Natural Sky Research Articles

Why flying insects gather at artificial light

Explanations of why nocturnal insects fly erratically around fires and lamps have included theories of “lunar navigation” and “escape to the light”. However, without three-dimensional flight data to test them rigorously, the cause for this odd behaviour has remained unsolved. We employed high-resolution motion capture in the laboratory and stereo-videography in the field to reconstruct the 3D kinematics of insect flights around artificial lights. Contrary to the expectation of attraction, insects do not steer directly toward the light. Instead, insects turn their dorsum toward the light, generating flight bouts perpendicular to the source. Under natural sky light, tilting the dorsum towards the brightest visual hemisphere helps maintain proper flight attitude and control. Near artificial sources, however, this highly conserved dorsal-light-response can produce continuous steering around the light and trap an insect. Our guidance model demonstrates that this dorsal tilting is sufficient to create the seemingly erratic flight paths of insects near lights and is the most plausible model for why flying insects gather at artificial lights.

Toward a Spectrophotometric Characterization of the Chilean Night Sky. A First Quantitative Assessment of ALAN across the Coquimbo Region

Light pollution is recognized as a global issue that, like other forms of anthropogenic pollution, has a significant impact on ecosystems and adverse effects on living organisms. Plentiful evidence suggests that it has been increasing at an unprecedented rate at all spatial scales. Chile—which, thanks to its unique environmental conditions, has become one of the most prominent astronomical hubs of the world—seems to be no exception. In this paper we present the results of the first observing campaign aimed at quantifying the effects of artificial lights at night on the brightness and colors of the Chilean sky. Through the analysis of photometrically calibrated all-sky images captured at four representative sites with an increasing degree of anthropization, and the comparison with state-of-the-art numerical models, we show that significant levels of light pollution have already altered the appearance of the natural sky even in remote areas. Our observations reveal that the light pollution level recorded in a small town of the Coquimbo Region is comparable with that of Flagstaff, Arizona, a ten times larger Dark Sky city, and that a mid-size urban area that is a gateway to the Atacama Desert displays photometric indicators of night sky quality that are typical of the most densely populated regions of Europe. Our results suggest that there is still much to be done in Chile to keep light pollution under control and thus preserve the darkness of its night sky—a natural and cultural heritage that it is our responsibility to protect.

Sky-like interior light settings: a preference study

This paper explores human observer preferences for various sky-like interior lighting scenarios realized by a combination of a blue-enriched indirect uplight component with a correlated color temperature (CCT) of 6,500 K up to 30,000 K and a 4,000 K or 5,500 K direct downlight component. Variations in the natural sky were mimicked by the indirect uplight component reflected from the ceiling of the experimental room. The settings for the direct lighting component, on the other hand, were selected based on the reported outcomes of previous preference studies in the field of interior lighting. The resulting lighting conditions were evaluated by a total of 29 observers, from which subjective ratings of brightness, sky-likeness, satisfaction, pleasantness, and general appeal were collected in an office workplace environment. In this experimental setting, the most preferred lighting conditions exhibited a direct-to-indirect lighting ratio of 50:50 with a CCT of 4,000 K in the direct component and 6,500, 7,500, and 9,000 K in the indirect component. For all examined combinations, none was rated as truly sky-like. Nonetheless, the study results showed that only the combination of a warmer CCT in the direct component and a cooler, blue-enriched CCT in the indirect lighting component leads to a maximum in the subjects’ preference ratings. In summary, the subjects preferred light settings with a white appearance on the work surface without any intense or noticeable blue cast or tint.

An enhanced version of the Gaia map of the brightness of the natural sky

The GAia Map of the Brightness Of the Natural Sky (GAMBONS) is a model to map the natural night brightness of the sky in cloudless and moonless nights. It computes the star radiance from the photometric data in Gaia and Hipparcos catalogues, adding the contributions of the diffuse galactic and extragalactic light, zodiacal light and airglow, and taking into account the effects of atmospheric attenuation and scattering. The model allows computing the natural sky brightness in any given photometric band for a ground-based observer, if appropriate transformations from the Gaia bands are available. In this work we present the most recent improvements of the model, including the use of Gaia EDR3 data, the inclusion of a wide set of photometric bands and the derivation of additional sky brightness indicators, as the horizontal irradiance and the average hemispheric radiance.

