back to panic.com

Panic Blog

From the desk of
Dave
Engineering Dept.

Q: “Once in a blue moon” is a rare event. But what does “blue moon” really mean?

calendar-iconThis was the final question last week at Pub Trivia, and our team won the evening with the same answer to this question that almost everyone else gave, probably the answer you’ve heard before: a blue moon means two full moons in one calendar month. This month, December 2009, has a blue moon on the 31, since it also had a full moon on the 2nd. But I had the nagging feeling that I’d read or heard somewhere (probably on QI) that the popular definition is wrong, that the real blue moon isn’t that straightforward. When I got home, Google confirmed it: We were wrong, quizmasters and all.

This idea that a month with two full moons is unusual points out the incongruity between the lunar cycle and our familiar 12 month, 365 day calendar. Month and Moon are nearly the same word–why don’t they mean the same thing? The answer, of course, is that they once did. But nature seems to hate an integer* as much as it reportedly hates a vacuum, so a year isn’t an even 365 or 366 days or 12 orbits of the moon around the Earth; neither does the moon take exactly 29 or 30 days to go around. So the problem is one of successive approximation, dividing one number by another and dealing with the remainder.

A first approximation: the lunar calendar

The moon was a useful timekeeper, despite its reluctance to synchronize with the cycles of the seasons. Instead of having to remember “day 237 after the winter solstice is wheat-planting day”, a farmer simply knew that the eighth moon is the wheat-planting moon. The wheat didn’t care much if one year’s full moon was 11 days earlier (or 18 days later) than the last year’s. Before the unveiling of the Julian calendar in 45 BC (more on that later) nearly all cultures on the planet used a calendar based on the cycles of the moon. (The Mesoamericans are a notable exception, and that 2012 thing is complete malarkey. Just saying.)

calendarjoke2

So here’s the first rounding error: Twelve cycles of the moon take between 354 and 355 days, but it’s 365 or 366 days between winter solstices. Or in other words, every two or three years will have thirteen full moons in it instead of twelve. And this is where the “blue” moon comes in, the moon that’s different from the others. (Though at this time, months and moons were the same thing so there’s not any question of having two of one in the other.) If you have a Stonehenge or other primitive-but-awesome celestial observatory handy (robed druids included) to tell you exactly when the solstice occurs you’ll notice that the phase of the moon and the solstice just about line up every 19 years. (But not exactly. Again, nature versus integers.) And even if you don’t, you’d eventually notice that, on average, 7 of every 19 cycles of the seasons need 13 months instead of 12 to keep everything in sync, or you’ll planting your wheat at the wrong time of year.

One interesting exception to this is the Islamic calendar, which has twelve lunar months and doesn’t adjust to match the seasons. This calendar is used for civil and religious purposes; a separate solar calendar must be used for agriculture. An Islamic, or hijri, year is on average 354.36 days long, so their calendar shifts forward by around 11 days each year relative to the seasons. Ramadan isn’t just on different dates every year (which would be more alarming to us if we didn’t have to deal with Easter), it crosses the seasons and returns to the same time of year every 33 years. This also means their years are counting up around 3% faster than seasonal years: It’s currently 1430 AH in the Islamic calendar, but in the year 20874 (AH and AD) their years will “catch up” with ours.

Next approximation: the solar calendar

In the Roman Republic the months were determined, as in most other cultures at the time, by priests (or imams, robed druids, etc.) who announced the presence of a new moon–and thus a new month–just after sunset. (The word “calendar” comes from the Latin calare “to call out, announce”, by way of kalends, the first day of the Roman month.) The Pontifex Maximus, high priest of the Roman religion, had the additional task of determining how long the year would be, 12 or 13 months, and therefore which day of the week important holidays would fall on–important stuff for a superstitious lot like the Romans. Originally, this was a purely religious role, but by the middle of the first century BC Roman politics had become particularly nasty and the Pontifex Maximus could use the calendar as a political tool, choosing to add an extra month to years when allies were in power or denying an extra month to his enemies.

