Thursday, May 29, 2008
Libra And The Light Polluted Sky
I ended my post yesterday by saying:
I’m really looking forward
to observing Libra . . .
I urge everyone
to check out Libra.
It should be hard to see
but easy to find
in the southern sky.
And it’s worth tracking down!
Last night around sundown I got to thinking about those words and worrying. Saying something is going to be “easy to find” can jinx you.
Last summer when I was looking for Uranus in Aquarius I had planned one sequence of star hops that proved totally impossible in my light polluted skies and I had to abandon that route.
Last night, however, things went reasonably smoothly.
Even while there was still some twilight in the western sky, I found Spica (Alpha Virginis) in the southern sky. This proved to be good news and bad news. Using Spica as a reference point, I knew that Libra was just a few binocular fields to the east. My binoculars capture about a six degree field. I started scanning to the east and relatively quickly found a couple of stars that seemed to be in the proper relationship as Alpha and Beta Librae. And there appeared to be ancillary stars around the pair in roughly the proper layout of Libra.
But the magnitude relationships didn’t seem right and I wasn’t seeing double stars where I should have been seeing double stars.
I just kind of frowned and went inside to have dinner and wonder what might be happening. It is easy to misjudge magnitudes under urban skies. It is easy to miss close doubles with a small telescope under low power. It is also easy to misjudge patterns among dim stars because you can see ‘triangles’ and ‘parallelograms’ just about everywhere you look.
After dinner the sky was much darker and Spica had moved about 15 degrees to the west, bringing more of the eastern sky into view for me. I located the stars that I thought were Alpha and Beta Librae and then I looked around a little more. Nudging my view east a little bit, looking at stars that had been screen by a tree in the alley an hour earlier, I actually said, ‘Whoa!’ out loud as I saw a reasonably bright binocular double with one bright and one dim component.
‘That’s got to be Zubenelgenubi,’ I said, out loud. ‘And if it is, I should be able to look up and to the left slightly, just outside and above the one binocular field anchored by Zubenelgenubi at the bottom, and see Zubeneschamali.’
I panned upward and slightly to the left and there was Zubeneschamali.
I had found Libra.
I had briefly confused some of Virgo’s eastern-most stars and some of the field stars between Libra and Virgo for Libra because I hadn’t really known what to expect, visually, for the two Z-stars. But once I saw them they were clearly brighter than the field stars. And once I saw Zubenelgenubi, its binary arrangement of bright star and dim companion was clear.
Sadly—damn it!—I can’t report seeing any colors in Libra. I didn’t see even a tint of yellow to Zubenelgenubi. I didn’t even see a tint of green (or blue) to Zubeneschamali.
I suspect my location might be playing a big part in my seeing. Looking south from my back yard, our neighbor two doors down has a bright nightlight on the side of his house. Our neighbor across the alley has a bright nightlight. And there are two sodium vapor street lights in our alley. (I can actually read the text of a star map at night in my back yard.) I suspect my eye is adapting to the light pollution and that is masking my ability to see subtle details of the sky.
Last summer when I looked at Uranus and Neptune and did see subtle blues and greens I did much of my viewing late, when the nightlights were out. And I actually constructed a paper glare shield to tape to the front of my telescope.
So, last night I was disappointed that I didn’t see any notable colors in Libra. But I did observe some cool things.
The most extraordinary experience of the evening was late, around eleven or eleven-thirty. As I was scanning east of Zubenelgenubi, following the dimmer stars tracing out the scale pattern of the constellation, I followed the stars all the way into Scorpius. I hadn’t even realized Scorpius had risen, but there in the sky was the unmistakable ‘head’ of the Scorpion and, looking farther east, I saw Antares itself through the trees in the alley.
I was able, for the first time, to really put Libra in perspective, between Spica in Virgo to the west and Antares in Scorpius to the east. Libra falls almost directly between the two and the two Z-stars are bright enough to hold their own, visually, with the brighter stars.
Panning back and forth across the southern sky with my wide angle binoculars from Spica, past the two Z-stars of Libra, to Antares, I actually had the feeling that I was looking up at a ‘sky show’ at the Adler Planetarium in Chicago. Only I was standing in my back yard and everything was real, not a light show projected onto an auditorium ceiling.
That was a pretty cool feeling.
Under skies like mine, wide angle binoculars seem to generate more magic than a telescope.
And the distance from Zubenelgenubi to Antares is an interesting distance. If you star hop from the Z star to Antares and then continue about the same distance past Antares toward Sagittarius, that distance takes you—just before you actually get to Sagittarius—to a view of the center of the Milky Way galaxy, our galaxy.
Sadly, dust lanes prevent us out here in a spiral arm of the Milky Way from seeing the center of our galaxy in all its glory. And—damn it, again!—light pollution here south of Chicago prevents me from seeing the Milky Way at all. But with a visual measure like comparing the distance from Zubenelgenubi to Antares to the same distance past Antares, I can visually ‘mark the spot’ in my mind and know, at least, that I’m looking at something very special even if I can’t really see it.
And—much more to the point—drilling these visually landmarks into my observing mind is very important because when I do get under rural skies (or off-shore ocean skies!), when I do get good seeing, I’ll have a better understanding of where to look than if I simply ‘wrote off’ the light polluted skies and never tried to see what was available.
I want to mention one unexpected thing I saw last night.
Under Spica in Virgo I saw a sequence of six or seven field stars so dim that they didn’t even have Greek letters assigned to them on the charts, just numbers. But they formed a very attractive pattern—they looked almost like an open cluster—all the way down to Gamma Hydrae. It was a beautiful pattern of stars and it led me to a whole new constellation—Hydra (the water snake or sea monster!)—that I hadn’t even expected to observe.
Again, wide angle binoculars generate more fun and magic than a small telescope.
So, that was my adventure with Libra.
It’s a pretty cool constellation that actually looks a little like the diagrams you see on star charts. But, sadly, from my back yard it doesn’t create thoughts of alchemy in my mind with its beautiful colors because all the stars I saw just looked white to me.
But, now that I know where to look, I’m going to come back to Libra under better skies. I don’t give up easily and I almost never forget. I’m going to keep checking Zubeneschamali.
I want to see the green!