Monday, October 03, 2011

About Not Seeing Comet Garradd

Last night I took my binoculars out into the back parking lot and looked for Comet Garradd. I didn’t see it, but I’ll be looking again. Comet Garradd reaches perihelion on December 23, so the comet may be getting a little brighter over the coming months.

This is an interesting situation for an urban astronomer.

Comet Garradd is a dim comet. It never gets very close to the Sun. Even at perihelion it will be roughly around the distance of the orbit of Mars. But at about sixth magnitude, it should be visible to binoculars, even under city skies. However unlike a star which is a point source of light or a planet which is tiny disk, a comet is kind of a little smudge. Because the dim light is spread out against an urban sky which itself is light, the lack of contrast makes a comet harder to see than a point source or disk.

Now, normally, it might not be much fun to try to hunt down a little smudge against a city sky at night. But Comet Garradd is in an almost perfect position for star-hopping. And because of the comet’s orbit, it will remain in a good position—in fact, its position against the stars will even improve—for the next three or four months. That’s very unusual for a comet.

And it’s very tempting for a would-be urban comet hunter. It’s like the comet is teasing us.

I’m going to describe the star-hopping I do to try and track down this comet.

First, here is a finder chart. If you click on the chart, the image will get a little bigger. And here is a link to a story about the comet at Sky and Telescope. The story includes a link to an even larger finder chart.

Here’s how I look for this comet.

First of all, the comet is in the constellation Hercules. Hercules isn’t a hard constellation to find, but there are no really bright stars and here, south of Chicago, I can’t see any of the stars without binoculars.

To find Hercules, I usually start at the very bright star Arcturus and go “up and to the left” through the beautiful Corona Borealis and into Hercules.

But Arcturus sets pretty early now, so I approach Hercules from the opposite direction.

After the Sun goes down, almost directly overhead the brightest star is Vega. If you start at Vega and come down almost due west—just a touch, just a tad, south of due west—the first bright stars you see are the edge of Hercules, the Omicron, Xi and Mu stars. (If I’m reading the Greek letters correctly.)

In the sky, the three stars look almost exactly like a small version of the handle of the Big Dipper, and there is the little star, Nu Hercules, just offset from Xi Hercules as a kind of indicator that you’ve found the right place. When Hercules is in the west, those three stars appear to be almost a vertical line.

In my wide angle binoculars, Omicron, Xi and Mu (along with Nu) all just barely fit into one field. So that’s a great landmark in Hercules.

And as it happens, right now Comet Garradd is almost exactly twice that distance—twice the distance from Omicron to Mu, one binocular field for me—away from Omicron Hercules.

So once I find those stars in Hercules, I can position my binoculars on Omicron and shift them almost directly south two field-widths and know that there—right there—is Comet Garradd. It may be too dim for me to see, but this star-hopping sequence makes looking for Comet Garradd a bit of fun. (Because Nu Hercules is the little star just offset from Xi Hercules, you can even tell which way is south, because you know the comet is in the opposite direction from little Nu.)

And everything is just going to get better.

Comet Garradd may get a little brighter. And over the coming months Comet Garradd will move almost directly in-line with Xi and Mu Hercules, so those two “easy” stars will be like an arrow pointing right at the comet.

Comet Garradd may be dim, but it is right in the neighborhood of some easy stars. The idea is to star-hop over to the easy stars, and use the easy stars to judge the field-width of your binoculars, and the distance on the finder chart to the comet. Then you use background stars as guideposts to carefully shift your field of view from one of the easy stars, one field-width at a time, over to where the comet is.

Then you’ll either see it or you won’t, depending on the sky.

(That’s not exactly true. Sometimes you will see a little shimmering patch, a kind of haze that seems to blink into existence and then out, and all you can do is wonder—“Was that it?!” It probably was, but, really, you have to wait for darker skies or a bigger telescope to make sure.)

I haven’t taken out my four inch refractor. I’m kind of hoping to track down Comet Garradd with my binoculars. But as Hercules sinks lower in the evening sky—the constellation will become a morning object soon, but Comet Garradd still will be there in Hercules in the morning sky—I may buckle down and take out my telescope.

So there’s a comet out there in the sky, just kind of hanging around and daring everybody to find it and have a look.

To anyone who takes up the comet’s challenge and looks for it: Good luck!

And if you see it, blog about it!

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I’ve changed some of the colors
on the blog. Fall colors.

-- Mark

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