To what extent Julius Caesar did this after becoming Pontifex Maximus in 63 BC isn’t clear, but 17 years later, in 46 BC, Caesar reformed the calendar, taking the power to define the year away from the position. The Julian reform declared that the Roman Republic would use a 365 day calendar with an extra day every 4 years. (Sound familiar?) In other words, the year was approximated at 365 1/4 days. To align the new calendar with the equinox, though, he had to add another two months to the already extended year, making 46 BC 445 days long. Cicero called this “the last year of confusion”.

One last tweak: our modern calendar

calendarjokeBy the end of the Middle Ages, it had become apparent that the Julian calendar had drifted away from the seasonal calendar. This distressed the Catholic Church terribly because the most important thing in the world for the Catholic Church was celebrating Easter on the correct day. It had spent the previous thousand years trying to sort it out and now it found that their calendar, the calendar that Caesar had set in motion 1600 years before, was missing the equinox by more than a week.

During this time, some very clever people had figured out that the Julian calendar was moving forward around 3 days every 400 years, and came up with a modification to the leap year scheme: Every 100 years would not be a leap year, except for every 400 years. (You may remember that 2000 was a double exception year–it wouldn’t have been a leap year, but it was. So if you weren’t paying attention, you didn’t really miss anything.) In 1582, Pope Gregory XIII announced a reform of the calendar, the new leap year scheme along with a 10-day shift to get the calendar synced back up with the seasons.

The Church and a number of Catholic countries adopted the new calendar on Friday, October 15th of 1582, one day after (Julian) Thursday, October 4th. Protestant Europe resisted this papist conspiracy until the 18th century; the British Empire (including the American colonies) saw Thursday, September 14th follow Wednesday the 2nd in 1752. Sweden tried to make a gradual change by ignoring leap years until their calendar matched the Gregorian, but lost its nerve and switched back to the Julian calendar after 12 years. To make up for a day gained in the interim, they added a 30th of February to 1712. In Alaska, the change took place in 1867 when US took possession of the territory from Russia, which still used the Julian calendar. Because the international date line shifted at the same time, Friday, October 6th was followed by another Friday, the 18th of October.

Time in the future

Modern astronomy tells us that a year is, to within one day in a million years, 365.242199 days long. The Julian calendar put 1 year as 365.25 days, for an error of 7.8 days per 1000 years. The Gregorian calendar put 1 year as 365.2425 days, for an error of 0.3 days every 1000 years. The astronomer Sir John Herschel proposed that the year 4000 and every 4,000th year after should not be a leap year in order to reduce this remaining error to about half a day every 10,000 years, but the standard has not yet been adopted. New calendars have been proposed (mostly to solve the problem of figuring out which day of the week any given date is) but the Gregorian calendar’s ordering of leap years is sufficient to keep the calendar aligned to the seasons as far into the future as we need.

Well, almost. At this scale we begin to see that the speed at which the Earth rotates on its axis isn’t constant, or even predictable. The pull of the moon’s gravity is slowing the rotation of the Earth, making each day longer by a fairly constant 2.3 milliseconds per century, but other factors, most notably shifting mass on and inside the planet, add to or subtract from this by a different amount every year. Since 1967, when the length of a second was standardized in precise atomic terms, days have been between 0.3 and 1 ms longer than the traditional 24 * 60 * 60 = 86,400 seconds. As a result, every once in a while astronomers add a “leap second” to UTC, Coordinated Universal Time. Surprisingly, while just about every year between 1972 and 1999 required a leap second to keep the clock synchronized with the Earth’s rotation, there have only been two leap seconds added in the last ten years. The Earth has sped up just a bit, and no one knows why.

Time in Cocoa

Since this is a Panic Engineering post, I should probably write something about calendars and programming. If you’re not a Cocoa programmer (and even if you are), feel free to skip this–it won’t be on the test.

First, here’s the code I used to find the year that the Islamic and Gregorian calendars meet. The NSCalendar and NSDateComponents classes were introduced in OS X 10.4 to replace NSCalendarDate, and are also available on the iPhone OS. They provide calendar operations such as converting NSDate objects to and from units of years, months, days, etc., and finding the difference between two NSDates. (Since we mentioned it above, it’s worth noting that as far as the OS is concerned every day is 86,400 seconds long. NSCalendar does not know about leap seconds.)

