Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

3rd quarter

  1. Aug 7, 2012 #1
    I may not know everything about astronomy, but I thought I had a pretty good understanding of the moon.

    How come it is the 7th of the month, and the moon is already in its 3rd quarter? I thought the months match up with the moon fairly closely.. Mistaken?
  2. jcsd
  3. Aug 7, 2012 #2


    User Avatar
    Gold Member

    "Brief Explanation of the Moon Phases

    The phases of the moon are caused by the relative positions of the earth, sun, and moon. The moon goes around the earth in 27.3 days, or 27 days 7 hours 43 minutes, on average. This measurement is relative to the stars and is called the sidereal period or orbital period. However, because of the earth's motion around the sun, a complete moon cycle (New Moon to New Moon) appears to earthbound observers to take a couple of days longer: 29.5305882 days to be exact. This number is called the synodic period or "lunation", and is relative to the sun."

    There is more expanation of why the moon appears the way it does to us at this same website:
  4. Aug 7, 2012 #3
    So it's rare that the new moon actually starts at the beginning of every month?

    Also it would be inaccurate for females to base there cycle off the moon also than, correct?

    Thanks for the link, about to check it out.
  5. Aug 8, 2012 #4


    User Avatar
    Science Advisor
    Gold Member
    2017 Award

    yes of course it is :) .... think about it ... orbital period of the moon 27 days .... calendar month 28 or 29 ( feb. leapyear), 30 or 31 days depending on the month

    so it would be very coincidental for the 2 to start at the same time with any regular short term periodicy

  6. Aug 8, 2012 #5
    Yes. When it does happen, it's pretty much a "broken clock is right twice a day" thing.

    Correct, in general - while the menstrual cycle is typically approximately the same as the lunar cycle, it is rare for the two not to drift, relative to each other, over the course of a year.

    That being said, there are environmental factors which do influence the menstrual cycle. If, for example, one's sleeping patterns are influenced by the lunar cycle (full-moon insomnia, say), then it does not seem implausible that a menstrual cycle could genuinely synchronize to those patterns. Some studies suggest that this does occur, others suggest that the statistics are inconclusive. See e.g.
    "What's the link between the moon and menstruation?" @ The Straight Dope

    Fundamentally, the situation is too complex and, at this time, insufficiently well-understood to be able to readily confirm or rule out influences from any periodic phenomenon that resembles the menstrual cycle. For another example, see "Do the menstrual cycles of women living together tend to synchronize?" @ The Straight Dope.
  7. Aug 8, 2012 #6
    Was it is a misunderstanding that months, are based off the Moons cycle?

    I thought ancient people made calendars based on the orbit of the Moon, but this wouldn't make sense because no month is 27 days.
  8. Aug 8, 2012 #7


    User Avatar

    Staff: Mentor

    No, the ancients did roughly base the 12 month calendar on the lunar cycle, but they also had to make it jive with the length of the year. So they fudged a bit. Note the similarity in the words "month" and "moon". And see the wiki: http://en.wikipedia.org/wiki/Month
  9. Aug 9, 2012 #8
    There were many ancient cultures which used a true solar year and a true lunar month as their basis for timekeeping - in that case, months drifted with respect to the start of the year. Just as our weeks drift in that respect, which is why New Year's Day can be any day of the week.

    If the months were tied to the seasons, which was usually the case, this needed to be adjusted, so every once in a while there would be a "leap month", a thirteenth month which would bring the two cycles back into rough alignment. Just as we have leap days to keep our year in alignment with the seasons without having to make it a non-integral number of days long.

    It just so happened that people decided at some point that having stable months with respect to the seasons was more important than having true lunar months, and so schemes like the one we use today were born. Lots of things about our timekeeping are arbitrary, after all.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook