How hard is it to track moon cycles

[SNOUSSI Anis]

I know this may be extremely basic to almost everyone on the forums,
but I really can't tell if moon cycles are predictable or not !
like can we tell in advance when is the full moon ?
I feel like the answer sould be a definite YES, but my confusion is based on what I see each year.
I live in an islamic country, and each year the first day of ramadan is marked by a full moon which indicates the new lunar month, but almost each year, people have to gather with their telescopes looking for the full moon, and what's even worse, someyimes sightings in different countries are different, so they have the first day of ramadan on a different date and that doesn't make any sense for me !
so as simple as this question can be: Can we predict accuratly moon cycles ?

 

[davenn]

Hi,
welcome to PF

but I really can't tell if moon cycles are predictable or not !
like can we tell in advance when is the full moon ?

yes, of course they are... many daily newspapers give that info often in the weather section
there's books on it by the zillion, there's info all over the internet

And because of that, we can predict solar and lunar eclipses 100's of years into the future.
The phases of the moon have been known for centuries ... probably several 1000 years

I live in an islamic country, and each year the first day of ramadan is marked by a full moon which indicates the new lunar month, but almost each year, people have to gather with their telescopes looking for the full moon, and what's even worse, someyimes sightings in different countries are different, so they have the first day of ramadan on a different date and that doesn't make any sense for me !

I can't really follow that. Am not sure what you are getting at ??Here's just one www site for phases of the moon for 2019
https://www.timeanddate.com/moon/phases/

there's 100's more for any year you chooseDave

 

[SNOUSSI Anis]

thanks! all clear !

 

[russ_watters]

The lunar month is about 29.530588 days long. That should keep you going for a few thousand years.

 

[Bystander]

See "ephemeris/ephemerides."

 

[DrClaude]

I live in an islamic country, and each year the first day of ramadan is marked by a full moon which indicates the new lunar month, but almost each year, people have to gather with their telescopes looking for the full moon, and what's even worse, someyimes sightings in different countries are different, so they have the first day of ramadan on a different date and that doesn't make any sense for me !
so as simple as this question can be: Can we predict accuratly moon cycles ?

Ramadan actually starts on the new moon, not the full moon. For religious reasons, no matter how well you can calculate the date of the new moon, ramadan will only start when an imam as actually seen that crescent moon.

 

[phyzguy]

Ramadan actually starts on the new moon, not the full moon. For religious reasons, no matter how well you can calculate the date of the new moon, ramadan will only start when an imam as actually seen that crescent moon.

This makes more sense. The moon is not visible to your naked eye when it is new, because it is not sufficiently lit up and is too close to the sun. When you can actually see the moon with the naked eye for the first time after new moon depends on a lot of things (clouds, how clear your western horizon is, ...). So this explains why it could be different in different countries. Amateur astronomers actually compete to see who can see the 'youngest' new moon. I think the record without using a telescope is about 15 hours after new moon. This website has some good explanations and pictures, including a picture (through a telescope) of the moon at the instant of new moon.

 

[russ_watters]

Ramadan actually starts on the new moon, not the full moon. For religious reasons, no matter how well you can calculate the date of the new moon, ramadan will only start when an imam as actually seen that crescent moon.

So you're saying Ramadan always starts a day or more late? Interesting.

 

[DrClaude]

So you're saying Ramadan always starts a day or more late? Interesting.

The discrepancy between different countries can be up to three days.

I should have mentioned that my knowledge of this comes from discussions that took place more than a decade ago with muslim friends of North African origins.

Looking at a few things on the web, it appears that some countries, such as Saudi Arabia, Bahrain, and Qatar, actually use a calculated calendar. http://www.staff.science.uu.nl/~gent0113/islam/ummalqura.htm

 

[sophiecentaur]

I can't really follow that. Am not sure what you are getting at ??

Bearing in mind that you can not see the Moon for all day and that the first instant that the leading edge of the Moon could be seen from Earth to be illuminated whilst the Moon is below your horizon, someone else could see the first hint of a crescent Moon on the same day. You would have to wait until your tomorrow to see it.

The discrepancy between different countries can be up to three days.

I would like to have an explanation of that. I have accounted for just one day's discrepancy but I cannot see how there could be more than that. Anyone, anywhere would see the Moon for some minimum time but not near the poles, I guess. Not a problem for the original Muslim calendar makers, though.

 

[russ_watters]

I would like to have an explanation of that. I have accounted for just one day's discrepancy but I cannot see how there could be more than that.

Rain.

 

[sophiecentaur]

Rain.

