Father Sky

Harvest Moon September 6, 2011

Every full moon has a name from Beaver Moon to Worm Moon but by far the most popular is Harvest Moon. This particular moon has a couple of special traits that earn it it’s popularity.

The first is the legend that it’s illumination was used by farmers to extend their harvesting day into the night hours before the age of artificial light.

This story has been around for all of my life and probably many generations before. But am I the only one who has a hard time believing it? Have you ever tried to do something rather detailed with only the light of the moon? Yes, it’s quite bright, certainly bright enough to walk around and even read something with somewhat large text but it would seem to fall short of providing the light necessary for harvesting most crops.  Everything is shades of gray in moonlight so how do you determine which item is ripe and which isn’t? Perhaps the story is true for a limited group of crops but certainly not all. I’d love to hear from a farmer to set the record straight.

But it is the second trait that is really quite fascinating. If you follow the motion of the moon at all you’ll know the moon rises roughly 50 minutes later each successive night (on average).  That’s the rate it takes for the moon to make one complete orbit around the earth every ~29 days.  But the Harvest moon rises pretty close to the same time each night which allows for more of those prolonged days (for harvesting or whatever) than a typical full moon. How can that be?

The answer actually has more to do with the earth than the moon. I am unable to insert a map that illustrates this but if you could take a look at this link…

http://stars.astro.illinois.edu/sow/cm2.html

… you will see a map of the night sky as seen from earth during our Northern Autumn (or Southern Spring). I wish to draw your attention to the straight line running from the lower right to the upper left with the name “Ecliptic” in white just below it. That is the path that the  Sun and all of the planets take through the celestial vault from our earthbound perspective. This includes the moon.  The journey all of the celestial bodies take is from right to left (West to East). During the other seasons of the year the path flattens out, curves downward and then curves back up. Sort of like a lazy “S” on it’s side. (the reason for this non-linear path is due to our 23.5 degree tilt)

The moon makes one complete circuit of this path every month but it’s only during our Northern Autumn that we have a full moon on this steep upward portion of the path.

So while the moon is moving West to East it is also climbing North a significant amount each day (or night). So instead of rising 50 minutes later each night, the moon may rise with as little as a 20 minute delay night over night providing more nights of mostly full illumination.

So how ever you use the extra moon time, be sure to get out and enjoy the show this fall.