Before I get to the main topic, let me just do a quick recap of my observing session last night. I went out to take another look at Jupiter, to see both the Great Red Spot, and a transit of Europa. According to this listing from Sky & Telescope, the Great Red Spot was supposed to be at meridian at 3:24 am Universal Time (the time in London) on December 16, which works out to be 5 hours earlier here in New York, or 10:24 pm on December 15. That means that it would be right smack in the middle of the planet then, although slightly south of the main equatorial bands.
Since Jupiter’s day is about 9 hours and 50 minutes, that means that we can only see the GRS at a maximum for less than 5 hours at a time when it’s on the side that’s facing us. The amount of time is actually significantly less than that, maybe a couple of hours, because when it’s at either extreme limb of the planet, you won’t be able to see it because of the distortion that naturally comes with looking at the edges of a globe. Additionally, the time given at the Sky & Telescope site is merely a prediction, made months in advance. The GRS is a weather system, not an orbiting body subject to the unceasing laws of physics – like Europa’s orbit around Jupiter is. For example, this site predicted that the GRS would reach the meridian 40 minutes earlier than the Sky & Telescope prediction.
I went out to observe at 10:15, so I could begin to get dark-adapted and be ready at 10:24 – or at least by 10:30 or so. However, that was still very early for Jupiter, because it had only risen 10 degrees above the horizon at that time, and was sitting right over the roof of a building across the street from me. To paraphrase George Costanza, the sky was angry last night, my friends. At that level just off of the horizon, Jupiter was a roiling white blob, a big mess, because at that shallow elevation, I was looking through practically the maximum amount of atmosphere. I tried using the 8mm TeleVue on it at 194x, but that was far too much power – there was very little detail to see as Jupiter shimmered and glimmered.
So, I went on to observe some other things, mostly some of the brighter open clusters in and around Gemini and Orion. There was also a second Jupiter event to see, the transit of Europa’s shadow across Jupiter, which, according to this website, was to start at 11:04. As mentioned, orbital mechanics make this a precise figure that can be calculated far in advance with great accuracy. (In the olden days, sailors could set their clocks by astronomical events like this – such as when a moon would cross in front of or behind Jupiter. Knowing the precise time was important to be able to determine your longitude.) But Jupiter was still pretty low in the sky even at that time, and it would take a lot of magnification to see that little black dot cross the face of Jupiter. By about 11:15, when I looked at Jupiter again, I could only get the power up to 103x with my 15mm Paradigm before the surface was a mess again. That was not enough magnification. And no GRS to be seen.
Again, I tried for some other objects. I went Messier hunting, looking for M48 and M46, but they were too low in the sky and behind the building next door. M79, a rare winter globular cluster, was also very low in the sky, but it’s only an 8th magnitude object, and I couldn’t make it out with all the skyglow coming from the south. M44, the Beehive, was pretty, though.
Finally, it was 11:45, close to my witching hour. By now, Jupiter was fairly high in the sky, approaching 30 degrees above the horizon. I started off slow, moving up in power gradually. Then I popped in the TeleVue, and there it was! A tiny little black dot, just off the limb, in between the two equatorial belts! Whoa! Here I was, over 400 million miles away, and I could see the shadow created by a moon less than 2,000 miles wide on the surface of another planet. Incredible! It reminded me of when I saw the Transit of Venus in 2012 through someone else’s scope, but obviously on a much smaller scale. Except, of course, the transits of the Galilean moons occurs slightly more often than once every 122 years. Just slightly, though. This one was all mine, though, through my very own scope.
But was I just deluding myself? Was I actually seeing that dot, or was it just an illusion created by an overactive imagination that happened to know precisely where that dot was supposed to be? I decided to look at something else to be sure, to let the dot have time to move, and to let Jupiter get just a little higher in the sky. Orion was already high in the sky, so M42 beckoned, as always. I’ve been trying to get the E or F stars in the Trapezium, but without luck. The B star is 8th magnitude, and is difficult enough; the E and the F are 10th magnitude and are basically impossible in the City.
Now it was midnight. my last chance to see it, or see if I saw it. Yup, it was there, all right! That tiny little dot had moved further along on Jupiter’s surface. Amazing! And with that, I packed it in for the night.
What is light-polluted astronomy anyway? Well, as I alluded to with my discussion of the Trapezium, it’s doing everything possible with my 5-inch Mak that it’s capable of doing from Manhattan, and more particularly, from the roof of my 16-story apartment building. So, one thing is to try to push my scope and my observing skills as much as I can.