Not a bad answer in some ways.
It would make sense if the Moon's cycle were not very predictable, year by year. I can't believe that the early Muslims (pretty damn good astronomers, aamof) wouldn't accept the validity of just counting days from one new moon to the next. As I understand it, the Science / Religion divide was not marked in early Islam so it seems strange that an available solution, based on Science, wasn't accepted.

 

[SNOUSSI Anis]

I should have mentioned that my knowledge of this comes from discussions that took place more than a decade ago with muslim friends of North African origins.

I didn't expect this much engagement in the topic, thanks guys.
I'm also from North Africa, and this situation bothered me so long, so I decided to post it here, and somewhat, it all makes sense now.

 

[256bits]

Not a bad answer in some ways.
It would make sense if the Moon's cycle were not very predictable, year by year. I can't believe that the early Muslims (pretty damn good astronomers, aamof) wouldn't accept the validity of just counting days from one new moon to the next. As I understand it, the Science / Religion divide was not marked in early Islam so it seems strange that an available solution, based on Science, wasn't accepted.

Like how easy it is to calculate Easter
https://en.wikipedia.org/wiki/Computus
Excerpt

Thus the lunar month took the name of the Julian month in which it ended. The nineteen-year Metonic cycle assumes that 19 tropical years are as long as 235 synodic months. So after 19 years the lunations should fall the same way in the solar years, and the epacts should repeat. However, 19 × 11 = 209 ≡ 29 (mod 30), not 0 (mod 30); that is, 209 divided by 30 leaves a remainder of 29 instead of being a multiple of 30. So after 19 years, the epact must be corrected by one day for the cycle to repeat. This is the so-called saltus lunae ("leap of the moon). The Julian calendar handles it by reducing the length of the lunar month that begins on 1 July in the last year of the cycle to 29 days. This makes three successive 29-day months.[25] The saltus and the seven extra 30-day months were largely hidden by being located at the points where the Julian and lunar months begin at about the same time. The extra months commenced on 3 December (year 2), 2 September (year 5), 6 March (year 8), 4 December (year 10), 2 November (year 13), 2 August (year 16), and 5 March (year 19).[26] The sequence number of the year in the 19-year cycle is called the "golden number", and is given by the formula

GN = Y mod 19 + 1

See section 4. Algorithms.

Seems to me that the Islam way of determining their special date is simplified.

 

[LURCH]

Not a bad answer in some ways.
It would make sense if the Moon's cycle were not very predictable, year by year. I can't believe that the early Muslims (pretty damn good astronomers, aamof) wouldn't accept the validity of just counting days from one new moon to the next. As I understand it, the Science / Religion divide was not marked in early Islam so it seems strange that an available solution, based on Science, wasn't accepted.

If I understand correctly, Ramadan commemorates when the Quran was first revealed to the Prophet, so I would guess that there is significance in the Imam actually seeing the moon (revelation of the moon).

 

[sophiecentaur]

I didn't expect this much engagement in the topic,

You poke a wasps' nest with a stick and they all fly out.
As with many other 'simple' topics, there's a lot to be said and PF will say it.

 

[Helios]

The lunar month does shorten or lengthen as the Earth goes round the Sun. Otherwise the pattern calendar months should make, given that they have either 29 or 30 days, are called yerms. Here is a 17-month yerm;
30 29 30 29 30 29 30 29 30 29 30 29 30 29 30 29 30

Here are two 17-month yerms with a 15-month yerm;
30 29 30 29 30 29 30 29 30 29 30 29 30 29 30 29 30
30 29 30 29 30 29 30 29 30 29 30 29 30 29 30
30 29 30 29 30 29 30 29 30 29 30 29 30 29 30 29 30

and the pattern gets more complex as you go for precision.

 

[Klystron]

[snip!] As I understand it, the Science / Religion divide was not marked in early Islam so it seems strange that an available solution, based on Science, wasn't accepted.

Let us not neglect the gulf between the privileged/educated classes and the bulk of humanity. As I understand early theology, any old soldier with a taste for haranguing the multitude could declare himself Imam, holier than thou, and begin preaching. Niceties such as reading and writing Arabic much less Arithmetic and Algebra were not required, particularly among the Ummah.

 

[Nik_2213]

Tracking moon cycles...

With respect to the discussions above, beyond the classic 'Metonic Cycle', the answer is surely 'Yes, But'.

If you look at tidal prediction, there is a ruddy zoo of effects to consider about the Earth, Moon and solar motions. Even with two tides a day and long, long records, took the best minds and new maths to gradually identify and quantify enough 'residuals' to get from qualitative via semi-quantitative to reliable prediction...

