Quantification of Evening and Morning Twilight Angle in Malaysia: A Suburban-Rural Areas Comparison - sersc
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International Journal of Advanced Science and Technology
Vol. 29, No. 3, (2020), pp. 14995 – 15001
Quantification of Evening and Morning Twilight Angle in
Malaysia: A Suburban-Rural Areas Comparison
Ngadiman, N.F.1,2,3*, Shariff, N.N.M.1,2,3 and Hamidi, Z.S.1,3,4
1
Islamic Astronomy & Solar Astrophysics (IASA) Universiti Teknologi MARA,
Shah Alam, Malaysia
2
Academy of Contemporary Islamic Studies (ACIS) Universiti Teknologi MARA,
Shah Alam, Malaysia
3
Institute of Science (IOS) Universiti Teknologi MARA, Shah Alam, Malaysia
4
Faculty of Applied Sciences, Universiti Teknologi MARA, Shah Alam, Malaysia
1
nfn132@gmail.com, 2nnmsza@uitm.edu.my, 3zetysh@uitm.edu.my
Abstract
Studies on the effects of sunlight scattering during twilight phenomena yield
information about solar altitude or twilight angle value for determination of Isha and
Subh prayer times. This study set out to a quantify the evening and morning twilight angle
based on long time observation and compare the similarities as well as dissimilarities of
results obtained between suburban and rural areas. Hence, data acquisition was
performed in selected areas throughout Malaysia from April 2018 to September 2019 by
recording sky brightness readings during twilight every day, utilizing Sky Quality Meter
Data Logger (SQM-LU-DL). Good data visualization criterion and third-degree
polynomial models have been applied to the collected data to obtain the best data. This
study has found that the tendency values acquired from observation data appear to be in
the range -18o for evening twilight angle and -17o to -18o for morning twilight angle.
Besides, there is no specific pattern that distinguished between twilight angle variations
in suburban and rural areas. The only discernable difference is that suburban area with
higher population density has given a wider range and variability in twilight angle
values.
Keywords: twilight angle, Sky Quality Meter, polynomial measurement
1. Introduction
The existence of the atmosphere surrounding the solid Earth prevents the sky from
completely dark or bright during the transition of sunrise and sunset. This is due to the
atmospheric molecules that scatter the sunlight, inducing illumination, or a faint glow at
the sky. This phenomenon called twilight which directly associated with Muslim prayer
times determination, Isha and Subh whereby shafaq (the afterglow of sunset at a certain
angle) and fajr (first light before sunrise) both are astronomical phenomena part of
evening and morning twilight stage respectively [1]. As stated by U.S. Naval
Observatory, twilight is classified into three perceptible phases in proportion to a solar
altitude below the horizon or named as twilight angle, i.e., a) civil twilight (-6oInternational Journal of Advanced Science and Technology
Vol. 29, No. 3, (2020), pp. 14995 – 15001
definitive as the disperse and intensity of light on the horizon incrementally changes
over time, resulting in the pinpoint phenomena determination open to interpretation
(ijtihad). Hence, various twilight angle values that have been used for both prayers
throughout the world. The variation of this parameter value is also attributable to the
different duration of twilight depending on the latitude of a location in which the
parallactic angle for the diurnal path of the Sun relative to the horizon will vary at
different locations [3]. Consequently, it affects how quickly or slowly the Sun’s
movement below the horizon. For equatorial regions such as Malaysia, the Sun’s
path nearly parallel to vertical to the horizon caused the twilight period in this
country shorter than higher latitude regions.
