SAG: Stars and Galaxies Vol. 6, id. 10, 2023 December 28
c 2023. Center for Astronomy, University of Hyogo.
⃝

Long term variability of light-pollution in Bisei Town
Ryosuke I TOH1, 2∗
and
Syota M AENO1
1

2

Bisei Astronomical Observatory, Bisei, Ibara, Okayama, 715-1411, Japan
Hiroshima Astrophysical Science Center, Hiroshima University, Higashi-Hiroshima, Hiroshima 739-8526, Japan∗
itoh@bao.city.ibara.okayama.jp(Received 2023 November 4; accepted 2023 December 15)

Abstract
Bisei town, located in the west part of Japan, is known as a place where the local community protects its beautifulnight sky from light pollution through its unique ordinances and the efforts of the local residents. It is also important
to monitor in the quantity and quality of light pollution for precise measurement of astronomical observations.
The ﬂuorescent lamps in the city were gradually replaced with light emitting diode (LED) lamps. In order toinvestigate how much light pollution is affecting astronomical observation, we analyzed the archival photometricand spectroscopic data taken by the 101 cm telescope that has been installed at Bisei Astronomical Observatory(BAO) since 2006. As a result, we found that there is no signiﬁcant variability in sky brightness in optical bands,
but from spectroscopic observation, we observed a blue humps around 4500 Å originating from LED lights from2017 to 2023. The brightness of light pollution observed at BAO is not varied but the origin of light has graduallychanged from ﬂuorescent lamps toLED lamps.
Key words: light pollution
1. Introduction
Light pollution originates from the excessive or improperly
arranged usage of artiﬁcial light. It is suggested that it affects not only astronomical observation but also ecosystems(e.g. Wesołowski 2023; Davies et al. 2013; Holker et al. 2021).
Kyba et al. (2017) reported that Earth’s artiﬁcially lit outdoor
area and brightness rate grew by ∼ 2% per year, from 2012
to 2016. The author also claim that only a few countries keep
or decrease the sky brightness. For astronomical observation,
having a bright sky as a background affects the limiting magnitude that can be observed. This has a signiﬁcant negative
impact on the observations of faint, distant galaxies etc. The
impact is signiﬁcant not only from a scientiﬁc perspective, butalso from an educational one. The experience of observing thestars with the naked eye is one of the ﬁrst step toward scientiﬁccuriosity, but that opportunity is greatly reduced by light pollution. In order to minimize the effects of light pollution, it is
crucial to conduct studies on its nature over time.
Sky brightness has been measured in various ways: by photographing the night sky with a digital camera ﬁtted with awide-angle lens; by using SKy Quality Meter with Lens (SQML) provided by Unihedron; by a CCD camera attached to atelescope, and so on. Each has its advantages and disadvantages (ease/accuracy of measurement, etc...), but in this paper,
we use the third method, which accurately measures brightness. When monitoring the variability of light pollution, notonly imaging is important but also the spectroscopy of the sky
especially since the lights in the city have changed over the past

decade from ﬂuorescent lamps to light emitting diodes (LED).
Lights emitted from the city are scattered by the atmosphere and can be observed as light pollution even from a
distance of 100 km (Kocifaj et al. 2021). There are a variety of physical processes that cause diffusion of light pollution; Mie or Rayleigh scattering, depending on he size/shapeof aerosols/particles, and not only scattering but also someaerosols/particles could be absorbers. However, the theoreticalprediction of light pollution is difﬁcult since there are several
types of light sources, as well as the presence of ﬂoating clouds
and atmospheric inhomogeneity (Kocifaj et al. 2023a). There
are several studies of light pollution, especially by photometricobservation (e.g. Onoma et al. 2009), but spectroscopic observations alone are not enough in Japan, one reason for thismay be the lack of spectrometers among amateurs(Iye et al.
1991; Kato & Kongou 2020). As the ﬁrst step toward unraveling this mechanism, it is crucial to measure the spectrum oflight.
Bisei Astronomical Observatory (BAO) is a public observatory that opened on 7 July 1993, and is located in Bisei Town,
Ibara City, Okayama Prefecture in Japan. The town is also
known as the ﬁrst local government in Japan to enact a light
pollution prevention ordinance in 1989. The observatory has
an optical telescope with a 101 cm aperture, and is open to
the public on weekends and at night, attracting many visitors.
The telescope is also open to amateurs on weekend evenings.
BAO has been using the 101 cm telescope for 30 years since
its opening, and has accumulated a large amount of imaging
observation data. By reanalyzing these data, it is possible to2

