Toward a new standard for astronomical observatory site protection

Image credit: KPNO/NOIRLab/NSF/AURA/B. Tafreshi (CC BY 4.0​)
1707 words / 7-minute read

​A visit to an astronomical observatory at night is a magical experience. On a far mountaintop, the starry vault overhead, they are like temples to the study of the cosmos. Telescopes silently scan the heavens all night, shutting their eyes before the first light of dawn. But with increasing frequency, these distant sites are under threat from artificial light at night. What were once thought to be the most defensible locations are less so now. Some suggest that a new management paradigm is the only way to save them.

To the ends of the earth

Astronomical discovery depends on access to faint cosmic light. It is the one physical attribute of the universe directly accessible to science over very large distances. Information about physical conditions in faraway stars and galaxies is encoded in the light we collect with telescopes. Absent this fact, we would know very little about outer space. Because the intensity of light decreases with the distance to its source, these signals are very faint when they reach Earth. Telescopes with massive mirrors collect this light in enough quantity to make sense of the information it brings.

Yet it is now often the case that cosmic light must compete with light from artificial sources on the ground. This began as early as the 18th century, when astronomical observatories were still situated in cities. The installation of gas lighting systems, later replaced by electric ones, began brightening the night sky. Astronomers moved out of cities and took their telescopes with them to the countryside. Many such locations, especially in Europe were unfavorable for observations. Bad weather got in the way, and turbulence in the atmosphere made for poor-quality images. They then moved telescopes to the summits of mountains far from cities. In the 20th century, they started launching them into space. All this increased the difficulty and cost of collecting astronomical data.

Well into the last century, many important observatories in and near cities continued their work. Some sought to hold back the rising tide of light pollution with public policy interventions. Lowell Observatory, in Arizona, U.S., pioneered this approach in the late 1950s. It convinced the council of the city of Flagstaff, its home since the 1890s, to enact what became known as the "Searchlight Law". [1] It aimed to reduce the impact of the use of searchlights for advertising purposes on the observatory's work. But this could only go so far. Flagstaff's population now over four times that in 1960, and the observatory has moved its operations out of town.

Rising night sky brightness over observatories means that scientists need more time and money to achieve the same science outcomes. Consider a case in which the brightness of the night sky at an otherwise unpolluted observatory site doubles. The exposure time for a given telescope and camera needed to reach some science goal then doubles. [2] Given the dollar cost associated with operating a modern observatory, the cost to achieve that goal also doubles. Light pollution thus not only threatens to slow the pace of discovery, but it also makes that discovery more expensive.

There are some recent successes. In 1998, the Chilean National Congress enacted the "Norma de Emisión para la Regulación de la Contaminación Lumínica" ("Emission Standard for the Regulation of Light Pollution”). It required a few basic restrictions on the operation of outdoor lighting in three northern provinces of the country. The goal was to protect observatories in the mountains above the Atacama Desert. Successive updates to the 'Norma Lumínica' have proven rather successful. Yet even the most remote observatory sites there are now threatened. Light can travel hundreds of kilometers through the atmosphere, fouling the night sky far from where it is emitted. Now, no observatory is entirely safe from light pollution. [3] 

Listening for a whisper in the cacophony

Astronomers began confronting this problem in a systematic way over a half-century ago. In 1973, the International Astronomical Union (IAU) established a 'commission' tasked with identification and protection of existing and potential observatory sites. At its 1976 General Assembly, the IAU adopted this statement of concern:

The IAU notes with alarm the increasing levels of interference with astronomical observations resulting from artificial illumination of the night sky, radio emission, atmospheric pollution, and the operation of aircraft above observatory sites. The IAU therefore urgently requests that the responsible civic authorities take action to preserve existing and planned observatories from such interference. To this end, the IAU undertakes to provide through Commission 50 information on acceptable levels of interference and possible means of control.

​It issued guidelines for what it considered 'acceptable' levels of light-pollution interference in 1977. By the following year, the issue attracted the attention of the International Commission on Illumination (CIE). As the international authority on light and illumination, it took note of the increasing problem faced by observatories. It adopted a statement acknowledging the problems caused by uncontrolled outdoor lighting near the best observatory sites.  Further, it urged authorities to take all possible action to protect these sites.

The IAU and CIE joined forces in 1980, releasing the joint publication "Guidelines for minimizing urban sky glow near astronomical observatories". [4] Among its main findings, the document recommended that:

The increase in sky brightness at 45° elevation due to artificial light scattered from clear sky should not exceed 10 per cent of the lowest natural level in any part of the spectrum between wavelengths 300 and 1000 nm.

