October has always been my favorite month for observing, as the month usually offers the best observing weather of the year. The longer, cooler nights also allow for almost an entire year’s worth of sky to be seen in one night—if you can stay awake!
What’s New
Almost everything in the solar system orbits the Sun in a narrow band called the ecliptic. Think of it this way: If all the planets were orbiting the Sun like marbles swirling on a plate, then the thickness of the plate would define the ecliptic. From Earth’s vantage point, the ecliptic traces a line across our sky that mirrors the Sun’s path (makes sense, doesn’t it?).
Another fact of the solar system is that everything reflects the Sun’s light to some extent. Again, this makes sense, but a lot of people don’t realize it. Take moonlight, for instance. The moon doesn’t “shine.� What we call “moonlight� is simply reflected sunlight.
Orbiting the Sun with the big planets and asteroids are billions of tons of microscopic rock and dust. Like everything else, this stuff orbits in the ecliptic. And, like everything else, it reflects light. Not enough to see directly, but when you look through a lot of it, the collective light adds up to a faint glow that is visible.
So, what does all this have to do with the month of October?
Because the Earth’s axis is tilted 23.5 degrees, then the ecliptic is tilted by the same amount with respect to the equator. It intersects the celestial equator at two points, which we call the equinoxes. Therefore, near the equinoxes (for example, in October) that line (the ecliptic) stands almost perpendicular to the horizon.
This alignment means that the faint glow of all the solar system’s dust is most easily visible, rising straight up from the horizon. Astronomers call this the Zodiacal Light (named for the group of constellations the ecliptic appears to travel through). Ancient mariners called it the False Dawn.
From a dark sky site, with no Moon in the sky to wash the darkness away, the zodiacal light can be seen about two hours before morning twilight as a large, cone-shaped glow standing straight up in the eastern sky. This month, the mornings surrounding Friday, the 20th, are the best ones to try seeing the elusive zodiacal light.
Coincidentally, that same night is the peak of the Orionid meteor shower, our annual gift from Comet Halley. Almost all meteor showers are gifts from a single comet. As a comet orbits the Sun, it leaves behind parts of itself, like a boat that’s falling apart as it sails onward, leaving bits and pieces of itself in its wake. Along comes the Earth, on a slightly different – but still intersecting – path, and plows through this debris field left by the comet. When the Earth smashes through the field, those little pieces of comet (each of which is usually about the size of a Rice Krispy) are vaporized by our atmosphere, and seen as meteors. Some comets leave very thick debris fields, and we have lots of meteors. Some leave very sparse fields, and only few shooting stars can be seen.
Comet Halley came through in 1986, and left behind an “average� debris trail. That translates to about 20 meteors per hour at best, right before sunrise (never sunset) on the 21st. Why meteor showers are always best before dawn is a topic for a future column! But to give you a hint, consider the following questions: When you drive in a snowstorm, does your front windshield get hit by more snowflakes than your rear windshield? Why? How does that apply to Earth plowing through a cometary debris field? And how does “sunset” and “dawn” apply in that analogy?
What’s Old
Believe it or not, there is only ONE Messier object visible in the month of October between R.A.: 22h and 24h. It is the open cluster M-52, located in Cassiopeia. M-52 has a rough triangular shape, with a yellow-orange star at its apex that presents a beautiful color contrast with the other field stars in the cluster. At a declination of +61 degrees, M-52 will be close to zenith at local midnight—an angle that is certainly annoying for you Dobsonian users! The cluster is about 15 minutes of arc, so use low power eyepieces. It looks fantastic in binoculars, too.
There are a few NGC objects of note, however. Try for the “Blue Snowball� in Andromeda, NGC-7662. It’s about half the size of the more famous M-51, and located about 10 degrees or so west of the Andromeda galaxy.
One object I love to look for is NGC-7331, a spiral galaxy in Pegasus. When looking at this galaxy, note all of the fainter smudges and “stars� in the background—most of them are galaxies as well!
Glorious image of NGC 7331 by Bob and Janice Fera.
NGC-7317, 18,19, and 20 are just a half-degree SSW of 7331. These are better known by the name “Stephen’s Quintet�
If you happen to be under extremely dark skies this month, you might try for NGC-7293, the “Helix Nebula� in Aquarius. Using filters will make this very large planetary a cinch to see. I recommend you try to see it without filters first, using binoculars. If you see a patch of sky that’s just a tad brighter than the sky background, you’ve nailed it.
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