8 research outputs found
The photometric observation of the quasi-simultaneous mutual eclipse and occultation between Europa and Ganymede on 22 August 2021
Mutual events (MEs) are eclipses and occultations among planetary natural
satellites. Most of the time, eclipses and occultations occur separately.
However, the same satellite pair will exhibit an eclipse and an occultation
quasi-simultaneously under particular orbital configurations. This kind of rare
event is termed as a quasi-simultaneous mutual event (QSME). During the 2021
campaign of mutual events of jovian satellites, we observed a QSME between
Europa and Ganymede. The present study aims to describe and study the event in
detail. We observed the QSME with a CCD camera attached to a 300-mm telescope
at the Hong Kong Space Museum Sai Kung iObservatory. We obtained the combined
flux of Europa and Ganymede from aperture photometry. A geometric model was
developed to explain the light curve observed. Our results are compared with
theoretical predictions (O-C). We found that our simple geometric model can
explain the QSME fairly accurately, and the QSME light curve is a superposition
of the light curves of an eclipse and an occultation. Notably, the observed
flux drops are within 2.6% of the theoretical predictions. The size of the
event central time O-Cs ranges from -14.4 to 43.2 s. Both O-Cs of flux drop and
timing are comparable to other studies adopting more complicated models. Given
the event rarity, model simplicity and accuracy, we encourage more observations
and analysis on QSMEs to improve Solar System ephemerides.Comment: 23 pages, 5 appendixes, 16 figures, 7 table
Night-sky brightness monitoring in Hong Kong - a city-wide light pollution assessment
Results of the first comprehensive light pollution survey in Hong Kong are
presented. The night-sky brightness was measured and monitored around the city
using a portable light sensing device called the Sky Quality Meter over a
15-month period beginning in March 2008. A total of 1,957 data sets were taken
at 199 distinct locations, including urban and rural sites covering all 18
Administrative Districts of Hong Kong. The survey shows that the environmental
light pollution problem in Hong Kong is severe - the urban night-skies (sky
brightness at 15.0 mag per arcsec square) are on average ~100 times brighter
than at the darkest rural sites (20.1 mag per arcsec square), indicating that
the high lighting densities in the densely populated residential and commercial
areas lead to light pollution. In the worst polluted urban location studied,
the night-sky at 13.2 mag per arcsec square can be over 500 times brighter than
the darkest sites in Hong Kong. The observed night-sky brightness is found to
be affected by human factors such as land utilization and population density of
the observation sites, together with meteorological and/or environmental
factors. Moreover, earlier night-skies (at 9:30pm local time) are generally
brighter than later time (at 11:30pm), which can be attributed to some public
and commercial lightings being turned off later at night. On the other hand, no
concrete relationship between the observed sky brightness and air pollutant
concentrations could be established with the limited survey sampling. Results
from this survey will serve as an important database for the public to assess
whether new rules and regulations are necessary to control the use of outdoor
lightings in Hong Kong.Comment: 33 pages, 13 figures, Environmental Monitoring and Assessment, in
pres
Multiple Angle Observations Would Benefit Visible Band Remote Sensing Using Night Lights
The spatial and angular emission patterns of artificial and natural light emitted, scattered, and reflected from the Earth at night are far more complex than those for scattered and reflected solar radiation during daytime. In this commentary, we use examples to show that there is additional information contained in the angular distribution of emitted light. We argue that this information could be used to improve existing remote sensing retrievals based on night lights, and in some cases could make entirely new remote sensing analyses possible. This work will be challenging, so we hope this article will encourage researchers and funding agencies to pursue further study of how multi‐angle views can be analyzed or acquired
Quark-Hadron Phase Transitions in Brane-World Cosmologies
When the universe was about 10 seconds old, a first order cosmological
quark - hadron phase transition occurred at a critical temperature of around
200 MeV. In this work, we study the quark-hadron phase transition in the
context of brane-world cosmologies, in which our Universe is a three-brane
embedded in a five-dimensional bulk, and within an effective model of QCD. We
analyze the evolution of the physical quantities, relevant for the physical
description of the early universe, namely, the energy density, temperature and
scale factor, before, during, and after the phase transition. To study the
cosmological dynamics and evolution we use both analytical and numerical
methods. In particular, due to the high energy density in the early Universe,
we consider in detail the specific brane world model case of neglecting the
terms linearly proportional to the energy density with respect to the quadratic
terms. A small brane tension and a high value of the dark radiation term tend
to decrease the effective temperature of the quark-gluon plasma and of the
hadronic fluid, respectively, and to significantly accelerate the transition to
a pure hadronic phase. By assuming that the phase transition may be described
by an effective nucleation theory, we also consider the case where the Universe
evolved through a mixed phase with a small initial supercooling and
monotonically growing hadronic bubbles.Comment: 13 pages, 8 figures, accepted for publication in Nucl. Phys.