  1. NSCalendar* gregorian = [NSCalendar currentCalendar];
  2. NSCalendar* islamic = [[NSCalendar alloc] initWithCalendarIdentifier:NSIslamicCalendar];
  3. NSDateComponents* components = [[NSDateComponents alloc] init];
  4.  
  5. [components setDay:1];
  6. [components setMonth:11];
  7. [components setYear:20874];
  8.  
  9. // Always use a format string in NSLog, even if it's just @"%@"!
  10. NSLog(@"%@", [gregorian dateFromComponents:components]);
  11. NSLog(@"%@", [islamic dateFromComponents:components]);

And here’s a snippet showing the jump from the Julian to the Gregorian calendar on October 15th, 1582. Be aware that the default NSCalendar uses the Julian calendar before this date. (For “fun”, check out October 1582 in iCal…)

  1. [components setDay:15];
  2. [components setMonth:10];
  3. [components setYear:1582];
  4.  
  5. NSDate* date = [gregorian dateFromComponents:components];
  6. NSLog(@"%@", date); // 1582-10-15 00:00:00 -0800
  7.  
  8. NSDateComponents* offset = [[NSDateComponents alloc] init];
  9. [offset setSecond:-1];
  10.  
  11. date = [gregorian dateByAddingComponents:offset toDate:date options:0];
  12. NSLog(@"%@", date); // 1582-10-04 23:59:59 -0800

OS X 10.4 provided Gregorian, Buddhist, Hebrew, Islamic, Islamic Civil, and Japanese calendars. 10.6 added Republic of China (for calendrical operations only, not formatting), Persian, Indian, and ISO8601, the traditional calendar of ISO8601istan. NSDateFormatter can format dates in any of these calendars:

  1. NSDateFormatter* formatter = [[NSDateFormatter alloc] init];
  2. [formatter setDateStyle:NSDateFormatterFullStyle];
  3.  
  4. NSLog(@"Today's Gregorian date: %@", [formatter stringFromDate:[NSDate date]]);
  5.  
  6. NSCalendar* buddhist = [[NSCalendar alloc] initWithCalendarIdentifier:NSBuddhistCalendar];
  7. [formatter setCalendar:buddhist];
  8.  
  9. NSLog(@"Today's Buddhist date: %@", [formatter stringFromDate:[NSDate date]]);
  10.  
  11. NSCalendar* islamic = [[NSCalendar alloc] initWithCalendarIdentifier:NSIslamicCalendar];
  12. [formatter setCalendar:islamic];
  13.  
  14. NSLog(@"Today's Islamic date: %@", [formatter stringFromDate:[NSDate date]]);

Blue Moons

The article that told me I was wrong about what “blue moon” means (and hilariously refers to that definition as “trendy”) explains that the “two full moons in a month” definition is due to a misreading of the Maine Farmers’ Almanac: the real Blue Moon is the third full moon in a season that has four full moons. Why the third? The first full moon of the season is particularly significant; e.g., the Easter moon. The moons before and after have names as well–the Lent Moon precedes the Easter Moon, whether it’s the third or fourth moon of the winter–so the third moon of four is the extra one. By this definition, 2009 does not have a blue moon, since the full moon on December 31st is after the solstice and belongs to next year’s winter.

For an example of the importance of the sequence of the moons to daily life, consider Charlemagne’s naming of the months of the year (a solar year, but even so), used for over 700 years after his death:

Wintarmanoth, winter month
Hornung, the month when the male red deer sheds its antlers
Lentzinmanoth, Lent month
Ostarmanoth, Easter month
Wonnemanoth, love-making month
Brachmanoth, plowing month
Heuvimanoth, hay month
Aranmanoth, harvest month
Witumanoth, wood month
Windumemanoth, wine month
Herbistmanoth, autumn harvest month
Heilagmanoth, holy month

One etymology claims “blue” comes from belewe, “betrayer”. Imagine if an extra moon showed up and delayed Wonnemanoth for 29 days. Betrayer moon!

A: The third of four full moons in a season

The next day I emailed the quizmaster to let her know how we were all terribly, horribly wrong, that we’d need to recount the scores, possibly have a do-over. She said they’d take either answer.