R. Itoh and S. Maeno

accurately measure the sky brightness and spectrum from past
to present.
2. Observation
The main telescope of BAO is a 101 cm optical telescopemounted on an equatorial mount. The telescope has fourtypes of focus; the main Cassegrain focus is for visual observation, the north-folded Cassegrain focus has an opticalimaging monochromatic CCD for photometry, the west-folded
Cassegrain focus is where a (commercial) digital camera can bemounted, and the south Cassegrain focus has an optical spectrograph. Our observatory has observed a wide variety of objects; super novae, novae, active galactic nuclei, electromagnetic counterparts to gravitational wave events, variable stars,
minor planets, artiﬁcial satellites and so on. We have a vast
amount of archived data, most of which are available for themeasurement of sky brightness. From these data, we are able
to carry out the long-term monitoring of light pollution.
2.1. Photometry

Vol. 6,

resolution mode in order to investigate the variability in thewide wavelength range. The data was selected based on thefollowing criteria: (1) Altitude of the Sun is less than -22 degrees (end/before the astronomical twilight); (2). Altitude of
the Moon is less than 0 degrees; (3) Altitude of the target staris more than 45 degrees. In addition to the above, we selected
data which had high S/N that deﬁned as the ratio of continuum
ﬂux between 4800 and 5200 Å and its variance, and low countratio of sky brightness. Finally, we obtained 18 spectroscopic
data from October 2006 to July 2023.
The data reduction was performed under the standard procedure of CCD spectroscopy; bias subtraction, ﬂat-ﬁelding, cutting out the sky region, conversion to one-dimensional spectrum, wavelength calibration and ﬂux calibration. We selecteda sky region which did not contain the starlight of spectroscopic
data (regions at least 30 arcsec away from star) with a 50-pixel(∼ 40 arcsec) width against the direction of the dispersion axis,
which determined from the limitation that no distortion or rotation of the emission lines caused by the optical system. Then
we took the median value for the spatial direction axis and obtained one-dimensional data. The calibration of wavelength
was performed with atmospheric emission lines (typically 10
lines) for each frame. The ﬂux was calibrated using observations of a spectrophotometric standard star. Note that absolute
ﬂux were not reliable since collected spectrum are taken bydifferent observers with different method (altitude, time interval between observed frame and standard star, weather condition and so on). We used two different CCD; ANDOR DU440BV (2006–2021) and DU-940P-BV (2021–2023) for the
spectroscopy with the same optics, and also conﬁrmed that thedifference of the wavelength sensitivity was quite small andtherefore negligible (difference is less than ±2% between 4000
and 7000 Å).

Photometric observation of the sky was performed with amonochromatic CCD camera (SBIG STL-1001E) ﬁtted to the
101 cm telescope from 2008 to 2021. We selected data withthe following conditions: taken (1) with object altitude over
60 degrees; (2) with B-, V- or RC -band ﬁlter; (3) when Moon
altitude is less than 0 degrees; (4) when Sun altitude is lessthan -22 degrees (end/before the astronomical twilight); and(5) when the atmospheric transparency is more than 40% (typical atmospheric transparency is ∼ 60% at BAO). The data reduction involved standard CCD photometry procedures - darkcurrent subtraction, ﬂat ﬁelding and aperture photometry. We
calculated the zero-magnitude for each frame with the stars
cataloged in UCAC-4 within the ﬁeld of view. We used theAB magnitude system for the ﬂux calibration. For the calculation of sky brightness, we calculate the median from the valuesof all pixels in the FITS images, then we converted the countvalue to the unit of magnitude arcsec−2 using zero-magnitude
and pixel-scale for each image. We did not apply to any starmask but conﬁrmed that even in large clusters such as M13, we
found that whether or not a mask is applied has little effect on
the results (less than 1%). We also conﬁrmed that the standard
deviation for the difference between our measurement of thestar ﬁeld and the cataloged value in UCAC-4 was ∆V ∼ 0.18
[mag] for the CCD + 101 cm telescope. This value was added
to the photometric errors. The atmospheric transparency arecalculated by the ratio of observed brightness of stars used in
the ﬂux calibration and theoretically predicted photon amountfrom telescope optics. It also contains device efﬁciency (quantum efﬁciency of CCD, reﬂectance of Mirrors and so on) but
the main contribution of variability for the value is atmosphericcondition.