Natural phenomena such as twilight have their dynamism, which requires
continuous study to observe the consistency of its pattern over time. In Malaysia, we
can see few quantitative types of research utilizing various equipment besides using
naked eyes, such as Sky Quality Meter photometer [4], [5], [6], [7], [8], [9], [10],
[11], [12], handmade photometer [13] and DSLR [14] in measuring the morning
and evening twilight angle. In some way, all these studies indicate the different
value and wide-ranging of twilight angle, ranged from -17 o to -20o . Thus far, there
are no satisfactory studies of the observational and data collection to justify the
twilight angle values in the context of Malaysia. This study, therefore, set out to a
quantify the evening and morning twilight angle based on long time observation and
compare the similarities as well as dissimilarities of result s obtained between
suburban and rural areas. Besides, this study also serves as a scientific document for
quantitative review of evening and morning twilight in Malaysia.
2. Methodology
2.1. Data Collection
Data acquisition was performed in four (4) different locations throughout
Malaysia involving the east and west of Peninsular Malaysia and Sabah from April
2018 to September 2019. These locations represent suburban and rural areas as
shown in Table 1. This classification is based on the Department of Statistics
Malaysia, which defines urban areas as having a combined population of 10 000 or
more at the time of the Census 2010 with at least 60% of the population were
involved in non-agricultural activities.
Table 1. Location of Data Collection
Type of
Observation Site Coordinate Elevation (m) Population of District
Location
Selangor Observatory 3.8192° N,
7 103153
(BCS) 100.8143° E
Suburban
5.8082352° N,
Kuala Besut (BST) 7 140952
102.5814666° E
4.177633° N,
Kuala Lipis (LPS) 66 85341
102.08035° E
Rural
6.021572° N,
Kundasang (PLI) 1642 94092
116.603637° E
Several methods currently exist for the twilight angle measurement. In this study,
sky brightness during twilight phenomena was recorded each minute in magnitude
per square arc second (MPSAS) unit every day utilizing portable photometer, Sky
Quality Meter Data Logger (SQM-LU-DL). The selection of this equipment is since
there is a good agreement between the photoelectric measurement and the naked eye
[15]. This photometer which composed of high sensitivity TAO TSL237S sensor
ISSN: 2005-4238 IJAST 14996
Copyright ⓒ 2020 SERSCInternational Journal of Advanced Science and Technology
Vol. 29, No. 3, (2020), pp. 14995 – 15001
was set to 10 MPSAS thresholds and oriented towards the zenith because of the least
contaminated area of the sky.
2.2. Data Selection
The selection of the best data is based on good data visualization and root mean
square error value (RMSE). Theoretically, a graph with good data visualization will
form a light curve, whereas, for evening twilight, the brightness of the sky would
increase as the Sun is getting away from the horizon and would reach a point of
maximum brightness which would continue to remain constant throughout the night.
On the contrary for morning twilight, at nighttime, the brightness is constant
because the sunlight no longer affects the sky and the night sky brightness becomes
decreases as the Sun approaches the horizon.
To support the criteria of good data visualization, the sky brightness data will be
filtered by fitting the data to the third-degree polynomial models. The polynomial
function as following:
(1)
where y is the recorded night sky brightness readings during twilight in MPSAS unit, x is
the altitude of the Sun or twilight angle in degree unit and a, b, c, d are the polynomial
parameters. We used the RMSE as standard statistical metric to select the good data,
which a good light curve will produce RMSE valueInternational Journal of Advanced Science and Technology
Vol. 29, No. 3, (2020), pp. 14995 – 15001
Figure 1. Percentage of Good Sky Brightness Data at Each Observation
Site
It is important to mention that Figure 2(a) and (b) are the identical pattern of
night sky brightness during twilight phenomena at each observation site on the same
date (note that the triangle marks in the graph represent suburban area and circle
marks for rural area). It can be seen during evening twilight the sky brightness
readings continue increasing as the Sunsets until reaching the maximum point and
remain steady throughout the night. Similarly, the graph readings during morning
twilight will portray the same pattern in which the brightness of the sky starts to
decrease gradually from the constant readings as the Sun begins to move closer to
the horizon.