A lot has changed in 45 years. For one thing, we know more about by how much the natural night sky varies in brightness. [5] That's true on timescales ranging from minutes to years. For example, the 1980 CIE-IAU report presumed a typical night sky brightness at unpolluted sites that is only found sometimes near the minimum of the 11-year solar activity cycle. At the cycle's maximum the brightness can be over 50% brighter, even in the absence of light pollution. Natural sources of light in the night sky varying from one night to the next can yield even bigger changes.

​The bottom line in all this is that the natural night sky itself is a dynamic system "alive with its own light," as the American astronomer Al Grauer says. The night sky, apart from stars and other sources, is not a pure black due to the absence of light. It varies in brightness and color like a noisy audio signal varies in intensity and tone. To continue the analogy, light pollution is like a loud sound atop this noise, with a pure timbre that stays mostly constant from night to night. In the midst of this racket, astronomers are trying to sense a mere whisper.

A panoramic view of ESO’s Paranal Observatory in Chile, one of the naturally darkest observatory sites in the world. The four Unit Telescopes of the VLT, seen just right of centre in this panorama, are posing in front of the huge expanse of the Milky Way galaxy, which appears almost like a rainbow made of stars, arching over the site. 'Light domes' from distant villages can be seen toward the horizon at lower left. Image credit: ESO/P. Horálek (CC BY 4.0​)

An evolving landscape of protection

In 2019, astronomers reacted to a new threat from above: the light of thousands of new satellites launched into orbit around the Earth. They held an international conference in 2021 on the emerging idea of preserving "dark and quiet skies" (D&QS). The report of this conference make a series of recommendations for protecting observatory sites from encroaching light pollution. Some of these are technical in nature, while others leverage the power of public policy to regulate outdoor lighting installations.

The current IAU view, echoing the D&QS report, is:

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.
Its new guidance is to "keep the total contribution to skyglow from ALAN substantially below the 10% dark site limit defined by the IAU" (emphasis in the original). At the same time, it recognizes that there is no "one size fits all" approach:

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.

We can monitor the light situation near observatories from the ground and space, but this involves making long-term observations. Changing the trajectory of that situation requires the commitment not only of governments, but also of the public they serve. There are various proposals for how to do this. We previously wrote about one such idea: the active management of so-called "lightsheds" near observatories.

Another approach is to incentivize reducing light pollution through means such as cap-and-trade schemes. But this still requires treating observatory sites as a special kind of reservation. "For example," the IAU writes,

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.

There is now movement in the astronomical community to revise the existing 1980 standard with new information gleaned during the intervening decades. It is also informed by radical changes in the way the way the outdoor world is lit at night since the introduction of LED technology. The high directionality of LED and the ease by which active controls manipulate its light is a game-changer. And non-white light sources, such as amber LED, are more available than ever before. These sources emit light in a limited range of colors, leaving much of the rest of the spectrum dark.

​Astronomers are optimistic that these factors enable a tightening of the guidance that governs outdoor lighting. Over time, it could make a real difference for the wellbeing of the world's observatories. It may also extend their useful lifetimes. Controlling light pollution in this way could lead to a renaissance of science's "cathedrals of the night". 

References

1. Portree, D. (2002). Flagstaff’s Battle for Dark Skies. Griffith Observer (Vol. 66, No. 10, pp. 2-16) https://www2.lowell.edu/users/wes/GriffithObserver1crop.pdf
2. Barentine, J. C., Venkatesan, A., Heim, J., Lowenthal, J., Kocifaj, M., & Bará, S. (2023). Aggregate effects of proliferating low-Earth-orbit objects and implications for astronomical data lost in the noise. Nature Astronomy (Vol. 7, Issue 3, pp. 252–258). https://doi.org/10.1038/s41550-023-01904-2
3. Green, R. F., Luginbuhl, C. B., Wainscoat, R. J., & Duriscoe, D. (2022). The growing threat of light pollution to ground-based observatories. The Astronomy and Astrophysics Review (Vol. 30, Issue 1). https://doi.org/10.1007/s00159-021-00138-3
4. CIE 001-1980, https://cie.co.at/publications/guidelines-minimizing-urban-sky-glow-near-astronomical-observatories-joint-publication
5. Barentine, J. C. (2022). Night sky brightness measurement, quality assessment and monitoring. Nature Astronomy (Vol. 6, Issue 10, pp. 1120–1132). https://doi.org/10.1038/s41550-022-01756-2