October 2006

by Deane Morrison

It's harvest moon and Halloween month, so get ready for lots of light, natural and otherwise. There may even be a bonus light show, courtesy of the Orionid meteors.

But let's begin with the harvest moon. Traditionally the nearest full moon to the autumnal equinox, it falls this year on October 6. When it rises it will be nice and round, just four hours short of the moment of fullness. The moon will also be near perigee, the closest point to Earth in its orbit, but the increased size won't be noticeable to a casual observer. The harvest moon gets its name from the fullish moon's habit of rising relatively earlier than usual from night to night at this time of year, giving farmers extra light to bring in their crops.

October is a good time to study the geometry of the sky. Not its 3-D structure¿let's leave that to physicists. But looking to the west, you'll see the Summer Triangle of Vega, Deneb and Altair still shining brightly. Front and center in the south is the Great Square of Pegasus, the mythical horse. Below the Great Square, the Circlet of Pisces is easy to find in a dark sky. Finally, a second triangle appears east of the Great Square, just below Andromeda, as the appropriately named constellation Triangulum.

Planets make themselves scarce this month, and not just because Pluto has fallen from their ranks. Mars and Venus are both hidden as they pass directly behind the sun¿Mars on the 23rd and Venus on the 27th. Between those days, our two nearest neighbor planets, the sun and the Earth will be arrayed in a nearly straight line. After emerging from behind the sun, Mars will be a morning planet; that is, it will appear to the right of the sun and both rise and set ahead of the sun. But Venus, after its passage, will move to the evening sky, meaning it will appear to the left of the sun and will rise and set after the sun.

Hold on¿all the planets move in the same direction around the sun. Therefore, it seems that after emerging from behind the sun, Mars and Venus should move together, to either the right or the left, either leading or trailing the sun on its journey across the sky. But there's a big difference: Venus is closer to the sun than we are, and Mars is farther, and this makes them move in different directions from our point of view.

To visualize the situation, imagine the sun at the center of a round running track. You are running in the middle lane, with speedier Venus on the inside track and pokier Mars on the outside. Now suppose both Venus and Mars are half a lap away from you, directly behind the sun on the far side of the track. If you keep looking toward the sun, you'll see Venus gaining on you, and you will perceive her moving to the left¿or, if you prefer, from the 12 o'clock position toward 11 o'clock. At the same time, you will gain on Mars, and he will seem to move to the right, or toward one o'clock.

If you like figuring out stuff like this, imagine the opposite situation: Venus and Mars in line with Earth on the same side of the sun. Using the racetrack analogy, you'll see Venus move into the morning sky (toward five o'clock) and Mars into the evening sky (toward seven o'clock).

Saturn, as dim as it gets this year, rises well before dawn just ahead of Leo. As the month progresses, Saturn and Leo rise earlier, with Saturn drawing nearer to Regulus, Leo's brightest star. Jupiter, the only other easily visible planet, lingers in the southwest after sunset but drops nearly out of sight by month's end.

Pluto, now a "dwarf planet," trails Jupiter as both head toward the western horizon. For a story about the rise and fall of Planet Pluto featuring University of Minnesota astrophysicist Terry Jones (with guest appearance by planetary physicist Robert Pepin), visit www.umn.edu, type "A planet by any other name" in the "search" bar and click on the first result.

Meteors fly like sparks from the club of Orion during the Orionid meteor shower, which is best seen on the 21st. The show starts around 10 or 11 p.m. Moonless conditions make this an ideal year for the Orionids, although meteors may come only every two minutes or so.

October ends, of course, on Halloween, with a waxing gibbous moon to light the way for trick-or-treaters as they complete their appointed rounds. Halloween began as one of four Celtic cross-quarter days, falling halfway between an equinox and a solstice. It marked the beginning of the dark half of the year, when evil spirits held sway. After six months' confinement, the spirits came out on Halloween, or Samhain (rhymes with COW-en), as it was called, and had to be warded off with night lights and offerings of food.

Standard time returns at 2 a.m. Sunday, the 29th. Clocks should be set back an hour. The changeover shifts an hour of daylight from the evening to the morning.