Dark Skies Their Importance and the Need to Protect Them

Dark skies are the pristine state of nocturnal nature preserved in sites where the darkness of the night sky should be free of artificial light interference. In Mexico, there are many dark skies that are not officially recognized and they deserve to be protected because of their natural values and their scientific, educational and cultural potential. However, Mexican dark skies have been affected due to heterogeneous urbanization processes in which light pollution has had a differentiated role; therefore, it is necessary to determine the environmental impact of its causes. 

Light Pollution
Light pollution is the emission of luminous flux streaming from artificial sources with excessive intensity or at unnecessary places, hours or spectral ranges meant for performing activities planned in the area in which lights are installed. Light pollution manifests in the night sky brightness mainly due to the reflection and diffusion of light in air molecules, aerosols and particles of small size suspended in the atmosphere. The inappropriate and disproportionate use of artificial light is not only affecting our view of the universe but also having a major impact on human beings and wildlife. Light or the absence of it—darkness—are environmental factors that influence greatly the living conditions of biological species, including humans. Throughout evolution, the biological systems of our planet have adapted to the accompanying astronomical cycles and have developed light and dark detection techniques that ensure the integration of their behavior into the seasons’ annual progression. The low illumination level at night is part of a natural cycle with which most organisms composing biodiversity on earth are familiar.

The most common consequence of light pollution is the sky brightness resulting from an unjustified escalation of artificial lighting propagation into the night atmosphere that reduces contrast between sky luminance and celestial bodies’ light in the affected area. The impact is also evident on astronomy and on the observation of nocturnal atmospheric processes. That is why astronomers were the first to provide evidence of this phenomenon. In their warnings, they described the disturbances on the natural state of night sky and the close-to-total reduction of celestial bodies visibility because of the unwanted luminous fluxes of artificial light sources.

Studies on light pollution effects from the ecological and biological perspectives are increasingly better known and more frequent. Some of them address issues related to artificial light impact on insects and vertebrates (Moore et al., 2001; Anisimov, 2003; Vera & Migaud, 2009; Fox, 2012; Cho et al., 2015; Solano Lamphar & Kocifaj, 2015), including ecosystem services (Lyytimaki, 2013). Animals are not the only organisms influenced by the night sky light (Neil & Wu, 2006) thus, a significant line of research has emerged focusing on the effects of sky brightness on plants. Plants photoreceptors play different roles in leaf and stem growth, flowering and fruit production and they have an influence on other life developments as well (Briggs, 2014). These organisms produce their own food through the photosynthesis process by means of which they capture radiant energy from different light sources through chlorophyll and convert carbon dioxide and water into sugar to be used as energy source (Krause & Weis, 1991). This is a crucial mechanism in plant development that also takes place in aquatic ecosystems (Kirk, 1994).

As in the atmosphere, the temporal and spatial fluctuations in light pollution are caused by light emissions, whether from private and public sources, which result in the collective effect of all artificial lighting distributed all over the city. Unfortunately, there are scarce theoretical or experimental studies that explore pollution sources and their consequences due to the lack of specialized measuring devices and statistical data regarding the amount of public and private lighting available in urban areas. Therefore, a diagnosis of Mexican cities is needed in order to identify dark sky places for their protection and restoration and turn the city into a promotion mechanism for environmental protection, finding geographical capacities for the use of regulated night lighting, and thus, to foster and lead sustainable and innovative actions that provide environmental, economic and social benefits by protecting dark skies areas and all the species that inhabit them.

City lighting is a complex network that does not only allow us to carry out activities at night, but also conveys a series of meanings of industrialized societies development. Officials considered it a means for providing security; so, better-lit streets and parks are expected to contribute to lowering the rates of crime and vandalism. Furthermore, urban lighting is an engine of economic development that favors nightlife and creates feelings of pride and belonging since it fosters the conditions needed for the appropriation of public space at night. According to Calvillo Cortés (2010), urban lighting goes beyond its functional aspects and creates an emotional response in the observer. Consequently, it influences the way we value nocturnal urban spaces providing a sense of emotional well-being to citizens and visitors. Some authors (Sveikauskas, 1975; Garrocho Rangel, 2017) consider that the concentration of the population in urban centers is the engine of economic growth and human development. However, city populations also experience problems like insecurity, mobility, high costs of life, among others. We may argue that the growth trend of cities ensures that urban settlements will concentrate the world’s population in the coming years. In mid-twentieth-century Mexico, for example, urban population did not surpass 35% of the country’s population (Unikel, 1968); by the 1970’s, this percentage had increased to 58.7 and by 2010, to 77.8% (INEGI, 2010). This growth has been accompanied by the development of public lighting which has increasingly spread out since the emergence of electricity at the beginning of the 20th century. Previously, “the night was synonymous with insecurity, as it was the refuge of criminals, the marginals, it was also the propitious time for immoral activities” (Contreras Padilla, 2014). This may be why artificial lighting is related to population growth and urbanization. Having all these aspects in mind, it is convenient to do a thorough review of dark skies in relation with the need of city lighting from a multidisciplinary and interdisciplinary view.

Actions to Protect Darkness
At present, actions have been put into place by both the scientific and the public and private sectors to favor the protection of the night sky. In Chile and Spain, laws have been enacted to protect astronomical observation spaces. In 2007, under UNESCO’s auspices, the Canary Islands Institute of Astrophysics promoted the first conference “on the defense of the night sky and the right to the light of the stars”. An international campaign was then started for the preservation of night skies quality in favor of biodiversity, economy and the right to stargazing; as a result, the Starlight Declaration was drafted (see box).




