earlier in the following month (except the February to March roll-over, for obvious reasons). So the Lunar phases drift through the months.

For example, a Full Moon that starts on the 1st of a given month it will take roughly 30 months (or 2.5 years) to occur on the 1st again, (again, this get messed up a bit by 28 day February).

At least part of our surprise is due to the vast majority of us not remembering the previous blue moons 2.5 years in the past (hey, I can’t remember what I had for breakfast, so remembering that is a bit of an ask), and partly our disinclination to remember Solar and Lunar system data and do long division in our heads.

We express that surprise with a special name for the second of two Full Moons that occur in the same month[2], a "Blue Moon", this has come to be synonymous with something occurring rarely (Once in a Blue Moon). Although, at an average of once every 2.5 years it’s not that rare.

But why don’t we have a special name for the second of two First Quarter Moons? Or for two consecutive New Moons, or Last Quarter Moons in a month? After all, they will be as rare as two consecutive Full Moons (roughly every 2.5 years). In part, it’s due to the importance of the Moon in our lives, or for those of us in the industrialised world, the lives of our grandparents. In the days before artificial lighting was wide spread and of high quality, the light of the Full Moon was needed if you wanted to hold an event at night. The Full Moon was a signpost to agricultural activities (Harvest Moon anyone), and a visible calendar. So of course people will take notice when two Full Moons occur in a month.

When two New Moons occur in a month, who but the astronomers will know or care? Likewise for two Last Quarter Moons, unless you are an astronomer, fisherperson or a farmer you are not likely to be up early enough to see a Last Quarter Moon, let alone realise that there are two of them in a month. But First Quarter Moons, we all can see them, and for many of us they are companions on our way home from work. True, the impact of Last Quarter Moons on our collective psyche is less (notice how in movies or paintings or cartoons the Moon is almost always Full, or a narrow crescent, First Quarter Moons feature very rarely), but still as an easily observable Moon Phase I think we should have a name for them.

How about a “Half-Blue” Moon? Any suggestions?

Dates of “Blue Moons”, ie months were two of any given lunar phase fall in the same month (not just two full Moons in a month the standard definition). Note that every year has at least one “Blue Moon” of some type.

2001 -AprNov-
2002 ---August

[1] Because we run a solar calendar, and the Lunar “year” is shorter than the solar one. People seem to object to the seasons moving through the year more than the Lunar phases moving through the year. Also, rather inconveniently the solar year is 365.24 days long, making it impossible to design a stable calendar of integer day length, hence months with different days in them and leap years.

[2] This usage is actually due to a writer from Sky & Telescope, earlier usages seem to have been “the third of four full moons in 3 months”.

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Half a blue moon
that would be a blon 1/2 blue 1/2 moon. OR a moue again 1/2 moon 1/2 blue.
where could I find a current photo of what the moon looks like right now (your night time) in Australia? I am an astronomy student in San Francisco CA, USA and this is a part of an assignment we are working on: researching how the night sky varies in the lower hemisphere. If anyone could help, I'd really appreciate it.
Well, the java applet should show the Moon as seen from Australia, but for some reason it's showing it the wrong way up. I have no photos from tonight (it's cloudy - again), but you might like to look at the Double Moon image which shows the moon at a com parable time back in June.
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