<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>I understand that, but the chart indicates that they occur, on average, every 4 years, and on the last day of the month everytime. If the lunar month is about 27 days, then it should be impossible for a blue moon to occur on the last day of the month everytime. Sometimes is more believable, but not everytime.That doesn't make sense. What makes this 2nd full moon any more special than any other full moon? Does the atmosphere change or something if there 2 full moons in 1 month? That's another thing, months is something humans made up in order to keep track of time. Blue moons have been there for at least a couple billion years.Can you guys explain what all of those acronyms mean? Because you guys definitely lost me. <br /> Posted by CometPhoenix</DIV></p><p> </p><p>The lunar cycle is actually 29.53 days. The 27 days you are referring to is the Sidereal Month. The Sidereal month is when the moon return the the same spot using the fix position of the stars. Because the earth is moving in relation to the stars, it only 27 days.<br /><br />Let's see if some numbers will help clear it up for ya.<br /><br />There are 365.24 days in a year. Multiply this times 4 to account for the leap year (366 days once every 4 years). This gives us a 4 year cycle of 1460.96 days. We'll go with 1461 days to make it a bit easier. The lunar cycle is 29.53 days. This give us 12.36 full moons per year or an average of 1.03 per calendar month. This give us 49.4 full moons every 4 solar years or 48 calendar months. With this, we can say that, on occasion, there will be 2 full moons in 1 month. </p><p>With 12 calendar months in a year, we get an average of 30.44 day for each month. So, we can say, with the lunar month being 29.53 days, it is nearly 1 day less than the calendar month. Every month, the full moon starts about 1 day earlier than the previous month.<br /><br />Given the fact that a full moon can fall on any day of the month, the only time you will see 2 in 1 month is when the first full moon falls on the 1st of the month. 29.5 days later will be the 31st. Given that there are not 2 months in a row with 31 days, it will not be common to see one fall on the 30th day of a month with 31 days. If you extend the chart out far enough, you probably would see the occasional blue moon on the 30th day of a 31 day month, but it wouldn't be very common. </p> <div class="Discussion_UserSignature"> <div> </div><br /><div><span style="color:#0000ff" class="Apple-style-span">"If something's hard to do, then it's not worth doing." - Homer Simpson</span></div> </div>