Wednesday, February 25
I comet hunted four times this month. Among them, I had a fulfilling session under the wonderful sky condition on Saturday, February 21. Having said so, upon the arrival I noticed higher degree of light pollution than ever in the southeast, the direction of Nobeoka and Hyuga City with the total population of some 170,000.
Location of the site in Takamori town, Kumamoto.
https://www.lightpollutionmap.info/
I observed 29P Schwassmann-Wachmann (magnitude 11.5, diameter 2’, DC=3) and 24P Schaumasse (mag. 10, dia. 7’, DC=4）prior to the session. The estimated magnitudes were not based on formal measurements but determined by intuition. After enjoying the two comets, I observed additional objects. Seeing was rather good and M13 showed a noble brilliance, while each individual star in Omega Centauri flickered with scintillation as if each is asserting something. M106 shines, surrounded by numerous galaxies. I counted seven spiral arms in M101. After enjoying a 40 min observation, I began to comet searching session at 4:30am local time, one hour before the beginning of the twilight.
After the start of the session, I recognized that the light in the southeast, which I believed to be light pollution, was actually the Milky Way in the constellation of Sagittarius. Why could not I notice that in spite of having the same experience every year? It stems from the fact that steadily increasing light pollution year by year. I felt that the sky in the southeast that is in the direction of Nobeoka, the west Kumamoto, and the northeast Oita, all became brighter since 2018 when I moved to Kumamoto and started to visit this observation site.
Variation in light pollution during 12 years at the site in Takamori, Kumamoto Prefecture.
Distribution of light pollution across the entire sky in (a) 2012 and (b) 2024. South is down, east is left. (c) rate of annual change is +2.62%/year.
https://www.lightpollutionmap.info/
Recent version up of Light Pollution Map boosted the function. That reveals light pollution level at the observing site in Takamori, Kumamoto Prefecture deteriorate as I felt, and the rate is 2.65%/year. The rate around Ozu Town where I live is as high as 3.65%, and it had become worse rapidly especially in 2022 and after (data are not shown). It coincides with the rapid development that taken place since Taiwan Semiconductor Manufacturing Company. Light pollution from Nobeoka and Oita City also increase that made the entire sky brighter.
Light pollution at the observation site at Tokamachi, Niigata Prefecture with the altitude of 963 m. Annual increase rate is quite small, +0.65%. (a) Entire sky in 2024, (b) annual variation. https://www.lightpollutionmap.info/
Of all the locations where I conducted regular observation to date, the plateau with the altitude of 963 m in Tokamachi City, Niigata, had the lowest light polluted level. Although the site is inaccessible during winter due to snow, I made two new “discoveries” during limited non-snowy seasons. First, I saw the filament structure and the feature as if a crab was walking moving the legs due to scintillation in M1, the Crab Nebula, for the first time as I wrote in the diary on October 24, 2015. Second, in the observation site I noticed a planetary nebula, IC1295, which I had never observed as described in the diary on May 18, 2016. The annual increase rate of the light pollution at the site is quite small, at 0.65%.
Until now, numerous comet hunters have given up to visual searching because of light pollution, and have been forced to change to photographic methods. The number of visual hunters will continue to decline due to increase in light pollution globally. Even so, I intend to keep visual hunting, because a chance of visual discovery remains.
Saturday, January 31
Although we usually have many days with clear sky from October to December, there were fewer clear skies last year. However, in January I conducted comet hunting 10 times thanks to sudden increase in clear skies. Especially, I comet hunted six consecutive days at predawn from January 14 through 19. There were some more moonless starry nights other than the days I conducted the hunting sessions, but I was to fatigue to search.
On Thursday, January 1, I observed three comets, 3I ATLAS, 29P Schwassmann-Wachmann, and 24P Shaumasse. The transparency was poor on Saturday, 17, which exacerbated light pollution, especially, emitted from Nobeoka City, some 55 km southeast from the observation site. Even if it were not for it, light pollution becomes worse in the past few years due to rapid development in Kikuyo town and Ozu town, east of Kumamoto City, because Tiwan Semiconductor Manufacturing Company (TSMC) has established.
Moon rose during taking down my telescope on Saturday, January 17. The sky was red due to yellow sand from the continent.
On a warm day air temperature was +5°C, while when cold wave came it was as low as -9°C that was the same with the record low at the site. Nonetheless, perceived temperature cannot be determined by the air temperature alone. One feels colder at near 0°C with wind than at -9°C (Monday, Jan. 26) with windless. On Monday, January 26 the amount of snow was large. The depth was around 10 cm at the place where snowmelt has not yet melted.
After completed the comet hunting session on Monday, Janudary 26. Beyond remaining snow, the majestic mountain, Mt. Sobosan, came into view.
On Wednesday, January 28, I started comet hunting with M12, a globular cluster in the constellation of Ophiuchus. After that, M10 and M14 came into the FOV. The next deep sky object I identified was NGC6353! This globular cluster with the magnitude of 9.3 is a memorable celestial object identified just before the discovery of C/2002 E2 Snyder-Murakami. At the time of discovery, I captured another globular cluster, NGC6426, with the magnitude of 10.9, but I overlooked it this session. Although the altitude above the horizon for NGC6426 at the hunting session this time was about 26° that was sufficiently high, I overlooked. The cause was poor transparency and light pollution. If a new comet that had the same appearance and brightness with C/2002 E2 had been in the area of searching, I could not have detected it.
The next object came into the FOV was NGC6712, a globular cluster in the constellation of Scutum. I spotted a possible new comet of 7th magnitude near this cluster on February 26 in 2004, while I was seeping the sky using 15x110 mm binocular. However, it disappeared in the twilight before I confirmed the motion. I believe the object must be a NEO. I reported the discovery, but the next day no one in the world including me could observe the object.
Open cluster, IC4665, in the constellation of Ophiuchus. The image is 60' wide.