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B How hard is it to track moon cycles

  1. Feb 5, 2019 #1
    I know this may be extremly 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 ?
     
    Last edited by a moderator: Feb 5, 2019
  2. jcsd
  3. Feb 5, 2019 #2

    davenn

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    Hi,
    welcome to PF


    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 cant 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 choose


    Dave
     
  4. Feb 5, 2019 #3
    thanks! all clear !
     
  5. Feb 5, 2019 #4

    russ_watters

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    The lunar month is about 29.530588 days long. That should keep you going for a few thousand years.
     
  6. Feb 6, 2019 #5

    Bystander

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    See "ephemeris/ephemerides."
     
  7. Feb 6, 2019 #6

    DrClaude

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    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.
     
  8. Feb 6, 2019 #7

    phyzguy

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    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.
     
  9. Feb 6, 2019 #8

    russ_watters

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    So you're saying Ramadan always starts a day or more late? Interesting.
     
  10. Feb 6, 2019 #9

    DrClaude

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    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
     
  11. Feb 7, 2019 #10

    sophiecentaur

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    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.

    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.
     
  12. Feb 7, 2019 #11

    russ_watters

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    Rain.
     
  13. Feb 7, 2019 #12

    sophiecentaur

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    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.
     
  14. Feb 7, 2019 #13
    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.
     
  15. Feb 7, 2019 #14

    256bits

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

    See section 4. Algorithms.

    Seems to me that the Islam way of determining their special date is simplified.
     
  16. Feb 8, 2019 #15

    LURCH

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    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).
     
  17. Feb 8, 2019 #16

    sophiecentaur

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    You poke a wasps' nest with a stick and they all fly out.:wink:
    As with many other 'simple' topics, there's a lot to be said and PF will say it.
     
  18. Feb 8, 2019 #17
    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.
     
  19. Feb 8, 2019 #18

    Klystron

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    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.
     
  20. Feb 8, 2019 #19
    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.
     
  21. Feb 9, 2019 #20

    sophiecentaur

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    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. :smile:
     
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