Shine on, shine on harvest moon

  • Fri Sep 17th, 2010 12:30pm
  • Life

I can’t help being entranced by the full moon even though it gives all but the brightest stars and celestial treasures a lunar whitewashing.

My favorite is the harvest moon that officially takes place at 4:19 a.m. Thursday. In reality, the moon will be more or less “full” from Tuesday evening through Saturday evening.

Astronomically speaking it’s called a harvest moon because it’s the closest full moon to the date of the autumnal equinox, when autumn officially begins. That’s when the sun, on its annual path among the background stars called the ecliptic, slips below the celestial equator in the sky, which is a projection in the heavens of the Earth’s terrestrial equator.

That simply means that from now until the first day of winter in late December the sun’s daily path will take a lower and lower track in our sky, bringing on colder and colder weather.

This year the exact time of the full moon is only six hours after the exact time of the autumnal equinox, which takes place at 10:09 p.m. Wednesday. That makes the exact time of the full moon this month and the exact time of the autumnal equinox the closest they’ve been to each other since 1991, and the closest they’ll be again until 2029.

What makes the harvest moon special is the unique celestial mechanics this time of year. The moon migrates eastward in its orbit around Earth, rising about 40 to 60 minutes later from night to night.

Around harvest time, the moon rises only 20 minutes later each night. Why this happens has to do with the angle of the ecliptic, the sun’s path among the stars, and where it intersects the horizon.

One of the great things about it is the absolutely stunning orange hue. At first glance you might think that a fire has broken out in the distance.

The orange color is caused by our atmosphere. The white light that we see from the moon when it’s high in the sky is just reflected sunlight, and is actually a conglomeration of all the colors in the rainbow.

When the moon is close to the horizon, the light is filtered through the Earth’s atmosphere.

Its gigantic size is a complete optical illusion. The moon just seems bigger when it first rises because you’re comparing it with land objects: trees or buildings. It’s the same size whether it’s high or low in the sky.

When you first see the moon above the horizon, hold the eraser end of a pencil at arm’s length against the rising moon. Compare the size of moon with the eraser head. About two hours later when the moon seems smaller, do it again and you’ll see that the moon is no smaller compared to the diameter of the eraser.

By the way, this coming Wednesday night there’s an extremely bright star just below the full moon. That’s actually the planet Jupiter, at its closest point to Earth in 2010.

I’ll tell you more about the Goliath of our solar system next week in Starwatch.

Mike Lynch is an astronomer and professional broadcast meteorologist for WCCO Radio in Minneapolis and is author of the book, “Washington Starwatch,” available at bookstores. Check his website, www.lynchandthestars.com.

The Everett Astronomical Society: www.everettastro.org/.