Caoimhín's Content

Today, we are going to talk about light pollution and dark skies, our view of the sky and my view of the sky in particular. We’re going to start with a little bit of a correction.

I generally discuss the night sky as if it was around Bortle Class 5, that’s the level of light pollution that I generally show. For example, looking at the Plough or the Big Dipper, you can see all of it, but the elbow star joining the handle to the bucket, Megrez, is a little tricky to see. It’s visible, it’s maybe on the edge of visibility, but it’s there. It’s the same with the very end of Cassiopeia, most of the W-shape is visible but the last star, Segin, only barely. Similar things are true for a lot of the famous constellations that we’d see during wintertime, constellations like Orion that are very recognizable. Almost all of the important stars are visible, maybe a couple are on the edge of visibility or hard to see. Bortle Class 5 is considered a suburban sky, and I’ve often said where I base my view from, Cork City, is a city, but we’re not a particularly big city. We don’t have that much light pollution, so what would be considered a suburban sky is still pretty accurate for us.

Or at least it was. Maybe the level of light pollution in Cork City has changed by a whole Bortle class in the three years that I have been making these videos, that is certainly not out of the question. There is also the possibility that even three years ago, when I began this website and my YouTube channel, I was a little bit nostalgic for how the night sky was when I was slightly younger. Over the course of four or five years the light level in a location can definitely change by one, or even two or three Bortle classes, and that can go in both directions. It now looks like we have a night sky that is closer to Bortle Class 6, closer to what would typically be called an urban sky. This makes sense, Cork City, as I say, is a city. Under these conditions we’re really missing that star Megrez entirely, even though we can still see the rest of the Plough. It’s the same for Cassiopeia, we’re still seeing most of the W, but that star Segin is pretty much out of view. We’re far from winter now as we come through May, but the winter sky is often more familiar. The nights are longer in winter and the night sky in winter often gets noticed more than the night sky in the summer, particularly here in Ireland, where we don’t get much night during the summer at all.

Looking at the winter sky in Bortle Class 6, looking at Orion we really can’t see Orion’s Sword, anymore, maybe the tip of it, barely, with averted vision. The pelt of the lion, what looks like a bow and arrow being held out in front of Orion, is definitely not visible here either. If we take a slightly closer look we’ll start to see maybe the tip of the sword, maybe one of the stars here in the bow, but that is a level six sky and it is probably closer to the sky that Cork City currently actually has. Still in the winter, now in almost opposite places to the summer, we’re still missing the corner of the Plough and that little bit of Cassiopeia. The sky has changed. Generally, light pollution tends to go up, places get more light polluted generally as time goes on rather than less, although light pollution can be reduced as well. Light pollution has gone down in some places, areas that have specifically strove to reduce their light pollution (I was very close to saying and writing “have striven”, which almost sounds fine to me, but is surely dialectical). Those areas that have tried to have reduced it, it is possible. In the same way that we can add more lights, we can take some lights away or point them more intelligently. The same way we can waste electricity by shooting beams of light into the sky, we can save electricity by directing those lights down and into areas where they are more useful, areas that would need to be lit up anyway.

Bortle Class 6 is of course a slightly worse view, but there are still plenty of areas around the world where the sky is better. Bortle Class 3 also isn’t a level of light pollution that I talk about much. I normally talk about Bortle Class 1 and 2, the fantastic skies visible in Dark Sky Parks, those that are now quite rare, especially ones that are officially recognized. For example, the Iveragh Peninsula around Cahersiveen in County Kerry, and the Wild Nephin Wetlands National Park in Mayo, these are areas where the night sky is even better than Bortle Class 3. However, it’s still definitely a lot better than the urban sky, definitely a lot better than the Cork City sky. I was lucky enough to be on holidays recently in a more rural area where this kind of sky was visible. Not only was all of the Plough visible, with that elbow star Megrez being quite clear, but the general shape of Ursa Major leaps out almost immediately. It’s not a few faint stars that are difficult to spot or something you have to imagine. Around the Plough are the legs and the head of the bear, they’re really visible. It’s the same even with the Little Dipper. All of the Little Dipper is visible, with the arc leading down to the wider section, not just the North Star, Polaris at the tip. All of the W of Cassiopeia is visible, from one end to the other.

I got to see such a sky a few weeks ago, back in April. The Moon wasn’t quite full, but it was quite large. The light of the Moon is affects our view of the sky, it blocks out some of the stars. However, even low in the sky Corvus, was nice and easy to see. All of the Plough was still visible, along with those other stars in Ursa Major. Even when the Moon was right next to Leo, the Sickle was visible. Even practically in Leo, about to occult Regulus, , all of the major stars in Leo were still visible, whereas in a city like Cork, even without the light of the Moon, those extra details are missing. That is the kind of sky in which I saw a couple of the April lyrids on the 22nd, just one or two. They may have been random background meteors, but based on where I saw them in the sky and the direction they were moving, it seemed like their radiant came from more or less the right place. I hope some of you had a good sky for the Eta Aquariids meteor shower that is just after running, just after peaking, by the time this piece goes up.

We’ll very quickly move back into the winter time, to that wintry sky with so many famous things in it. Again, we’re not in a dark sky park, we’re not in a perfectly dark area free of all light pollution. The sky that I am basing this on is a rural town, but a town, not a village, not a hamlet. It is a town with street lights and it’s near some large urban areas, just not as near to them as I am to Cork City, I’m in Cork City. Even though those nearby urban areas are larger than Cork City, a sky of roughly Bortle Class 3 was still visible. Now, of course, I didn’t get to see the winter sky, I wasn’t in this location during the winter. Howere, based on the level of light pollution that was visible during summertime, I believe it is a fair estimation to assume it was the same. Unless a lot of people turn on extra lights during the winter. They might, maybe there is a seasonal fluctuation in light pollution, that could be a thing, but I don’t know it to be the case. Looking at Orion, not only is it clearly visible, two stars in the sword can be spotted. Part of that bow-shape at the front is visible. In fairness, the club is still a little bit tough to pick out, but that’s a tough area of Orion to see. There’s a full V shape of the Hyades and some of the Seven Sisters, I would say five to the naked eye. That’s great, five of the Seven Sisters is certainly better than the two or three that we get to see here in Cork City. Adjusting for the worse light pollution, it could just be one or two, I will need to review that, more honestly, on the next clear night that I get a chance.

A Bortle Class 3 sky is not a perfect dark sky, not a protected park, just an area that’s a bit more rural, a bit further from the large urban areas. That can still vastly improve what you can see the range of things you can see. Scorpius is so much clearer even at the same latitude. It’s almost more noticeable close to the horizon, where we have that atmospheric extinction that is making the stars close to the horizon fainter. The sky is so much darker, we can still see them. We can still see a good bit of the shape of Sagittarius, whereas in Cork City that’s pretty much invisible, and in many cities around the world. Stellarium is showing a faint glow of the Milky Way, but that may be a little much. It’s probably a bit more accurate to show the Milky Way as just barely visible. Still, a much better sky than what we normally see here in Cork City.

That is just a little bit about light pollution and how it’s changed, or at least how I seem to have seen it change. Those are the conditions I will assume when discussing it going forward, but also with just a little bit of hope. You don’t need to go into the middle of nowhere for better skies, even what is on the scale a small improvement, can be a huge improvement in what you see in the sky. I hope that you enjoyed this piece, if you did, please do like it. If you like this kind of content, then please subscribe to this website and my YouTube channel. Thank you very much for reading and hopefully I’ll see you back here next time.