* A Correction

“Nature hates integers” is a gross oversimplification and ignores not just all of chemistry, where everything happens in even ratios, but the surprising phenomenon of orbital resonance. Pluto and Neptune, for example, are phase-locked in a stable 2:3 resonance: Pluto orbits the sun exactly twice for every three of Neptune’s orbits, and has for millions of years. Our moon’s rotation is locked to its orbit around the Earth due to tidal forces, which is why we only ever see one side of it. For the same reason, the Earth’s rotation is also slowing down and will eventually match the orbit of the moon. At that point, billions of years from now, both a day and a month will last for around 47 of our current 86,400-second-long days. One side of the Earth will face the moon; the other will never see it again.

Posted at 1:52 pm 47 Comments

I didn’t see that coming…

Joseph McLaughlin

12/21/2009 2:09 PM

Great post. Question, did you hardcode the links to the Apple developer documentation or did you run it through a parser? I’m loving what I’m seeing here on the Panic blog, keep it up!

Jonathan Wold

12/21/2009 2:20 PM

Wow.. I wasn’t expecting that. Very well thought out, though – and informative. Looking forward to the next one!

Joseph: the blog runs code through the PHP-based GeSHI syntax highlighter (hacked on heavily to look nice). The links are an actual (optional) feature of GeSHI. I was impressed too!

That was far more than I ever knew I wanted to know about Calendar and time. I’ve actually almost always been interested in a discussion about time. Since I believe time is really obsolete because I beleive that the universe itself is really eternal, but that since we have a “beginning” and an “end” to our life (birth and death) we also feel that everything else NEEDS a beginning and an end.

Great write-up.

Incidentally, the process of adding or subtracting time from calendars goes all the way back to the Babylonians and even has its own name: “intercalation.” Not applying the correction, as you point out, would make the calendar fall out of synch with lunar periods and seasonal variations. It also made it so travelers from one region to another couldn’t know what month they would be arriving in since the intercalation periods varied even for the same calendar by region.

Judging by the variations in tracking estimates, I’m pretty sure all modern intercalators are now employed by UPS.

Harvard Irving

12/21/2009 5:07 PM

Interesting how Charlemagne has two plowing months in a row. Must have had particularly fertile fields.

Mitch Cohen

12/21/2009 5:09 PM

Ok, show of hands – how many of us actually brought up September 1582 in iCal? Neat.

Aaron Davies

12/21/2009 5:13 PM

Here’s what I’ve always wondered: why is the calendar set off ten days from the solar calendar? Why isn’t New Year’s on the winter solstice? Presumably Cæsar was preserving an existing Roman tradition in setting it that way, but does anyone know where it came from?

John Muir

12/21/2009 5:29 PM

Coming up next in the series: what relativistic reference frames mean for an atomic clock near you, and why it’s always tea time at Tau Ceti.

Rick Fletcher

12/21/2009 5:46 PM

Is that color scheme available anywhere as a .seestyle?

Aaron: I wanted to include that in the post, but I couldn’t find any answer other than “because that’s the way the Romans did it”. I did a bit more searching, and here’s what I’ve got:

According to http://www.firstscience.com/home/articles/origins/the-ides-of-march-in-january_41490.html the Roman year started, as many calendars do, on March 1st to match the spring solstice. (At the [northern hemisphere] spring solstice the sun in marching quickly southwards, while at the equinoxes it’s impossible to distinguish which day’s sunrise is northernmost or southernmost.) In 153, the Romans called elections two months early, so the year, which had traditionally been named after the consuls, also shifted forward two months. It seems like this would also explain the two-month name shift (September is the 9th, not the 7th month), but Wikipedia says that happened in 713 BC.

In the end, I think it was just luck of the draw.

Personally, I reckon we should have 4 seasons of 13 weeks each (divided into “months” of 4/4/5 or somesuch) with 1 or 2 ‘extra’ days at new year. This has the nice advantage that days, weeks and months are identical every year, and the moon still does its own thing just like it does now.

