Not the Harvest Moon
Published on Sep 7, 2006 at 10:42 am.
3 Comments.
Filed under calendars, moon.
A few months ago, I wrote a posting about Full Moon names while I was still at my old web site. I imported it into Astroprofspage, though, and it is here. I bring this up, because I was looking at a free calendar that I got at a local bookstore early in the year. Today is the Full Moon. The calendar marks it as “Harvest Moon.” This is incorrect! This is not the harvest moon. Let me explain.
Traditionally, the Full Moon closest to the Autumnal Equinox is called the Harvest Moon. The Autumnal Equinox is the time when the Sun appears directly overhead at the Earth’s equator. As the Earth goes around the Sun, it is tipped about 23.5 °. So, for about half the year the Northern Hemisphere is tipped towards the Sun, and for about half the year the Southern Hemisphere is tipped towards the Sun. On two days between, the Earth is oriented so that the Sun appears overhead at the equator. These are called the equinoxes, because legend has it that on these days you have equal lenght days and nights. Actually, that isn’t true, and I’ll say more about that when we get to the equinox in a few weeks. This year, the Autumnal Equinox happens September 23, at 04:03 UT (That turns out to be at 11:03 pm on September 22 here in Texas). The Autumnal Equainox is always about September 22. So, from today’s Full Moon until the equinox is about 15 and 1/2 days. The next full Moon will be October 7 (Actually late in the evening on October 6, here in Texas). That is 13 days after the equinox. Hmm. 13 is less than 15.5, so that means that this year October’s Full Moon is closer to the equinox than is September’s Full Moon. So, the Full Moon in October is the Harvest Moon, not today’s Full Moon.
So, why is my calendar wrong? Well, this is just sloppy work done by someone who doesn’t really understand the astronomical rules for determining the Harvest Moon. In the Full Moon in September occurs from September 9 or later, then it is closer to the equinox than the October moon. Since the calender isn’t perfectly in sync with the year, the equinox moves sllightly, so sometimes even a Full Moon on September 8 is just a shade closer to the equinox than an October Full Moon. So, if you work out the math, you see that this means that a Full Moon in September is going to be the Harvest Moon over 70% of the time. So, rather than figure it out, people sometimes just get lazy and always call the September Full Moon the Harvest Moon.
OK, so this isn’t really the Harvest Moon. As I said in my earlier posts, many of the Full Moons have names, so which one is this if it isn’t the Harvest Moon? Well, the Full Moon names vary by region, and so it varies. There is a semi-official definition of Harvest Moon, but few among the other moons. This Full Moon is sometimes the Fruit Moon, or the Dying Grass Moon (Oh, how that is really appropriate around here! We are in the midst of a long drought.). In the northwest part of the North America, it is the Salmon Moon. In the southwest of the US, it is the Sandstorm Moon. For much of Colonial America, it is either the Elk Moon, the Singing Moon, or the Wine Moon. There is little agreement, so I guess that we could put all three together and call it the Drunk Singing Elk Moon.
Incidentally, tides tend to be more extreme at Full Moon and New Moon. The Moon is actually closer to Earth than normal this Full Moon, so the tides will be extreme today, about as big of extremes as they will get this year.
So, if you get clear skies, go out and look at the Drunk Singing Elk Moon tonight!
-Astroprof
Rob Morris on September 14, 2006 at 3:29 pm: 1
Dear Astroprof,
I have been trying to find out why the equinoxal tides are more extreme than the solstice ones and came across this web site. You mention that the autumnal equinox tides are the biggest (? more so than the vernal?), and that this is caused by the fact that the moon is closer to the earth at this time; why is that so; could you please explain?
Thank you
Rob Morris
PS I live in Devon UK
Astroprof’s Page » Tides high and low on September 15, 2006 at 12:40 pm: 2
[…] A few days ago, I posted about the Full Moon last week not being the Harvest Moon. I got a comment from Rob asking about tides. I thought about it, and I decided that the answer was a bit much for a follow-up comment. So, I decided to do a posting on tides. […]
Astroprof’s Page » Harvest Moon on September 26, 2007 at 1:26 pm: 3
[…] The Full Moon closest to the Autumnal Equinox is called the Harvest Moon. The name of the Full Moon closest to the equinox presumably is because farmers could continue to harvest by moonlight after sunset. This is about harvest time for the people who gave the Harvest Moon this name. The Autumnal Equinox for 2007 just occurred about three days ago. The Moon takes about 29.5 days to go through a complete cycle of phases, so the last Full Moon prior to today’s Full Moon was August 28, at which time there was a lunar eclipse (you can see my photos of it here). Well, clearly September 26 is closer to the September 23 equinox than is August 28, so tonight is the Harvest Moon. Note that it is the Full Moon closest to the equinox that is the Harvest Moon. That happens to be during September just over 4/5 of the time. However, about 1/5 of the time, the Full Moon closest to the equinox actually occurs during the first week of October, as it did last year. Since the Harvest Moon occurs in September most of the time, sometimes people who don’t know the rules for the Harvest Moon mistakenly think that the Full Moon of September is the Harvest Moon. The Full Moon that follows the Harvest Moon (which will be October 26 this year, at 04:52 UT) is called the Hunter’s Moon. […]