Posted by John Nichols on November 02, 2004 at 14:27:34 from 165.91.196.110 user Mcneacail.
In Reply to: Re: Swallow Island- lunar eclipse. posted by Peter Ceresole on November 01, 2004 at 13:09:27:
I was incorrect a blue moon is rarer than a lunar eclipse.
Ah...............
Stumped again by the night sky.
John
A blue moon
Although the full moon occurring Saturday, July 31, 2004, will look like an ordinary full moon, it will actually be a bit extraordinary—a blue moon.
What is a Blue Moon?
There are in fact two definitions for a blue moon. According to the more recent definition, a blue moon is the second full moon in a calendar month. For a blue moon to occur, the first of the full moons must appear at or near the beginning of the month so that the second will fall within the same month (the average span between two moons is 29.5 days). July 2004 will have two full moons: the first on July 2, the second on July 31—that second full moon is called the blue moon.
The Other Kind of Blue Moon
An older definition for the blue moon is recorded in early issues of the Maine Farmer's Almanac. According to this definition, the blue moon is the third full moon in a season that has four full moons. Why would one want to identify the third full moon in a season of four full moons? The answer is complex, and has to do with the Christian ecclesiastical calendar.
Some years have an extra full moon—thirteen instead of twelve. Since the identity of the moons was important in the ecclesiastical calendar (the Paschal Moon, for example, used to be crucial for determining the date of Easter), a year with a thirteenth moon skewed the calendar, since there were names for only twelve moons. By identifying the extra, thirteenth moon as a blue moon, the ecclesiastical calendar was able to stay on track.
For a fuller explanation see http://www.inconstantmoon.com/cyc_blue.htm. For more background information on the controversy over the two definitions of blue moon, see the Sky and Telescope article, "What's a Blue Moon?" In it they explain how the two different definitions of a blue moon came about—including their own role in introducing the second, modern definition.
A Star Rating for the Modern Blue Moon
Although Sky & Telescope calls the modern blue moon definitition "trendy" and a "mistake," the fact that there is an older, preexisting (and more complicated) definition does not necessarily make it the more interesting or meaningful definition. Charting the "third full moon in four full moons" in a season isn't everyone's idea of an fascinating enterprise. The modern, "trendy" definition, however, points to an intriguing astronomical phenomenon—every so often two moons can manage to position themselves in the same month. Given that full moons occur once every 29.5 days, this is quite an accomplishment!
How Often Does a Blue Moon Occur?
Over the next twenty years there will be a total of 17 blue moons, with an almost equal number of both types of blue moons occurring. No blue moon of any kind will occur in the years 2006, 2011, 2014, and 2017.
The more recent phenomenon, where the blue moon is considered to be the second full moon in a calendar month, last occurred on Nov. 30, 2001. Two full moons in one month may occur in any month out of the year except for February, which is shorter than the lunar cycle.
The other, older blue moon event, which happens when there are four full moons in a season, last occured on Nov. 20, 2002. Since this type of blue moon is reckoned according to the seasons, it can only occur in February, May, August, or November, about a month before the equinox or the solstice.
Lunar eclipse
1996 April 3-4 - total lunar eclipse (see data sheet)
1996 September 27 - total lunar eclipse (see data sheet)
1997 March 24 - partial lunar eclipse (see data sheet)
1997 September 16 - total lunar eclipse (see data sheet)
1998 March 13 - penumbral lunar eclipse (see data sheet)
1998 August 8 - penumbral lunar eclipse (see data sheet)
1998 September 6 - penumbral lunar eclipse (see data sheet)
1999 January 31 - penumbral lunar eclipse (see data sheet)
1999 July 28 - partial lunar eclipse (see data sheet)
2000 January 21 - total lunar eclipse (see data sheet)
2000 July 16 - total lunar eclipse (see data sheet)
2001 January 9 - total lunar eclipse (see data sheet)
2001 July 5 - partial lunar eclipse (see data sheet)
2001 December 30 - penumbral lunar eclipse (see data sheet)
2002 May 26 - penumbral lunar eclipse (see data sheet)
2002 June 24 - penumbral lunar eclipse (see data sheet)
2002 November 19-20 - penumbral lunar eclipse (see data sheet)
2003 May 16 - total lunar eclipse (see data sheet)
2003 November 8-9 - total lunar eclipse (see data sheet)
2004 May 4 - total lunar eclipse (see data sheet)
2004 October 28 - total lunar eclipse (see data sheet)
2005 April 24 - penumbral lunar eclipse (see data sheet)
2005 October 17 - partial lunar eclipse (see data sheet)
2006 March 14-15 - penumbral lunar eclipse (see data sheet)
2006 September 7 - partial lunar eclipse (see data sheet)
2007 March 3-4 - total lunar eclipse (see data sheet)
2007 August 28 - total lunar eclipse (see data sheet)