Also, because of tidal friction, the Moon is gradually receding, so the quoted lunar month length is temporary, a nominal value only. Earth's rotation is slowing, too. Hence the leap-second at some New Years. Never mind going back to real-ancient corals etc which show a rather erratic evolution of day, month and year lengths as ocean levels rose and fell, sea-ways opened and closed, several historical eclipses could not have happened at dates and times observed if their day and month lengths were exactly the same as at present...

IIRC, most Astronomical software now includes a 'correction curve', along with fixes for the various calendar changes.

IIRC, there is still lively dispute over the detail of where and by how much ocean and land tidal dissipation occurs, and how such may be affected by mass-wastage from mega-icecaps, isostatic rebound and climate-related sea-level changes.

 

[sophiecentaur]

I didn't expect this much engagement in the topic, thanks guys.

You poke a wasps' nest with a stick and they all fly out

It's interesting to note that the OP was about the accuracy in terms of days but now we're into the long term results in seconds. That's PF for you.

 

[Helios]

several historical eclipses could not have happened at dates and times observed if their day and month lengths were exactly the same as at present

This is quite a claim. Aren't all eclipses that have happened historical? How many is several and which are the ones are you talking about that demonstrate your assertion?

 

[glappkaeft]

Not answering for Nik_2213 but just because there is a historical record doesn't mean that the date is well determined. One common problem was that dates where often written as "in the Nth year of the reign of king Suchandsuch". If you have a record saying in the 4th year of the reign of king Suchandsuch a dragon swallowed the Sun and you know that the approximate year is 1140 BCE and you have an accurate backprediction of a solar eclipse in the region in 1137 BCE you can use that to calibrate their calendar with ours. It's part of astroarchaeology and I had a astronomy professor who did that as a hobby.

 

[sophiecentaur]

just because there is a historical record doesn't mean that the date is well determined.

I'd certainly go for the date as calculated now as the date in an ancient manuscript. Does it really matter that much? We have good reasons for believing that our present system is pretty good for many years in either direction.

 

[Nik_2213]

@H:
There is a lot of excellent data if you have a really good hunt.
This reference deals in great depth with the records' observational and recording errors...
https://royalsocietypublishing.org/doi/full/10.1098/rspa.2016.0404
As the 'Science' journal commented,
"The oldest event in the catalog, a total solar eclipse that occurred in 720 B.C.E., was observed by astronomers at a site in Babylon (now modern-day Iraq). But, working backward, today’s astronomers would have predicted that the eclipse should have been seen a quarter of a world away, somewhere in the western Atlantic Ocean. The discrepancy means Earth’s rotation has gradually slowed since the 8th century B.C.E. "

( FWIW, I became very interested in lunar regression, day-length etc, while trying to convince a relative that the Apollo guys really, really had trod the Moon. Explaining the laser-ranged retro-reflectors left by Apollo missions plus those carried by several Russian rovers left her silenced, but unconvinced. I counted the former as a rare win 'On Points'...)

 

[sophiecentaur]

trying to convince a relative that the Apollo guys really, really had trod the Moon.

The most polite answer to those people is "Don't worry dear - you wouldn't understand it" Which is true.

 

[DaveC426913]

Otherwise the pattern calendar months should make, given that they have either 29 or 30 days, are called yerms. Here is a 17-month yerm;

Interesting. Citation?

I only find two references to this term via Google, involving a guy named Karl Palmen. One as a Wiki article and references the other, and the other says he invented the term himself.

https://www.hermetic.ch/cal_stud/palmen/yerm1.htm

 

[Helios]

I only find two references to this term via Google, involving a guy named Karl Palmen.

Yes, Karl came up with the term "yerm". I have corresponded with him.
The yerm is any alternating pattern like that, which begins and ends with a 30 day month. The 17-month yerm has 502 days, so you can see that its average is best for a single yerm.

 

[DaveC426913]

.. are called yerms.

OK. Are they called yerms by more than two people?

Because if you're going to bring unfamiliar terms into it, you got to expect people are going wonder why they're never heard of them and can;t find a reference for them. Just sayin'

 

[Helios]

@DaveC426913 The term was used for years on a calendar forum I was on. There won't be that much mention of yerms on the internet at large. It seems no has written a book on the subject. It's just a name for the apparent groupings that form when months are mapped out, evenly spread.
Karl claimed that Issac Newton already knew about the 3 yerm pattern of 1447 days;

30 29 30 29 30 29 30 29 30 29 30 29 30 29 30 29 30
30 29 30 29 30 29 30 29 30 29 30 29 30 29 30
30 29 30 29 30 29 30 29 30 29 30 29 30 29 30 29 30

for some calendar he invented.