Papers by Abdurrahman Özlem
The vertical color gradient of the twilit sky surface has been analyzed in this article. Through ... more The vertical color gradient of the twilit sky surface has been analyzed in this article. Through the red-to-blue alteration of the hue as moved up the horizon towards zenith, there happens an "achromatic" point, of which the elevation angle depends on the sight altitude besides the atmospheric turbidity. Graphical representation of this dependence enables to make a rough estimate about the altitude of the location of observation above the sea level, by fixing the vertical position of the achromatic band on the sky, either via photographic snapshot or just with the naked eye.
Abstract
A request is submitted to Honorable Minister of Religion of Brunei, Indonesia, Malaysi... more Abstract
A request is submitted to Honorable Minister of Religion of Brunei, Indonesia, Malaysia and Singapore (MABIMS) to deploy one or more fast airplane to chase the moon long enough to sight its development into the first crescent. Actual new crescent for Ramadan 1446 is expected be born over Indonesia on February 28, 2025, which is a rare occasion that allows us to establish the “Miqat”, an appointed time for a specific event, that the Quran directs us to use instead of just any crescent. This experiment will validate the accepted criterion of ARCL 6.4° and Altitude 3° by testing the theory of a single point for first visibility. All under the Faqih direction leading to an acceptable calendar.
To determine the start of any lunar month, the heliacal rising of the Moon is watched by trained ... more To determine the start of any lunar month, the heliacal rising of the Moon is watched by trained observers from favorable locations on the Earth. This paper describes a project for crescent sighting, which is based on the continuous chase of the Moon by an aircraft. The high altitude over the clouds enhances the possibility of discrimination. The trajectory of the plane will follow the special Best Visibility Line such that the earliest detection is assured. The timely progress of the lunar illuminated part, captured by the onboard video instruments, can either be transferred to the terra via a downlink for realtime assessment or saved locally to evaluate offline the validity of any altitude-compensated visibility criterion. Computation of instantaneous flight parameters such as latitude, longitude, speed and heading, necessary for navigation control, is demonstrated through a case study.
Along with this paper has been set forth a photometric heliacal visibility estimation technique, ... more Along with this paper has been set forth a photometric heliacal visibility estimation technique, based on the contrast threshold data by Blackwell. Background brightness onward the sight direction is calculated via the sky model of the author, which processes ample parameters including altitude, haze and ozone concentration and gives reliable spectral values for daylight, twilight and night. Stellar visibility is computed by regarding stars & planets as point sources of light, whereas the lunar visibility is reckoned by considering the illuminated surface of the Moon as an extended object. Lunar brightness is extracted from the topocentric rendering model of the author. Developed demonstration tools to generate spatiotemporal skydome iso-visibility charts and daily visibility diagrams for heliacal rise/set are introduced. Lunar visibility is validated against the semi-analytic Extended Crescent Visibility Criterion with five parameters, established by the author in 2017.
This paper presents a fast and realistic model for the synthetic reproduction of night sky appear... more This paper presents a fast and realistic model for the synthetic reproduction of night sky appearance. The basic elements of the nocturnal scene such as the airglow, the zodiacal light together with its Gegenschein, the integrated starlight as well as the scattered moonlight along with the effect of any artificial illumination are processed separately. Computation of the position and spectral luminosity of the visible planets and stars is demonstrated. Algorithm for a dynamic application of the Purkinje shift has been included. By integrating the model into the former daylight/twilight sky rendering framework of the author, this work enables a seamless infrastructure for a complete generation of 7/24 sky scenes with alterable turbidity and altitude, also comprising solar eclipses. Additionally, transition from false to true dawn is discussed through a deeptwilight analysis.
Lunar crescent visibility maps play an importing role in determining the Earth's regions where th... more Lunar crescent visibility maps play an importing role in determining the Earth's regions where the very young Moon can be spotted in the evening, thereby supporting aided or unaided observations and establish a lunar calendar. Construction of such a map requires heavy computing, since the visibility function based on the selected criteria must be evaluated for each latitude, longitude and time. Especially the preparation of high-resolution maps is very time consuming; execution times exceeding a minute are common. This paper proposes an analytic approach to solve the separator parabola function to achieve a calculation time of less than one second. Furthermore, the dependence of the lunar visibility on the site height above the sea level has been exercised; the generation of a topographic visibility map and the conformance of such an application to the existing criteria and observations have been studied. Benchmark among the available map software and the developed demonstration tool is included.
In embedded systems, the playback of pre-recorded voice data requires special hardware and/or ded... more In embedded systems, the playback of pre-recorded voice data requires special hardware and/or dedicated decoding software. The introduction of high capacity flash memories makes it possible to use higher bit rates and very long playback times. This paper introduces a cheap implementation of the Continuously Variable Slope Delta (CVSD) modulation on an 8bit microcontroller system using Secure Digital (SD) memory card.