The New Method To Calibrate Iso/cie General Skies modelled In Artificial Skies

Recently sky luminance distributions under fifteen ISO/CIE sky types were standardised in relative terms, i.e. all elementary sky luminance are normalised by the zenith luminance in nature. However, these standard sky luminance patterns can be characterised also in their physical units, see in CIE215:20014 which can be reduced by suitable intensity scales to simulate the very high natural sky luminance in laboratory under artificial sky domes. For the design of the electric illumination system of the artificial sky the appropriate zenith luminance for each sky type has to be derived respecting the indicatrix and gradation functions of each sky type. Thus, the sky patterns simulated in the artificial sky can be calibrated either by luminance measurements or by analysing fish-eye photos taken from the sky centre showing the smoothness and gradual changes of homogeneous sky type luminance patterns. The sky luminance images can be utilised to proceed with the adjustment or dimming of the LED electrical system to the final calibration check containing zenith luminance and exterior illuminance in the second calibration step.

Cryocooling technologies for the origins space telescope

The Origins Space Telescope’s (Origins) significant improvement over the scientific capabilities of prior infrared missions is based on its cold telescope (4.5 K) combined with low-noise far-IR detectors and ultrastable mid-IR detectors. A small number of new technologies will enable Origins to approach the fundamental sensitivity limit imposed by the natural sky background and deliver groundbreaking science. This paper describes a robust plan to mature the Origins mission, enabling cryocooler technology from current state-of-the-art (SOA) to Technology Readiness Level (TRL) 5 by 2025 and to TRL 6 by mission Preliminary Design Review. Entry TRLs corresponding to today’s SOA are 4 or 5, depending on the technology in question.

A multiband map of the natural night sky brightness including Gaia and Hipparcos integrated starlight

ABSTRACT The natural night sky brightness is a relevant input for monitoring the light pollution evolution at observatory sites, by subtracting it from the overall sky brightness determined by direct measurements. It is also instrumental for assessing the expected darkness of the pristine night skies. The natural brightness of the night sky is determined by the sum of the spectral radiances coming from astrophysical sources, including zodiacal light, and the atmospheric airglow. The resulting radiance is modified by absorption and scattering before it reaches the observer. Therefore, the natural night sky brightness is a function of the location, time, and atmospheric conditions. We present in this work the GAia Map of the Brightness Of the Natural Sky (GAMBONS), a model to map the natural night brightness of the sky in cloudless and moonless nights. Unlike previous maps, GAMBONS is based on the extra-atmospheric star radiance obtained from the Gaia catalogue. The Gaia-Data Release 2 (DR2) archive compiles astrometric and photometric information for more than 1.6 billion stars up to G = 21 mag. For the brightest stars, not included in Gaia-DR2, we have used the Hipparcos catalogue instead. After adding up to the star radiance the contributions of the diffuse galactic and extragalactic light, zodiacal light and airglow, and taking into account the effects of atmospheric attenuation and scattering, the radiance detected by ground-based observers can be estimated. This methodology can be applied to any photometric band, if appropriate transformations from the Gaia bands are available. In particular, we present the expected sky brightness for V (Johnson), and visual photopic and scotopic passbands.