Further data visualization analysis showed that in Figure 2(a), the sky at BST
reached the maximum brightness after 75 minutes of sunset (twilight angle = -
16.88o) which is earlier than two other sites, BCS and LPS, 79 minutes (twilight
angle = -18.68 o) and 78 minutes after sunset respectively (twilight angle = -18.53 o).
Meanwhile, Figure 2(b) indicates that LPS began to decline 75 minutes before
sunrise (twilight angle = -18.77 o), followed by BST and PLI which dropped 71
minutes before sunrise (twilight angle = -17.82 o and -17.84 o respectively) What is
striking about Figure 2 is no significant difference was found between obtained
twilight angle at suburban and rural areas. Apparently, rural areas are less exposed
to light pollution supposedly would detect the disappearance of shafaq later and
recognize the appearance of fajr faster than suburban areas. This has made the
acquired data do not seem to be in line with the theory. Howbeit, it is indisputable
that the overall sky brightness readings in rural areas are darker than suburban areas.
(a) (b)
18.9
18.6
18.3
MPSAS
18
17.7
17.4
17.1 BST BCS LPS
16.8
30 40 50 60 70 80 90 100
Time after sunset (minutes)
Figure 2. Sky Brightness During Evening Twilight and Morning Twilight on
19th June 2018 and 1st May 2019
ISSN: 2005-4238 IJAST 14998
Copyright ⓒ 2020 SERSCInternational Journal of Advanced Science and Technology
Vol. 29, No. 3, (2020), pp. 14995 – 15001
As for the statistical comparison of evening and morning twilight angle value
from each observation site. The results obtained from this measurement are
summarized in Tables 3 and 4. For overall 529 observations days, the mean values
of evening twilight angle measured in suburban areas were -18.22 o (SD=0.67) and -
18.3 o (SD=0.8) for BCS and BST, showing 0.1 o -0.4o (equivalent to 0.7-1.6 minutes)
slightly faster in detecting disappearance of shafaq, compared to the rural twilight
angle of -18.63o (SD=0.74) and -18.47 o (SD=0.81) at LPS and PLI. The pattern
shown during the morning twilight was unalike, with all suburban and rural sites
detecting the decrease in sky brightness at an average value of -18.1 o (equivalent to
about 72 minutes before sunrise), except for BST, which averaged -17.92o
(SD=1.23), slightly late in detecting the appearance of fajr. Considering the
acquired mean values, the tendency for evening and morning twilight angle
distribution is likely to be around -17 o to -18o .
Nevertheless, based on the observational data, the values of both twilight angles
were seen ranged from -20o to -14 o. There is no specific pattern that distinguished
between twilight angle variations in suburban and rural areas. What can be deduced
is that BST is the site with the widest range for both twilight angles. This is likely
due to the highest population density compared to other sites which resulting the
variability in twilight angle values.
Table 2. Descriptive Analysis of Measured Evening Twilight (o) at Each
Observation Site
Results BCS BST LPS PLI
Minimum -20.2172 -20.5527 -20.5647 -20.5269
Maximum -16.88 -15.2487 -16.0698 -16.6441
Range 3.3372 5.304 4.4949 3.8828
Mean -18.2296 -18.3097 -18.6327 -18.4652
Standard Deviation (SD) 0.667227 0.815565 0.737193 0.809961
Standard Error 0.108239 0.081967 0.074468 0.120742
Table 3. Descriptive Analysis of Measured Morning Twilight (o) at Each
Observation Site
Results BCS BST LPS PLI
Minimum -19.3598 -20.0157 -19.23 -19.7093
Maximum -16.9563 -14.5302 -16.5184 -16.2637
Range 2.4035 5.4855 2.7116 3.4456
Mean -18.176 -17.92391493 -18.10213188 -18.16737105
Standard Deviation (SD) 0.727426485 1.227708058 0.666131588 0.741452055
Standard Error 0.181856621 0.149988411 0.08019282 0.085050384
Besides, the pattern of measured data is further strengthened in Figure 3, which
represents the distribution of 0.5° width bars as the frequency of 529 evening and morning
twilight observations from all sites. While the concentration of the measured twilight
angle was delineated in the box-whisker plots graphs. It can be seen in Figure 3(a) that -
18o is the most commonly evening twilight angle observed at each site. The values of -
15o, -16o and -20o at each location had a frequency of less than 3%, except for the
frequency value of the -16o angle at BST which equals 8%. Morning twilight also exhibits
the same twilight angle pattern as depicted in Figure 3(b). The -14o, -15o, -16o and -20o
angles have frequencies less than 7%, not including -16o at BST with a frequency of 10%.