It quite interesting to note that nature actually loves integers as quantum mechanics (one of the most successful physical theories ever) tells us. However; the step between quantum mechanics and general relativity (the theory used to calculate orbits) is quite long and the two theories still hasn’t been merged.

Oh, by the way, the comics above were done by Vanja Mrgan (http://vanjamrgan.blogspot.com/).

I think the modern calendar should be 13 months of 4 weeks. All months are 28 days (28*13=364) and we should have an end of year total F’n throwdown at the end of the 13th month. Would it be so bad if the 1st, 8th, 15th, and 22nd were always Sunday?

Mat: I like the idea, except that for me (and many other europeans) Sunday is the last day of the week, not the first. The week starts on Monday for us.
We can’t even agree on meters and degrees… so the first day of the week… :)

Who wants to write a subclass of NSCalendar that supports the World Calendar? http://en.wikipedia.org/wiki/World_Calendar

Adam Rice

12/21/2009 7:49 PM

I look forward to your disquisition on the timezone database soon.

Fantastic post… I eat this kind of stuff up. Even though I knew a few of the details of the history of the calendar, this was a great summary filled with lots of interesting tidbits I was missing before. Thanks!

Holly Rose

12/21/2009 9:34 PM

Dave, I have to know, what were your sources? Please don’t say Wikipedia! It would break my information scientist heart!

Also, I’ve often thought that Christmas is in the wrong place, or else Thanksgiving should be at the end of February. There’s too much of one thing happening in the last two months of our calendar. Anyone who has lived more than one year in a seasonal climate knows that the time when you really need a party of lights, food, drinking, and cheer is in mid- to late February, when your whole world is gray and the chill won’t leave your bones. December is simply not cold or dreary enough anymore to warrant all-out celebrations of this nature.

Adam: Fellow Panic coder Ned (who implemented the calendar on the first-gen iPod!) pointed me to the Olson database. Crazy stuff! I was tempted to go into time zones here, but then it would have been twice as long.

Holly: Let me first say I fully understand and respect the importance of primary sources and independent research. But.. uh.. yeah, Wikipedia mostly. And the rest was web as well. This is pretty uncontroversial stuff, though. I did consider putting a big DO NOT CITE warning on the article, worried that I’d see it referenced on Wikipedia. I don’t think anyone will mistake this for a scholarly work.

A few notes on the Charlemagne naming and translation if you care. The Middle High German spoken around 800 is quite differnt from current German.
Hornung, hornunc means bastard, which here translates to month short in days (February)
Lenzinmanoth, lenz means spring, so spring month
Wunnimanoth, wunni means delight, in German May sometimes is still called “Wonnemonat”
Brachmanoth, brach means fallow
Herbistmanoth, simply fog or autumn month

I worked on a software project that involved timezone data. It was horrendous. We had to be backwards compatible with old versions of the project that I didn’t have the privilege of working on. A lot of science and business applications depend on an accurate record of time. Time zones and daylight savings time (which is a subset of the timezone problem) screws it all up! I got all these weird edge cases from different clients. If I had the power, I’d get rid of DST and make timezones based exactly on some formula for longitude and latitude. This post brought back nightmares and made me remember a time in my life when I wished that my eyeballs would explode.

“2.3 milliseconds per century” is actually 2.3 milliseconds per day per century. The earth as a clock which is running slow, but at a constant rate, would require adjustments, but since the rotation is slowing, the adjustments are actually accelerating, as the units suggest.

As for “no one knows why,” that’s not entirely accurate. Changes in mass distribution around the earth from actual changes in shape and effects of weather and climate contribute. “Hard to predict” is not the same as “qualitatively hard to explain.”

I wonder, does the stardate system from Star Trek sidestep any of these complications, or does it further muddle date-keeping?

In a certain cosmological schema it is posited that the earth originally had an orbit about the sun of 360 days and the moon had an orbit of 30 days. At some point, a world-wide catastrophe resulted in enough material being expelled from the earth into space to change the orbit to ~365.25 days and the moon, receiving a portion of the expelled material, changed its orbit to ~29 days.

John Goodman

12/22/2009 10:47 AM

I’ve built a kind of mechanical calendar that shows both the time of day and the time of year. As a calendar, it’s accurate to within 5 minutes per century. (The length of a year is 365.2422 days.) Have a look at http://www.annosphere.com

I’m happy to see so many people intrigued by this topic.