DISCRETE X-RAY SOURCES IN GALAXY CLUSTERS

Current studies of extragalactic X-ray sources using data from orbital observatory XMM- Newton are at the front line of X-ray astronomy. Resolution capacity of the observatory instruments enable to detect separate X-ray sources in the close galaxies. In this study we used data from recently published 4XMM-DR9 catalog. This catalog contains 550124 unique sources covering 2,85% of the sky. The main type of extended extragalactic X-ray source is hot halo of galaxy cluster. Such objects are important for astrophysics in the tasks of revealing dark matter distribution, galaxy formation and different cosmology studies. At the same time, some images of the galaxy clusters contain small scale pointlike X-ray sources scattered around the clusters or their halos. In many cases the nature of such objects is not clear yet and require more detailed studies. Furthermore, energy flux detection threshold of such objects is limited to a great extend not only by the instruments on board of the XMM- Newton observatory but the natural sky background radiation, which is at the level of 10 −13 Wm −2 sr −1 both for XMM cameras and the sky background. Using Hyperleda database we revealed 16 galaxy clusters among the bright XMM sources. Images of the two galaxy clusters demonstrate bright galaxy cores. These clusters are galaxy group NGC 507 and Coma cluster where nucleus of galaxy NGC 4889 was clearly detected. We also analyzed X-ray images of 14 other X-ray clusters but had not found AGNs there. Out of the above mentioned 16 clusters in the 7 of them we have found 30 pointlike X-ray sources. These sources are presumably mainly AGNs within these clusters. From the other side, they could be also more distant X-ray objects.

Specializations in the compound eye of Talitrus saltator (Crustacea, Amphipoda).

We investigated the eye regionalization in Talitrus saltator by morphological, electrophysiological and behavioural experiments. Each ommatidium possesses five radially arranged retinular cells producing a square fused rhabdom by R1-R4 cells; the smaller R5 exists between R1 and R4. The size of R5 rhabdomere is larger in the dorsal part and becomes smaller in the median and ventral parts of the eye. Spectral-sensitivity by electroretinograms were recorded from dorsal or ventral parts of the eye. The dorsal part possesses maxima at green and UV-blue region. The main response region in the ventral part is only from UV (390nm) to blue (430nm) decreasing at longer wavelengths. To evaluate the sandhoppers' celestial orientation, their eyes were painted black either in the dorsal or ventral part, under the natural sky or a blue filter with or without the vision of the sun. Sandhoppers with the dorsal region of the eyes painted and tested under the screened sun were more dispersed and their directions varied more than in other groups of individuals. Sandhoppers with this area of the eye obscured display considerable difficulties to head in a specific direction. This work suggests the existence of regional specializations in the eye of T. saltator.

Introducing the dark sky unit for multi-spectral measurement of the night sky quality with commercial digital cameras

Multi-spectral imaging radiometry of the night sky provides essential information on light pollution (skyglow) and sky quality. However, due to the different spectral sensitivity of the devices used for light pollution measurement, the comparison of different surveys is not always trivial. In addition to the differences between measurement approaches, there is a strong variation in natural sky radiance due to the changes of airglow. Thus, especially at dark locations, the classical measurement methods (such as Sky Quality Meters) fail to provide consistent results. In this paper, we show how to make better use of the multi-spectral capabilities of commercial digital cameras and show their application for airglow analysis. We further recommend a novel sky quality metric the “Dark Sky Unit”, based on an easily usable and SI traceable unit. This unit is a natural choice for consistent, digital camera-based measurements. We also present our camera system calibration methodology for use with the introduced metrics.

Toward an atlas of the number of visible stars

Modelling techniques for the propagation of light pollution in the atmosphere allow the computation of maps of artificial night sky brightness in any direction of the sky, involving a large number of details from satellite data. Cinzano et al. (2001a) introduced a method of mapping naked eye star visibility at the zenith from large areas based on satellite radiance measurements and Garstang models of the propagation of light pollution. It takes into account the altitude of each land area from digital elevation data, natural sky brightness in the chosen sky direction based on the Garstang approach, eye capability after Garstang and Schaefer, and atmospheric extinction in the visual photometric band. Here we discuss how to use these methods to obtain maps of the average number of visible stars when looking at the night sky hemisphere, finally answering, site by site, the question of how many stars are visible in the sky. This is not trivial, as the number of stars visible depends on the limiting magnitude in each direction in the sky, and this depends on sky brightness in that direction, atmospheric extinction at that zenith distance and the observer's visual acuity and experience. We present, as an example, a map of the number of visible stars in Italy to an average observer on clear nights with a resolution of approximately 1 km.