The influence of atmosphere on the salat times fajr, maghrib and isha is well-known and thoroughl... more The influence of atmosphere on the salat times fajr, maghrib and isha is well-known and thoroughly studied by Muslim researchers. However, its impact on asr time, though small, had not been a concern. This paper examines the effect of refraction upon the asr time and proposes a simple correction factor into the calculations.
The red & white dusk phenomena, also known as
shafaq ahmar & shafaq abyadh in the religious lit... more The red & white dusk phenomena, also known as
shafaq ahmar & shafaq abyadh in the religious literature, have
been important for the definition of night prayer time. Many
recent observation efforts result in lesser solar depression angles
for the red-to-white evolution of the twilight horizon when
compared to the early assumptions, leading to a debate among
various scholars. Consequently, the underlying physics of this
event has been initially analyzed in this work by applying the
principles of atmospheric optics. The outcome of this
examination reveals that the redness of the dusk horizon
continues nearly until the end of the astronomical twilight.
Thereupon, the human vision mechanism has been elaborated
such that the transformation from red to white has been
discovered to stem from the mesopic luminosity characteristics of
the ocular system, in connection with the Purkinje shift. A
photometric computation of the dusk leap threshold eventuates
to be around 10° of solar depression for clear air, which may
slightly deviate with altitude and ozone concentration.
Besides the simulation of daylight luminous intensity
distribution for interior architecture, s... more Besides the simulation of daylight luminous intensity
distribution for interior architecture, spectral rendering of the
sky hemisphere has been a rather important task for the video
game developers. A veridical panorama under vigorous
conditions in real-time requires a credible sky model as well as a
prompt algorithm. The main purpose of this work is confined to
the generation of an interactive WEB application which can run
on any general purpose device lacking dedicated graphics
processing but still can achieve sub-second response time. A
convincing clear/turbid sky reproduction for both daylight and
twilight was possible thanks to the employment of a single-pass
semi-analytical model without the necessity of any precomputing or recursive numerical integration effort. The highly
parameterized structure enables to experiment many
meteorological variations such as turbidity, altitude or ozone
layer thickness. The shadow of the Earth has been empirically
included into the algorithm in order to construct the Venus Belt.
Some rendered images are compared against real photographs to
evaluate the soundness of the proposed framework. The
imitation of the Martian sky appearance has further been
demonstrated in order to reflect the flexibility of the model to
diverse atmospheric compositions.
Moon rendering has been a rather substantial task for
any night sky simulation, not only for fl... more Moon rendering has been a rather substantial task for
any night sky simulation, not only for flight or vehicle
simulators, but also for planetarium shows. A geocentric Moon
reproduction necessitates the basic lunar orbital parameters in
order to estimate the brightness distribution of the Moon disc,
incl. the earthshine. A topocentric imitation on the other hand,
needs extra variables like the observer geographic location to get
the lunar and solar angular positions in azimuth & elevation in
addition to the local meteorological information, such that the
color, orientation and shape of the Moon as well as the spectral
luminance of the background sky can be figured out. Two maps
with equidistant cylindrical projection, adjusted for vertical &
horizontal libration, are provided for the implementation; the
albedo map visualizes the surface appearance with highlands &
maria, whereas the elevation map is used for shadowing.
Demonstration is performed by the generation of an interactive
WEB application which can run on any general purpose device
lacking dedicated graphics processing but still can draw the
image-mapped lunar disc onto the canvas within less than a
second. For a spatiotemporal fidelity, the solar & lunar positions
on the sky are computed from the entered local time-date and 3D
geographical data (latitude/longitude/altitude). Ozone & aerosol
concentration are used as additional parameters. All types of
lunar eclipses are included in the offered model. The single-pass
algorithm is suitable for daylight as well as twilight, thereby
favoring any verification of various lunar visibility conditions.
Some rendered images are compared against real photographs to
evaluate the soundness of the proposed framework.
Crescent visibility has been a concern for determining
the start of any lunar month. Various cr... more Crescent visibility has been a concern for determining
the start of any lunar month. Various criteria have been offered
by the astronomers since the Babylonians. The indigenous
criterion proposed in this paper uses the two reliable
parameters, altitude and crescent width, and makes it possible to
estimate the visibility for any phase of the Moon, not just limited
to thin crescents. Though very simple, the algorithm presented
here produces rather consistent results. Various visibility graphs
are included. In addition is introduced a tool for demonstration.
The Islamic Calendar is based on lunar visibility.
Many attempts have been made to realize an I... more The Islamic Calendar is based on lunar visibility.
Many attempts have been made to realize an International Hijri
Calendar to merge the dates of fasting and religious holidays
among Muslim countries, based on the hypothesis of “unity of
horizons”. To validate fasting, the “common night” principle is
stipulated, which necessitates global crescent visibility before
local dawn. This paper introduces an isha-based approach with
the argument of tarawih prayer time and shrinks the common
night down to the midnight at most. Hereby a regional sighting
concept with flexible dateline trimming and religious adherence
has been offered.
