Background

In 1979, the International Astronomical Union defined a maximum level of light pollution below which a professional site could be considered adequate for true dark sky observing. This standard was defined as an upper limit of artificial light contribution of 10% above the natural background at an elevation of 45° in any azimuthal direction. 

Considerable work has been done in recent years to advance the protection of astronomical sites. In particular, this issue has been reported and discussed at two international workshops: The Dark and Quiet Skies (D&QS) for Science and Society 1 and 2. The first D&QS workshop, held in October 2020, was sponsored by the United Nations Office for Outer Space Affairs (UNOOSA), the International Astronomical Union (IAU), the Instituto de Astrofisica de Canarias in Spain (IAC), and the National Optical-Infrared Laboratories (NOIRLab) of the United States (US) National Science Foundation (NSF). The second workshop, held in October 2021, was sponsored by the same four organisations and the European Southern Observatory (ESO). These two workshops ultimately inspired the creation of the IAU Centre for the Protection of the Dark and Quiet Sky from Satellite Constellation Interference (CPS) in 2022. 

The international working group for D&QS 1 consisted of nine astronomers, four technical experts in light pollution/modelling, seven lighting experts from the International Commission on Illumination (CIE), and two industry representatives, including one from the manufacturers' Global Lighting Association.  The working group worked on its findings and conclusions for three months prior to the workshop, led an active discussion at the workshop, and incorporated further insights into the final report. The conclusions in the report were unanimously agreed by the Working Group and adopted as the outcome of the workshop.

The international working group for D&QS 2 consisted of nine astronomers, 10 experts on various effects of light pollution, including on the natural world and public health, and a lighting engineer representing the CIE. Their task was to consider the broader aspects of artificial light at night and to develop further international guidelines based on the recommendations for site protection in D&QS 1. This working group invested a similar amount of effort (three months) prior to its workshop and followed a similar process to produce its report.  

The Summary Recommendation for the Protection of Astronomical Sites, presented below, is the result of the substantive deliberations of these UN/IAU-sponsored working groups with broad experience and expertise. The IAU is currently considering drafting a resolution based on these recommendations to be submitted for approval at its next general assembly in 2027. 

Summary of the Recommendations of the Working Groups

Present-day professional observatories are located in remote, high-altitude locations; a key selection criterion is the actual sky darkness being as close to the natural background as possible. These sites have an artificial light contamination significantly below the 10% limit recommended by the IAU in 1979. Hence, this limit is not appropriate for the protection of modern professional astronomical sites.

Adequate protection of major professional observatories requires actions to:

1. reduce, stop, and then reverse, on the timescale of a decade, the growth of Artificial Light at Night (ALAN);
2. keep the total contribution to skyglow from ALAN substantially below the 10% dark site limit defined by the IAU; and ultimately, 
3. control and reduce light pollution where it is mainly produced: in human population centres including cities and towns, and in industrial and agricultural production centres. 

To achieve the necessary level of protection, each major professional observatory and controlling governmental body should agree on the total amount of outdoor light allowable, based on measurement and modelling of the current level and rate of change of ALAN impacting the site.

Illustration of the recommendations

The core of the recommendations is that each major site has a unique limit that should not be exceeded by growing ALAN. It requires each observatory to know what its ALAN contribution is and the rate at which it is currently growing, quantities that can be easily measured. 

For example, if an observatory has a current ALAN growth rate of 0.04% per year, which is to be brought to zero within five to eight years, then the ALAN contribution will be less than 0.5% for the foreseeable future. The condition that the ALAN growth rate must be brought to zero and reversed at a site that now has an extremely low artificial contribution sets strong constraints; there will be no way to accommodate a new major artificial light source within these rules, as there is no offset for any sources that could be reduced.

More information

The IAU is the international astronomical organisation that brings together about 13,000 distinguished astronomers from around the world. Its mission is to promote and safeguard the science of astronomy in all its aspects, including research, communication, education and development, through international cooperation. The IAU also serves as the internationally recognised authority for assigning designations to celestial bodies and the surface features on them. Founded in 1919, the IAU is the world’s largest professional body for astronomers.

Links

• Recommendation on the Protection of Astronomical Sites: Studies by two UN/IAU-sponsored working groups recommend new guidelines (PDF, 127KB)
• Dark and Quiet Skies (D&QS) for Science and Society 1
• Dark and Quiet Skies (D&QS) for Science and Society 2

Contacts

Richard Green
Astronomer Emeritus, Steward Observatory
Former President IAU Inter-Division B-C Commission on the Protection of Existing and Potential Observatory Sites and Co-Chair of both UN/IAU sponsored working groups who have produced the recommendations
Email: rgreen@arizona.edu

Willy Benz
IAU President
Email: willy.benz@unibe.ch

Laura Hiscott
IAU Press Office Editor
Email: iaupressoffice@iau.org