Night sky brightness simulation over Montsec protected area

Night sky brightness over Montsec Observatory (north-east of Spain) has been computed and checked against measurements using Illumina numerical model [2]. In a previous publication [20] the methodology was validated and light pollution received in the observatory coming from a unique city was computed. Here we present a simulation that includes all the sources that has a significant impact over the quality of the night sky in this area. The decision of which sources should be included in the simulations was taken following the methodology explained by [6]: using a point spread function (PSF) as a simple approach to estimate which sources are brightening the sky dome over the observer. An ad hoc PSF derived with Illumina was used with the purpose of avoiding to have to rely on already existing empirical PSF. The resulting PSF can be used in any location with similar atmospheric conditions. Differences in the spectrum of the lamps can be accounted easily by adjusting a spectrum scale factor. Illumina simulates the artificial sky brightness received (W/sr/m2) by an observer from any direction. Adding the natural sky brightness allows to compare the simulations to measurements taken with different instrumentation. In our case simulations were checked against ASTMON, SQC and SQM measurements. They show a good agreement both in absolute values and in geographical patterns for the three filters studied, B, V and R. The methodology presented opens many possibilities, such as increasing the reliability of the maps that point out the light pollution main contributors for any location, and reducing the amount of time needed to perform an accurate simulation of the night sky brightness.

Re-envisioning the Nocturnal Sublime: On the Ethics and Aesthetics of Nighttime Lighting

Grounded in the practical problem of light pollution, this paper examines the aesthetic dimensions of urban and natural darkness, and its impact on how we perceive and evaluate nighttime lighting. It is argued that competing notions of the sublime, manifested through artificial illumination and the natural night sky respectively, reinforce a geographical dualism between cities and wilderness. To challenge this spatial differentiation, recent work in urban-focused environmental ethics, as well as environmental aesthetics, are utilized to envision the moral and aesthetic possibilities of a new urban nocturnal sublime. Through articulating the aspirations and constraints of a new urban nocturnal experience, this paper elucidates the axiological dimensions of light pollution, draws attention to nightscapes as a site of importance for urban-focused (environmental) philosophy, and examines the enduring relevance of the sublime for both the design of nighttime illumination and the appreciation of the night sky.

Association between nighttime artificial light pollution and sea turtle nest density along Florida coast: A geospatial study using VIIRS remote sensing data

Artificial lighting at night has becoming a new type of pollution posing an important anthropogenic environmental pressure on organisms. The objective of this research was to examine the potential association between nighttime artificial light pollution and nest densities of the three main sea turtle species along Florida beaches, including green turtles, loggerheads, and leatherbacks. Sea turtle survey data was obtained from the “Florida Statewide Nesting Beach Survey program”. We used the new generation of satellite sensor “Visible Infrared Imaging Radiometer Suite (VIIRS)” (version 1 D/N Band) nighttime annual average radiance composite image data. We defined light pollution as artificial light brightness greater than 10% of the natural sky brightness above 45° of elevation (>1.14 × 10−11 Wm−2sr−1). We fitted a generalized linear model (GLM), a GLM with eigenvectors spatial filtering (GLM-ESF), and a generalized estimating equations (GEE) approach for each species to examine the potential correlation of nest density with light pollution. Our models are robust and reliable in terms of the ability to deal with data distribution and spatial autocorrelation (SA) issues violating model assumptions. All three models found that nest density is significantly negatively correlated with light pollution for each sea turtle species: the higher light pollution, the lower nest density. The two spatially extended models (GLM-ESF and GEE) show that light pollution influences nest density in a descending order from green turtles, to loggerheads, and then to leatherbacks. The research findings have an implication for sea turtle conservation policy and ordinance making. Near-coastal lights-out ordinances and other approaches to shield lights can protect sea turtles and their nests. The VIIRS DNB light data, having significant improvements over comparable data by its predecessor, the DMSP-OLS, shows promise for continued and improved research about ecological effects of artificial light pollution.