Commentary: Multiple Angle Observations Would Benefit Visible Band Remote Sensing using Night Lights
The spatial and angular emission patterns of artificial and natural light emitted, scattered, and reflected from the Earth at night are far more complex than those for scattered and reflected solar radiation during daytime. In this commentary, we use examples to show that there is additional information contained in the angular distribution of emitted light. We argue that this information could be used to improve existing remote sensing retrievals based on night lights, and in some cases could make entirely new remote sensing analyses possible. This work will be challenging, so we hope this article will encourage researchers and funding agencies to pursue further study of how multi-angle views can be analyzed or acquired
Multiple Angle Observations Would Benefit Visible Band Remote Sensing Using Night Lights
The spatial and angular emission patterns of artificial and natural light emitted, scattered, and reflected from the Earth at night are far more complex than those for scattered and reflected solar radiation during daytime. In this commentary, we use examples to show that there is additional information contained in the angular distribution of emitted light. We argue that this information could be used to improve existing remote sensing retrievals based on night lights, and in some cases could make entirely new remote sensing analyses possible. This work will be challenging, so we hope this article will encourage researchers and funding agencies to pursue further study of how multi‐angle views can be analyzed or acquired.Plain Language Summary:
When satellites take images of Earth, they usually do so from directly above (or as close to it as is reasonably possible). In this comment, we show that for studies that use imagery of Earth at night, it may be beneficial to take several images of the same area at different angles within a short period of time. For example, different types of lights shine in different directions (street lights usually shine down, while video advertisements shine sideways), and tall buildings can block the view of a street from some viewing angles. Additionally, since views from different directions pass through different amounts of air, imagery at multiple angles could be used to obtain information about Earth's atmosphere, and measure artificial and natural night sky brightness. The main point of the paper is to encourage researchers, funding agencies, and space agencies to think about what new possibilities could be achieved in the future with views of night lights at different angles.Key Points:
Remote sensing using the visible band at night is more complex than during the daytime, especially due to the variety of artificial lights.
Views of night lights intentionally taken from multiple angles provide several advantages over near‐nadir or circumstantial view geometries.
Night lights remote sensing would benefit from greater consideration of the role viewing geometry plays in the observed radiance.EC H2020 H2020 Societal Challenges
http://dx.doi.org/10.13039/100010676Helmholtz Association
http://dx.doi.org/10.13039/501100009318Slovak Research and Development AgencyXunta de Galicia (Regional Government of Galicia)
http://dx.doi.org/10.13039/501100010801National Aeronautics and Space Administration
http://dx.doi.org/10.13039/100000104University of Hong Kong
http://dx.doi.org/10.13039/501100003803Fonds de recherche du QuébecEC Emprego, Assuntos Sociais e Inclusão European Social Fund
http://dx.doi.org/10.13039/501100004895Natural Environment Research Council
http://dx.doi.org/10.13039/501100000270City of Cologne, German