Figure 1 shows the variability of sky brightness in B, V, andRC band from 2008 to 2021 and Figure 2 shows the distributionof sky brightness for each band. The blue, green, and red data
points indicate B, V, and RC -band sky brightness, respectively.
The median values of sky brightness are B ∼ 20.9, V ∼ 20.3,
and RC ∼ 19.8 [mag arcsec−2 ] and standard deviation of skybrightness are B ∼ 0.5, V ∼ 0.5, and RC ∼ 0.6.
We examined the data for variability in the distribution ofsky brightness after every three year period, and found that no
signiﬁcant variability was detected in any year for all B, V ,and RC bands from 2008 to 2021 (Kolmogorov-Smirnov test,
C.L. = 95%). The darkest sky brightness levels were B = 22.0,
V = 22.0, and RC = 21.5 [mag arcsec−2 ]. The sky brightness
darker than 21.0 mag arcsec−2 in the V band accounts for about
5% of the total number of moonless nights observed.

2.2. Spectroscopy

3.2. Spectroscopy

The instrument has two different gratings (n = 300, 1800[mm−1 ]) thus two spectral-resolutions can be selected, one
low-resolution (R ∼ 1,500, λ = 4000–8000 Å) and the othermiddle-resolution (R ∼ 15,000, λ ∼ 300 Å width). We selected the data from our archival data taken with long-slit low-

Figure 3 shows all spectra of the sky from 2006 to 2023.
Spectra of the LED and ﬂuorescent lamps installed inside thedome was also taken with same apparatus and is shown forcomparison. For the sake of visibility, the ﬂux levels were
shifted. Figure 4 shows the close-up views of spectra around

3. Results
3.1. Photometry

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Long term variability of light-pollution in Bisei Town

3

Table 1. Spectroscopic observation

Date (UTC)2006

10/29 18:09

46.6

63.2

-40.4

0.6

2007

02/21 16:50

54.2

175.3

-58.2

0.5

2008

04/05 16:14

60.1

237.4

-46.1

2009

08/31 18:35

45.7

149.5

-24.4

0.2

2010

08/17 14:24

70.5

84.0

-40.8

0.1

2011

12/27 16:40

51.5

15.9

-66.6

0.0

2012

08/17 14:49

76.1

86.2

-41.9

2013

06/16 15:50

58.4

157.3

-31.0

2014

01/06 19:05

66.8

191.1

-38.0

2015

08/22 16:31

53.2

78.6

-39.7

2016

03/05 14:36

56.3

37.0

-59.6

2017

11/11 14:04

76.8

54.5

-70.1

2018

05/19 17:08

55.3

75.9

-27.7

2019

03/08 15:43

74.6

124.8

-59.5

2020

11/21 18:11

51.4

182.7

-43.8

2021

01/17 13:50

64.4

300.5

-66.5

2022

10/23 14:00

66.8

336.2

-64.3

2023

07/10 13:12

87.7

338.6

-26.5

a

telescope altitude [deg],telescope azimuth angle from the North to East [deg],c
Sun altitude [deg].
b

B
V
R

Sky brightness[mag/arcsec2]

18
19
20
21
22
2006

2008

2010

2012 2014 2016
Date [years]

2018

2020

2022

Fig. 1. Variability of sky brightness in B, V and RC bands from 2008
to 2021 taken at BAO. The data represents the darkest sky brightness
for that one night.