In Mexico, laws have been enacted from the 1980’s on to support environment protection and improvement such as the Federal Law on Environmental Protection, of 1982, and the General Law of Ecological Balance and Environmental Protection (LGEEPA, Spanish initials) of 1988; both of them are still in place today. It was in 2019 that the Senate of the Republic issued a decree to prevent light pollution and, in January 2021, several articles were modified to incorporate concepts such as intrusive light and light pollution. Both additions seek to prevent, reduce and control the effects caused by artificial lighting by promoting efficiency of outdoor lighting. In 2006, the Regulations for the Prevention of Light Pollution, were promulgated in the municipality of Ensenada, Baja California, and four years later, the state Congress included it in its Law of Environmental Protection. Through this regulation, a protection area was established for the National Astronomical Observatory, located in the San Pedro Mártir mountain range. This is the first step taken to protect the night sky. In 2019, sponsored by the ECOS network, a group of professionals, researchers and academics from different institutions came together to shape to the project “Lights Over the City”, looking to “propose strategies for the optimization of lighting benefits and the reduction of its negative effects on human health and the environment.” The mitigation of light pollution doesn’t have to do with controlling and eliminating the luminous flux emitted into the atmosphere or replacing technologies, but it involves urban planning processes and public and private lighting policies adapted to the geographical, social and economic conditions of every specific place. Most of problems related to city growth are not the result of the growth itself, but of “the inability of local, regional and national institutions to face the challenges of the rapid growth of cities” (Garrocho Rangel, 2017).

It is a fact that most people will live in urban areas by 2030. By that time, urban settlements in the developing world will account for 80% of the world’s urban population (Garrocho Rangel, 2017) and, according to UN estimates, two-thirds of the world’s population will live in cities by 2050. Thus, lighting systems in cities face great challenges ahead. Electricity consumption usually receives most attention as a result of the concern on climate change and the need to reduce greenhouse gases emission. The answer has been to replace traditional light sources, as fluorescent and discharge, by LED technologies. In Mexico, lighting is considered a fundamental public service and an unmet social need (CONUEE, 2019), so it has been required to implement actions that guarantee sufficient national coverage to provide security and mobility conditions by night.  

The problem of Measuring Light  
Outdoor lighting, whether public or private, on the roads and in the urban areas, for advertising or for decorative purposes, is the main cause of light pollution. There are also other sources of pollution such as vehicle headlights or the light escaping from the windows of buildings and houses (Darula, 2013; Bará et al., 2019). Therefore, to clearly understand the light pollution phenomenon, to establish its sources and quantify its impact, it is first needed to analyze the existing conditions in a given site to be able to identify the relationship between the glow of the night sky brightness and the artificial sources on surface. Currently, light pollution models are based on satellite data or astronomical observations. Detecting night lights from space offers many advantages, but also some limitations. Perhaps the main advantage is the possibility to observe the study area in its integrality, compared with methods based on specific measurements. An additional advantage is that direct observation minimizes atmosphere effect, except when cloudiness prevents observation (Sánchez de Miguel, 2015). VIIRS (Visible Infrared Imaging Radiometer Suite) images, generated by SUOMI-NPP satellite, allow to work on light maps based on daily satellite data that must be corrected by complementary observations on the ground (Cinzano & Falchi, 2014).

In addition, astronomical observation requires the use of professional telescopes and a photometer or a CCD camera (a silicon based photo-sensible technique) in order to establish instrumental constants to determine the darkness of the sky; this is a specialized and laborious task. There are other techniques used to assess light pollution like the measurement of the sky background brightness, frequently used by professional and amateur astronomers worldwide. This parameter, which accounts for the luminous flux coming generally from the zenith direction, can be measured by means of a special photometer, the Sky Quality Meter (SQM). The measurement is calculated in magnitudes per square-arcseconds (mag/arcsec²), an astronomical unit used to express sky brightness. Despite its simplicity and ease of use, this devices do provide accurate measurements and can be considered scientific instruments that have been used in several research projects (Zamorano Calvo et al., 2011).

Within the framework of the project “Lights Over the City”, an analysis group was created with the mission of preparing the draft of the Light Pollution and Intruding Light Standard. The group is meant to establish the scope, purpose, recipients and technical parameters of this document. In collaboration with the Architectural Lighting Laboratory of the School of Architecture at UNAM, this group will monitor, diagnose and characterize the night pollution exposure in urban, suburban and rural environments in Mexico City, by measuring the sky background brightness. The purpose is to determine existing levels of light pollution in specific areas and identify its sources and the characteristics of those sources. This project is intended to draw a light pollution map that will serve as the basis to propose urban lighting strategies and make recommendations for Mexico City. The sky background brightness map of the metropolitan area will provide key input to formulate regulations and policies relevant for the city and other urban centers with the aim of offering efficient lighting conditions that respect biodiversity and people’s well-being. This project would contribute to the country’s urban development plans and ensure that lighting becomes an axis of strategic planning targeting sustainability, quality of life and lighting conditions improvement in cities.

By learning about light pollution conditions in urban centers, we will be able to identify the best actions and policies to deal with it, favoring environmental protection and sustainable development in the cities and creating better living conditions for society as a whole. 

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