Shine on, shine on Harvest Moon

What do you think of when you hear the term "Harvest Moon?"

Some folks see a bare field full of corn shocks, with pumpkins, squash, and other autumn crops stacked around the bottom of them.

As for me, I see a giant orange moon rising over the horizon looking like a super-large pumpkin dominating the eastern sky just after dark.

Years ago for those engaged in agriculture, the harvest moon meant at least three days of extra moonlight to help gather the crops out of the fields.

Because the ecliptic is at such a shallow angle to the horizon, at this time of year, the moon rises very nearly at the same time each night, usually within a half-hour of the previous rising time, instead of the normal 40 minutes or more.

Thus the full moon appears brighter for those three days around the time of the full moon than any other time of the month.

The "Harvest Moon" is usually designated at the full moon nearest the autumnal equinox, which this year is on Thursday, Sept. 22 at 8:21 a.m. MDT, so the "Harvest Moon" will be on Friday, Sept. 16.

Another hallmark of the "Harvest Moon" is its apparent size. I say "apparent" because in watching a full moon rise, we are subject to what is called "The Moon Illusion." This illusion -- along with all other full moons--causes the moon to look a lot larger in our sight than it really is.

No one has ever really come up with a good explanation for the cause of this illusion, we just know that it is.

If you want to see the illusion, be outside watching as the moon rises and climbs above the horizon until it is fully visible.

Hold up a dime at arm's length next to the rising moon and you will see they are both about the same size. Then, if you can wait up until midnight when the moon is almost directly overhead, hold the dime up next to the moon again and you will notice that they are still the same size.

That is the "Moon Illusion."

Oh, and let's not forget the beautiful orange color of a Harvest Moon. That too is somewhat of an illusion, because what we are seeing is the sum total of the all the reddish light from all the sunrises and sunsets from all around the Earth.

Now, let me warn you, that the moon will also look very nearly full on the 15th and the 17th, because of that "shallow ecliptic" thingie, so don't be fooled.

Something else about the full moon is it rises at about the same time as sunset and sets about the same time as sunrise. So if you are an early riser and are outside before dawn on September 17th look east for an orange setting moon.

SKY WATCH:Full moon Friday, September 16th, Saturn and Mars are still holding forth in the southwest, about an hour after local sunset in the area of Scorpius. Mars is rapidly heading eastward against the background stars so they are no longer close, they are still close enough to enjoy the view. Best time for looking is about an hour after local sunset. If you want to get a scope on Saturn, now is a great time to do it, while it is still relatively high in the sky and not down in the atmospheric soup. We have new constellations joining the team these evenings, such as Pegasus and Andromeda, more about them later.

NEXT WEEK:Autumnal constellations, and more astronomical blathering.