R
V
B

0.8

Altitudea Azimuthb Sun Alt.c

Year

N

0.7

0.4
0.3

16

17

18
19
20
21
22
Sky brightness [mag/arcsec2]

23

24

Fig. 2. Normarized histogram of sky brightness in B, V and RC bands
from 2008 to 2021 taken at BAO.

4500 Å and 5700 Å, which corresponds to the LED bright partand lines from the ﬂuorescent (Hg) lamps and sodium vapor(Na) lamps. For the spectrum taken in 2006 (top brown line),
strong Hg (4358, 5460, 5770, and 5791 Å), Na (5683 + 5688
Å), is seen and no hump structure around 4500 Å is visible.
With the passing of time, a hump structure is seen around 4500
Å, and the Hg and Na line seems to decrease. Strong linesof Y(P, V)O4 :Eu3+ (around 6200 Å, Kocifaj et al. 2023b),
also originating from the ﬂuorescent lamps were also visible inearly observation and decreased in late observation. The characteristic hump around 4500 Å is also visible in the spectra of
the LED light (bottom blue line in ﬁgure 3) and therefore, theorigin of this hump is thought to be due to the LED lamp.
In order to investigate the transition of light sources, we derived the equivalent width (EW) of the blue hump around 4500
Å, Hg, Na, and [O I] lines for each spectrum and ﬁgure 5 shows
the time variability of these EWs. In order to avoid the contamination of the Hg 4358 line for LED hump, we ﬁtted the
4000–5000 Å (except 4300–4400 Å) spectra with a Gaussianfunction and linear straight line. EWs and errors of LED humpwere calculated based on the integration of Gaussian derived byspectral ﬁtting and propagation of ﬁtting error (1-sigma). If thedifference of chi-square value between linear + Gaussian ﬁtting and linear ﬁtting is less than 3-sigma, we marked the data
as non-detection and calculated the lower limit with searching
the parameter of Gaussian which makes 3-sigma difference of
chi-square. EWs for the other lines were derived by the simple integration of spectra around the target line (width of 30 Å
which covers ∼ 3-sigma region of the lines). No signiﬁcant (3sigma) LED hump was detected before 2017. After 2017, an
LED hump was detected and became more pronounced witheach passing year.
In contrast, the lines originating in the ﬂuorescent lamps (Hg4358, 5460, 5770, and 5791 Å) became weaker in later years.
There is a time lag between the beginning of decreasing of Hgline and appearance of LED hump. Since this hump is widely
spread compared with Hg lines, therefore it is difﬁcult to detectat early phase .

4

R. Itoh and S. Maeno

relative flux

2006
2007
2008
2009
2010
2011
2012
2013
2014
2015
2016
2017
2018
2019
2020
2021
2022
2023
LED
Flourecent

20

15

Equivalent width [Å]

25

0
200
400
600
800

LED(lower-limit)
LED

0
Equivalent width [Å]30Vol. 6,

100
200
300

Hg 4358
Hg 5460
Hg 5770
Hg 5791

400
500

10

2007.5 2010.0 2012.5 2015.0 2017.5 2020.0 2022.5
Year
Fig. 5. Time variability of equivalent width for LED (upper panel), Hg,
Na, and [O I] (bottom panel) line from 2006 to 2023.

5

4000

4500

5000

5500
6000
wavelength [Å]6500

4. Discussion

7000

Fig. 3. Sky spectra of BAO from 2006 to 2023. Different colors represent different years. Spectra of LED and ﬂuorescent lamps taken withsame instruments are also shown for comparison (bottom two spectra).