ISSN: 2005-4238 IJAST 14999
Copyright ⓒ 2020 SERSCInternational Journal of Advanced Science and Technology
Vol. 29, No. 3, (2020), pp. 14995 – 15001
Therefore, these clustered bars provide important insight into twilight angle patterns in
suburban and rural areas where values of -15o, -16o and -20o may be measured during
observation, but the frequency is extremely small and sporadic.
(a) (b)
Figure 3. Frequency Distribution of Measured Evening and Morning
Twilight Angle
4. Conclusion
In conclusion, this study has identified that the acquired mean of evening twilight
angle from all sites is within the range of -18 o, while the range for the mean of
morning twilight angle is slightly wider at -17o to -18o . These findings are in accord
with most of the twilight angle values in previous studies. Additionally, there is no
definite pattern that discerned between twilight angle variations in the suburban and
rural areas as indicated by the collected data. What can be inferred is that one of
suburban site, BST, has the widest range for both twilight angles. This is likely
attributable to the highest population density compared to other sit es, inducing the
variability in twilight angle values. Long term research with suitable integration of
various methods and equipment as well as the use of naked eyes on twilight
measurement would help us to establish a greater degree of accuracy of twiligh t
angle value.
Acknowledgment
This work was partially supported by the grant, 600-IRMI/FRGS 5/3 (100/2019),
UiTM grants and Kementerian Pengajian Tinggi Malaysia. Special thanks to the
Universiti Teknologi MARA and University of Malaya for supporting this study.
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ISSN: 2005-4238 IJAST 15000
Copyright ⓒ 2020 SERSCInternational Journal of Advanced Science and Technology
Vol. 29, No. 3, (2020), pp. 14995 – 15001
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Authors
Nurul Fathin Ngadiman, currently pursuing her MA in Contemporary
Islamic Studies besides being Islamic Astronomy & Solar Astrophysics
(IASA) research assistant at Universiti Teknologi MARA, Shah Alam,
Malaysia, who is working on Isha and Subh Light Curve Profile research.
She obtained her BA in Shariah: Islamic Astronomy in 2018 at University of
Malaya (UM). Her degree’s project is about atmospheric extinction and
night sky brightness using photometry technique. Project done at Langkawi
National Observatory, Malaysia.
Assoc. Prof. Dr. Nur Nafhatun Md Shariff received BA in
Shariah: Islamic Astronomy in 2006, MSc in Islamic Astronomy in
2009 and PhD in Science & Technology Studies in 2013 at University
of Malaya. She is currently at Universiti Teknologi MARA, Shah Alam,
Malaysia. Her research area includes astronomy (light pollution
monitoring, Isha and Subh prayer time determination, hilal – Islamic
new moon research), solar studies (solar monitoring – optical and
radio) and sustainability (sustainable agriculture).
Assoc. Prof. Dr. Zety Sharizat Hamidi received BSc in Physics in
2004, MSc in Physics in 2008 and PhD in Solar Astrophysics in 2014
at University of Malaya. Currently Senior Lecturer at Universiti
Teknologi MARA, Shah Alam, Malaysia. Her research area includes
solar astrophysics, Islamic astronomy, climate change and environment,
antenna, radio and optical astronomy.
ISSN: 2005-4238 IJAST 15001
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