“When the Stars are Glittering with Gold up in the Sky...”. Jewellery Metaphors in Descriptions of the Sky Phenomena in Roman Literature

The paper traces the use of metaphors and comparisons concerning jewellery in the descriptions of the sky in Roman literature, most of all in poetry. As it is shown in the paper, many of those poetic devices served as a means highlighting the vividness and perfection of the natural sky phenomena. Analysis of jewellery imagery helps also to demonstrate the occurrence of some changes in descriptive conventions and aesthetic attitudes in Roman literature.

Modelling of Daylight Sources in the Artificial Sky

Daylight in nature is characterized by daily permanent changes of sunlight and skylight. Real measurements of daylight availability are showing that different daylight exterior illuminance for building interiors can be expected each minute. This brings some complications in the window design and the choice of criteria for daylight evaluations. There are several ways how to find basic conditions and typical relations combining sunlight beam with diffuse skylight from the whole sky vault. The older assumption considered that interiors have to be sufficiently illuminated under the worst overcast conditions. The newer approach is based on the utilization of daylight in specific localities and the determination of changes in sunlight and skylight occurrence probability. Therefore, both daylight sources are researched in detail specifying sky luminance distributions and sun influences to find conditions for their simulation in laboratory facilities. The most sophisticated equipment to study daylighting in exterior and interior architectural spaces are artificial skies with the artificial sun. These have to be precisely calibrated with a verified zenith luminance and horizontal illuminance levels by theoretical calculations and checked by experimental measurements. Reference daylight conditions defined in the ISO/CIE 15469:2004 standard have to be respected with trials to determine natural sun and sky as sources of daylight in the real environment and modelling these in the artificial sky in a certain intensity scale. This paper presents the method and results of modelling daylight applying electrical light sources in artificial sky which is installed in the Institute of Construction and Architecture, Slovak Academy of Sciences (ICA SAS) and discusses possibilities of their simulation in laboratory conditions.

Photometric indicators of visual night sky quality derived from all-sky brightness maps

Wide angle or fisheye cameras provide a high resolution record of artificial sky glow, which results from the scattering of escaped anthropogenic light by the atmosphere, over the sky vault in the moonless nocturnal environment. Analysis of this record yields important indicators of the extent and severity of light pollution. The following indicators were derived through numerical analysis of all-sky brightness maps: zenithal, average all-sky, median, brightest, and darkest sky brightness. In addition, horizontal and vertical illuminance, resulting from sky brightness were computed. A natural reference condition to which the anthropogenic component may be compared is proposed for each indicator, based upon an iterative analysis of a high resolution natural sky model. All-sky brightness data, calibrated in the V band by photometry of standard stars and converted to luminance, from 406 separate data sets were included in an exploratory analysis. Of these, six locations representing a wide range of severity of impact from artificial sky brightness were selected as examples and examined in detail. All-sky average brightness is the most unbiased indicator of impact to the environment, and is more sensitive and accurate in areas of slight to moderate light pollution impact than zenith brightness. Maximum vertical illuminance provides an excellent indicator of impacts to wilderness character, as does measures of the brightest portions of the sky. Zenith brightness, the workhorse of field campaigns, is compared to the other indicators and found to correlate well with horizontal illuminance, especially at relatively bright sites. The median sky brightness describes a brightness threshold for the upper half of the sky, of importance to telescopic optical astronomy. Numeric indicators, in concert with all-sky brightness maps, provide a complete assessment of visual sky quality at a site.