The orbital period of the moon is actually about 27.3 days. Its synodic period (time between full moons) is about 29.5 days. The reason is because by the time the moon has completed its orbit, the earth has moved about a twelfth of its way round the sun and the sun’s relative position in the sky has moved. It takes another couple of days for the next full moon to “catch up”.

Ian Romanick

12/22/2009 12:47 PM

I think that last bit about the moon is not true. The moon is also getting farther away from the Earth. I believe the moon will escape its orbit before the day / month phase lock occurs

Could you share your GeSHI theme?

Yeah, I’m involved with date/time libraries at my work, and it’s a total mess :) Whether it’s governments that can’t decide whether or not they’re having Daylight Savings Time each year (I’m looking at you, Argentina), days that don’t have midnight in certain timezones (or rarely, days that don’t exist at all), or the fact that most people don’t understand that “a day” and “86,400 seconds” are two different measurements, it’s fun times.

I love this material! New to me was that Cocoa has built-in calendar conversions. Very convenient.

As for the names of September through December being off by two, I was under the impression that this was the result of July and August being jammed into the calendar to honor Julius and Augustus.

My favorite source of calendar information, and one that fits with a programming audience, is the book _Calendrical Calculations_ by Dershowitz and Reingold. (I have the “Millenium Edition.”) They also wrote a couple of papers on the topic, which you can find at

http://emr.cs.uiuc.edu/~reingold/calendars.shtml

The code in their books/papers is in Lisp, but it’s very easy to follow. Wouldn’t surprise me to learn that the Cocoa routines were based on it.

The calendar functionality in Emacs comes from the Dershowitz/Reingold code. Quite a while ago, I wrote a blog post about accessing it from the command line:

http://www.leancrew.com/all-this/2008/04/emacs-lisp-as-a-scripting-language/

Wish I’d known then about the Cocoa classes.

Aaron Elliott

12/24/2009 12:06 PM

This is a very interesting post. I’ve done some celestial research (which is very interesting and still exciting as of 2009) on ephemeris data… While the Earth is slowing to match the Moon’s orbital velocity, the Moon is actually speeding up because of drag from the Earth’s laggy tidal forces yanking at the moon ahead of it’s orbit and conservation of momentum. What happens here, is the same force that slows the Earth’s rotation accelerates the moon’s orbit around the Earth, thus diminishing the centripetal force and allowing the Moon to draw away from the Earth by a couple inches each year. Over enough time, millions of years, let’s say… even the lunar months will draw out because of the longer orbit the moon must take. As the Earth aligns with that cycle, it will eventually slow more and more, too, until either the Moon escapes orbit and the Earth becomes tidally locked with the Sun, or the tidal forces slow the Moon’s orbit, again, locking the Moon’s orbital ascent (which depends on those internal mass redistributions you mentioned above, or whether we “pick up” or eject any substantial extra masses between now and then altering the system).

All that being said, I have a couple of questions… in your article you mention a few geographically dependant acceptances to the new Gregorian Calendar, has anyone built an all-encompassing function to take these time changes (and expanded to other calendars and other international calendars and their localized acceptances to those calendars)? That would be uber.

And last of all, this is sort of a question and a suggestion… Is there a function to convert to Star Dates from Star Trek, which can be rational and would completely defeat adding or subtracting days… I mean, with the iPhone, and iCal, who needs to “know” the date, why can’t we just ask a device to take everything, localized laws, traditions, general relativity and our precession through the universe?

a blue moon is also what you see after a volcanic eruption because of ash in the atmosphere

ignores not just all of chemistry, where everything happens in even ratios Isomorphous substitution allows atoms of similar size and the same valence to (randomly) fit in the holes. Purifying lanthanides is a nasty example. Geologly is filthy with it, especially if substitution can be made two or more atoms at a time to balance charge. We have rocks and alloys with composition ranges.