35
20

25
relative flux

relative flux

Hg 5460[O I] 5577

30

15

10

Hg 5770
NaI 5683+5791+5688
NaI 5893

20
15
10

5

5
0
4000

NaI 5683
NaI 5893[O I] 5577

Hg 43584200

4400 4600
wavelength [Å]

4800

5000

0
5200

5400

5600
5800
wavelength [Å]

2006
2007
2008
2009
2010
2011
2012
2013
2014
2015
2016
2017
2018
2019
2020
2021
2022
2023

6000

Fig. 4. Close up view of spectra around 4500 Å (left panel) and 5700
Å (right panel).

From section 3, we found that there is no signiﬁcant variability of the sky brightness in B, V, and RC bands from the photometric observations, but found the appearance of LED hump
structure around 4500 Å in the spectroscopic observation from2017 highly signiﬁcant (this wavelength is corresponds to the Bband in photometric observation). This appears to be an odd result since the LED hump component should appear in the variability of B-band sky brightness. We also tested the differenceof the sky brightness distribution of the B band from the photometric data taken before 2017 and after 2017. The result wasthat the distribution was quite similar and we found there wasno signiﬁcant difference between the data set (KolmogorovSmirnov test, 95% C.L.). Assuming the same baseline lightintensity in B band, the estimated contribution from the LEDs
is about 40% (corresponding to 0.4 mag) brighter, even when
we take into account the reduction of the Hg 4358 line comingfrom ﬂuorescent lamps. This is clearly inconsistent with thephotometric results in B-band and actually implies that a decrease in the baseline ﬂux of 4000–5000 Å (there is a continuum ﬂux of ﬂuorescent lamps) and the increase of hump structures are occurring simultaneously. As a result, no signiﬁcantvariability in sky brightness can be seen. Note that absolute
ﬂux calibration for spectroscopic observation is very difﬁcultand our results are shifted to make it easy to see.
In Bisei town, the replacement of ﬂuorescent lamps with
LED lamps for the town’s street lighting was started around2011. In 2021, all of the streetlights and outside lamps installed at public facilities were replaced with light-pollutionpreventative LED lamps, which have a light source with a color
temperature of 3000 K or less, and severely limit upward luminous ﬂux. That being said, white LED lamps which include
the blue hump around 4500 Å are still being used and thelight from neighboring cities is very strong. There are threecities with populations over 450,000 within a 50 km radius

id. 10

Long term variability of light-pollution in Bisei Town

around Bisei Town; Fukuyama city (450,000 peoples, ∼ 30km), Kurashiki city (470,000 peoples, ∼ 25 km), and Okayama
city (720,000 peoples, ∼ 35 km). It is presumed that light pollution originating from these cities can be observed from the
Bisei town. Quantitative evaluation of how much light fromthese cities and Bisei town contributes to light pollution observed at BAO by comparison with theoretical atmosphericscattering models is a future work. In order to comparison withtheoretical light pollution models, both observation of absolutebrightness by photometric observation and contribution ratio oflight source (ﬂuorescent and LED) by spectroscopic observation. And now is a transitional period from ﬂuorescent lights to
LEDs, it is important to continue both photometric and spectroscopic observations to investigate the effect of the variationin light source on light pollution.
Observed sky brightness of V ∼ 20.5 mag arcsec−2 corresponds to class 4 (rural/suburban transition) of the Bortle darksky scale (Bortle 2001), and it is consistent with the value calculated with the artiﬁcial night sky brightness at Bisei, reported
in Cinzano, Falchi, and Elvidge (2001). Therefore we could
say that the night sky in Bisei town is not outstandingly dark,
and the sky is affected by light pollution. Since it is difﬁcult
to separate the effect of light pollution from other cities withthe data used in this paper, we could not investigate how Bisei
town s efforts (replacement of city light, light pollution prevention ordinance and so on) are helping to prevent light pollution.
But as mention in above, comparison with theoretical atmospheric scattering models, especially the observations with low
altitude and toward the direction of other urban cities might be
the key to separate the origin of light pollution.
Acknowledgement
We are grateful to the users of 101 cm telescope at BAO whoprovided the sky brightness data. We thank Nobuko Sakagawa
for digitization of vast paper observation logs. We also thankanonymous reviewer and Kazuya Ayani fruitful discussions.
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