Evaluating the summer night sky brightness at a research field site on Lake Stechlin in northeastern Germany

We report luminance measurements of the summer night sky at a field site on a freshwater lake in northeastern Germany (Lake Stechlin) to evaluate the amount of artificial skyglow from nearby and distant towns in the context of a planned study on light pollution. The site is located about 70km north of Berlin in a rural area possibly belonging to one of the darkest regions in Germany. Continuous monitoring of the zenith sky luminance between June and September 2015 was conducted utilizing a Sky Quality Meter. With this device, typical values for clear nights in the range of 21.5–21.7magSQM/arcsec2 were measured, which is on the order of the natural sky brightness during starry nights. On overcast nights, values down to 22.84magSQM/arcsec2 were obtained, which is about one third as bright as on clear nights. The luminance measured on clear nights as well as the darkening with the presence of clouds indicates that there is very little influence of artificial skyglow on the zenith sky brightness at this location. Furthermore, fish-eye lens sky imaging luminance photometry was performed with a digital single-lens reflex camera on a clear night in the absence of moonlight. The photographs unravel several distant towns as possible sources of light pollution on the horizon. However, the low level of artificial skyglow makes the field site at Lake Stechlin an excellent location to study the effects of skyglow on a lake ecosystem in a controlled fashion.

Unified model of radiance patterns under arbitrary sky conditions

The radiance/luminance patterns that simulate more realistic skies are urgently needed in lighting engineering applications to model daylight availability in exterior and interior spaces. Undoubtedly, the angular behavior of sky radiance/luminance is a key factor determining the daylight conditions in rooms with variable orientations and window sizes. Therefore, much effort is currently expended in search of a universal, scalable daylight model that accepts actual meteorological situations.The natural sky radiances are neither monotonic nor smooth functions of zenith/azimuth angle, primarily due to the presence of broken cloud arrays or single clouds scattered over the whole sky vault. In this paper we show how we have addressed the phenomena and developed a universal sky radiance model for a turbid, cloudy atmosphere. This model accommodates higher scattering orders, aerosol optics, surface albedo, and the statistically relevant contributions of single clouds. The radiance at ground depends on the bulk characteristics of cloud fields, such as spectral optical thickness, spectral reflectance, altitude, positions on the sky, sizes, and shapes. We have shown that single scattering approximation fails to reproduce the radiance in the circumsolar region when clouds block the direct solar beams. The single scattering concept also overestimates reflection by individual clouds. Incorporation of double scattering into a computational model generally makes the radiance patterns smooth and less steep at the edges of clouds as commonly occur in nature. The numerical demonstrations are based on UniSky Simulator (Kocifaj and Fecko, 2014) which allows for modeling of various cloud configurations and is available for public use.

Amplitude and dynamics of polarization-plane signaling in the central complex of the locust brain.

The polarization pattern of skylight provides a compass cue that various insect species use for allocentric orientation. In the desert locust, Schistocerca gregaria, a network of neurons tuned to the electric field vector (E-vector) angle of polarized light is present in the central complex of the brain. Preferred E-vector angles vary along slices of neuropils in a compasslike fashion (polarotopy). We studied how the activity in this polarotopic population is modulated in ways suited to control compass-guided locomotion. To this end, we analyzed tuning profiles using measures of correlation between spike rate and E-vector angle and, furthermore, tested for adaptation to stationary angles. The results suggest that the polarotopy is stabilized by antagonistic integration across neurons with opponent tuning. Downstream to the input stage of the network, responses to stationary E-vector angles adapted quickly, which may correlate with a tendency to steer a steady course previously observed in tethered flying locusts. By contrast, rotating E-vectors corresponding to changes in heading direction under a natural sky elicited nonadapting responses. However, response amplitudes were particularly variable at the output stage, covarying with the level of ongoing activity. Moreover, the responses to rotating E-vector angles depended on the direction of rotation in an anticipatory manner. Our observations support a view of the central complex as a substrate of higher-stage processing that could assign contextual meaning to sensory input for motor control in goal-driven behaviors. Parallels to higher-stage processing of sensory information in vertebrates are discussed.