Light pollution takes two forms. The first is the worst – the skyglow that exists for miles all around a major city due to all the lights from that city. These lights wipe out the sky, whether they are from car headlights, street lights, building lights, or even spotlights shining into the sky, like those from the 9/11 memorial that seem to have been shining every night for years now. And if that’s not what that light is, then I have no idea where that giant shaft of light that’s always to my southwest is coming from. But it’s always there, night after night.
Interestingly, yesterday’s New York Times published an article about this very subject just yesterday. The article contained pictures of what the sky would look like from various cities around the world if there were no light pollution. The article says, “In the last half a century, a large percentage of the inhabitants of our planet have ceased to be able to contemplate the beauty of the Milky Way. Almost the totality of its stars has become invisible in our skies.”
This skyglow changes the sky from black to grey. The dark-adapted eye can normally see stars in the sky down to sixth magnitude on a clear dark night. My sky is literally overwhelmed by the skyglow. My limiting magnitude is 3rd. And that means that objects that have their light spread out over an area, like nebula and galaxies, simply disappear. With very few exceptions, like Orion and Andromeda, I can forget about seeing any nebula or galaxies.
Because they’re bright enough, the moon and the planets are easy targets. And because their light sources are pinpoint stars, open clusters, globular clusters, and double stars are still visible even from light-polluted skies. I can also see the brighter planetary nebula, so there is still the Ring and the Dumbbell. However, I tried for the 8th magnitude Crab Nebula two nights ago when conditions were pretty good for me; no dice. But that list of objects is still enough to keep me going for quite a while. All of these objects punch through the light pollution.
I have found that my limiting magnitude from the roof through the scope is about the 9th magnitude on stars, but I’m hoping that the right conditions will someday allow me to get down to 10th magnitude and see those fainter E and F stars of the Trapezium. On the other hand, even though it’s just a smidge brighter than 9th magnitude, I could see the Ring Nebula last month, when it became fully dark right after dusk, when Lyra was high in the sky. Even then, I could just barely see it, and I had to use averted vision to do so. That was a great challenge.
The other type of light pollution is local sources of light right in the vicinity of where you are. These could be the car headlights, the street lights, the building lights, that are near you as you observe. In my case, it’s the lights from my rooftop’s garden, that are right near me, shining right onto me. These lights ruin your night vision by preventing you from becoming fully dark-adapted, a process that takes up to a half an hour. They also prevent your pupil from dilating fully to take in the maximum amount of light.
Light-polluted astronomy includes wearing an eye patch on my non-observing eye to take the strain off of my observing eye. It also includes draping a black t-shirt completely over my head, so that there is no stray light entering my eye from the silver-colored roof and surrounding lights on from the rooftop garden. It means using a dew shield solely to eliminate stray light from entering the telescope’s tube, bouncing around inside and eventually reaching my eye – because there’s no dew here in Manhattan, ancient Indian word for “island without grass.” (I keed, I keed.)
It means observing later at night so that the dust and schmutz floating around in the atmosphere have had a chance to settle out. This stuff causes stars to twinkle and objects to shimmer. The later you observe, the fewer people are driving – and are thus not using their headlights. Also the later you observe, the more people go to bed and turn out their house lights. I find observing after 11 makes a big difference. Unfortunately, they never turn off the street lights – or the spotlights shining into the sky downtown somewhere.
Light-polluted astronomy also means sitting down while you observe. It is more difficult to observe standing. Sitting will relax you, relieving strain, and allow you to observe longer and see more. I’ve read that sitting down while observing is the equivalent of getting two extra inches out of your scope.
As I mentioned in my last installment, it also means optimizing my optical system as much as possible; ordering a new 99% reflective diagonal to squeeze out every last photon; eventually buying an orthoscopic EP to really be able to get the best views of the planets I can; making the Bahtinov or Lord mask I discussed in the last installment to get that focus tack sharp. Yes, it is difficult and challenging, but I love astronomy, so this is all I can do.
Hopefully, what it will also mean will be getting the scope out of the City to less polluted skies. That’s a ways off yet, and not quite what I’m talking about here. But it will be very welcome to be able to get to some dark skies to see what this scope can really do. Although a short blackout wouldn’t be so terrible.
And for all you non-grammar mavens, that hyphen in the title is there because “light-polluted” is an adjectival phrase that is modifying the noun, astronomy. No less than the Chicago Manual of Style backs me up – see page 3, number 2, adjective + participle. Note that light pollution itself is not hyphenated, because it is a noun. So there.