Organic chemistry compositionally behaves (until polymers, capped nanoparticles, colloids arrive). Some fun there is building molecules with no static structure, like bullvalene (somebody didn’t believe until it was synthesized). Inorganic gives us IF7. Silyl enol ethers of 1,3-diketones, etc. The universe is a messy place. All the fun is in the footnotes

Shawn Dehkhodaei

12/29/2009 12:23 AM

Very interesting read, and I like the depth of information provided (and the commentary). I’m Persian (Iranian) so I just wanted to add that in the Persian Calendar (Jalali Calendar), the length of a year, is actually measured astronomically, meaning that our “calendar year” is not always 365.2425 or some fixed number; it changes slightly every year. To explain it a bit better, our “new year” count down is at a different time of the day, each year, meaning that the actual orbit of the Earth is measured down to the millisecond.

Another thing to note is that the Persian Calendar is completely synced to the movement of the Sun, and to the seasonal equinoxes (Spring, Summer, Fall, Winter). The first day of the calendar, is the Spring Equinox (March 21st). We also have a celebration on the Winter Equinox, where we observe the “longest” night of the year (generally December 21st). I’m always amused by the myriad of calendars out there, but I always find that the Persian Calendar seems quite accurate (my bias perhaps).

I found some interesting details on it here: http://en.wikipedia.org/wiki/Iranian_calendar
in case anyone is interested.

What I find really odd is the Islamic Calendar (I’m Muslim by birth but that’s about the length of my involvement). In most Arab countries, they do NOT have a Solar calendar to keep things sane. For example, when I was working in the UAE, they used the Islamic Calendar, for everything, but they also used the Gregorian calendar to celebrate the “New Year” ??!! There is also a joke that an Arab never knows how old he/she really is because of the rotating [lunar] calendar. Or as you put it, they’re getting older at pace of 3% faster than the rest of us !!! I’m not racist, and this remark is not meant to be racist; but this is a serious problem.

How can the Earth’s rotation be speeding up AND slowing down simultaneously?

“Surprisingly, while just about every year between 1972 and 1999 required a leap second to keep the clock synchronized with the Earth’s rotation, there have only been two leap seconds added in the last ten years. The Earth has sped up just a bit, and no one knows why.”

“…the Earth’s rotation is also slowing down and will eventually match the orbit of the moon.”

Maybe we should stop focussing on moons and calendars and think about cooking: French “bleu” means rare and is translated “blue” in American cookery. If we can understand why raw steak is called rare in English and bleu (blue) in French we may understand why something which happens less often than at least once a year; whether the 13th lunar month, the third full moon in a season of four, or two full moons in the same month – is called rare/bleu/blue.

What an excellent, fascinating post — thank you.

Best thing I read today. Thanks.

Alex Hilton

1/10/2010 1:23 PM

Matt,

You’re talking about an average slowing over billions of years being counteracted by a temporary speeding up.

Bit like having a couple of cold years in an otherwise above temperature decade not making a difference to a general warming trend

To bad, you didn’t include the maya calendars (http://en.wikipedia.org/wiki/Maya_calendar) because these are the only ones in sync with the moon / nature

elvis kovacic

5/19/2010 9:24 PM

Cool topic. I never real thought about the moon benig blue but now that I think of it, its blue alot.

for(int i=1890; i<1910; i++)
{
NSDateFormatter *df = [[[NSDateFormatter alloc] init] autorelease];
[df setDateFormat:@"yyyy-MM-dd HH:mm:ss"];
NSString *s = [NSString stringWithFormat:@"%02d-02-01 01:00:00", i];;
NSDate *aDate = [df dateFromString:s];
NSLog(@"s=(%@) dateFromString=(%@)", s, aDate);

// Why do some dates around the beginning of the 19th century show a GMT hour-offset of HUNDREDS of hours????
// 1902-02-01 01:00:00 -053211
}

Chas.K.Castle

3/19/2011 12:37 PM

I suppose that some of this will change a bit after the Japanese Earthquake/Tsunami last week. I understand that it shortened our day, again. Hmmmm. – Charly

Hershel Blacksmith

3/12/2012 10:19 PM

Hello there, just became aware of your blog through Google, and found that it’s really informative. I am gonna watch out for brussels. I’ll appreciate if you continue this in future. Many people will be benefited from your writing. Cheers!