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Papers by Abdurrahman Özlem
A request is submitted to Honorable Minister of Religion of Brunei, Indonesia, Malaysia and Singapore (MABIMS) to deploy one or more fast airplane to chase the moon long enough to sight its development into the first crescent. Actual new crescent for Ramadan 1446 is expected be born over Indonesia on February 28, 2025, which is a rare occasion that allows us to establish the “Miqat”, an appointed time for a specific event, that the Quran directs us to use instead of just any crescent. This experiment will validate the accepted criterion of ARCL 6.4° and Altitude 3° by testing the theory of a single point for first visibility. All under the Faqih direction leading to an acceptable calendar.
shafaq ahmar & shafaq abyadh in the religious literature, have
been important for the definition of night prayer time. Many
recent observation efforts result in lesser solar depression angles
for the red-to-white evolution of the twilight horizon when
compared to the early assumptions, leading to a debate among
various scholars. Consequently, the underlying physics of this
event has been initially analyzed in this work by applying the
principles of atmospheric optics. The outcome of this
examination reveals that the redness of the dusk horizon
continues nearly until the end of the astronomical twilight.
Thereupon, the human vision mechanism has been elaborated
such that the transformation from red to white has been
discovered to stem from the mesopic luminosity characteristics of
the ocular system, in connection with the Purkinje shift. A
photometric computation of the dusk leap threshold eventuates
to be around 10° of solar depression for clear air, which may
slightly deviate with altitude and ozone concentration.
distribution for interior architecture, spectral rendering of the
sky hemisphere has been a rather important task for the video
game developers. A veridical panorama under vigorous
conditions in real-time requires a credible sky model as well as a
prompt algorithm. The main purpose of this work is confined to
the generation of an interactive WEB application which can run
on any general purpose device lacking dedicated graphics
processing but still can achieve sub-second response time. A
convincing clear/turbid sky reproduction for both daylight and
twilight was possible thanks to the employment of a single-pass
semi-analytical model without the necessity of any precomputing or recursive numerical integration effort. The highly
parameterized structure enables to experiment many
meteorological variations such as turbidity, altitude or ozone
layer thickness. The shadow of the Earth has been empirically
included into the algorithm in order to construct the Venus Belt.
Some rendered images are compared against real photographs to
evaluate the soundness of the proposed framework. The
imitation of the Martian sky appearance has further been
demonstrated in order to reflect the flexibility of the model to
diverse atmospheric compositions.
any night sky simulation, not only for flight or vehicle
simulators, but also for planetarium shows. A geocentric Moon
reproduction necessitates the basic lunar orbital parameters in
order to estimate the brightness distribution of the Moon disc,
incl. the earthshine. A topocentric imitation on the other hand,
needs extra variables like the observer geographic location to get
the lunar and solar angular positions in azimuth & elevation in
addition to the local meteorological information, such that the
color, orientation and shape of the Moon as well as the spectral
luminance of the background sky can be figured out. Two maps
with equidistant cylindrical projection, adjusted for vertical &
horizontal libration, are provided for the implementation; the
albedo map visualizes the surface appearance with highlands &
maria, whereas the elevation map is used for shadowing.
Demonstration is performed by the generation of an interactive
WEB application which can run on any general purpose device
lacking dedicated graphics processing but still can draw the
image-mapped lunar disc onto the canvas within less than a
second. For a spatiotemporal fidelity, the solar & lunar positions
on the sky are computed from the entered local time-date and 3D
geographical data (latitude/longitude/altitude). Ozone & aerosol
concentration are used as additional parameters. All types of
lunar eclipses are included in the offered model. The single-pass
algorithm is suitable for daylight as well as twilight, thereby
favoring any verification of various lunar visibility conditions.
Some rendered images are compared against real photographs to
evaluate the soundness of the proposed framework.
the start of any lunar month. Various criteria have been offered
by the astronomers since the Babylonians. The indigenous
criterion proposed in this paper uses the two reliable
parameters, altitude and crescent width, and makes it possible to
estimate the visibility for any phase of the Moon, not just limited
to thin crescents. Though very simple, the algorithm presented
here produces rather consistent results. Various visibility graphs
are included. In addition is introduced a tool for demonstration.
Many attempts have been made to realize an International Hijri
Calendar to merge the dates of fasting and religious holidays
among Muslim countries, based on the hypothesis of “unity of
horizons”. To validate fasting, the “common night” principle is
stipulated, which necessitates global crescent visibility before
local dawn. This paper introduces an isha-based approach with
the argument of tarawih prayer time and shrinks the common
night down to the midnight at most. Hereby a regional sighting
concept with flexible dateline trimming and religious adherence
has been offered.