Going for the Gold : Quantifying and Ranking Visual Night Sky Quality in International Dark Sky Places
John B= arentine1,*
[1]<= /a> 1Inter= national Dark-Sky Association, 3223 North First Avenue, Tucson, AZ 85719- USA=
Received 05 December 2016, Accepted 21 Decem=
ber
2016
Abstract
Since the invention of electric lighting in the nineteenth century, the=
steadily
increasing use of artificial light at night in outdoor spaces has grown to
threaten the integrity of dark night skies and nocturnal terrestrial spaces.
The conservation community has gradually come to accept the need to protect
natural nighttime darkness, which finds expression in dark sky parks and
similar protected areas. As these places begin to reap tangible economic
benefits in the form of sustainable ¡®astrotourism,¡¯ the movement to activ=
ely
protect them gains strength. The International Dark-Sky Association designa=
tes
Dark Sky Parks and Reserves under a comparative ranking scheme that assigns=
night
sky quality tiers according to a combination of objective and subjective
characteristics, but shortcomings in the consistency of these ratings exist
that undermine the consistency and reputation of the designation program. H=
ere
we consider potential changes to the qualification regime to make the ratin=
gs
system more robust for the benefit of future designations.
Keywords: Astrotourism, Conservation, Parks and
protected areas
1.
Introduction
Access
to naturally dark night skies is threatened on a global scale as the result=
of
emissions of artificial light into the nocturnal environment; some 80% of t=
he
world and more than 99% of the U.S. and European populations now live under
light-polluted night skies. The visibility of the Milky Way, considered a
metric of night sky quality, is impaired by anthropogenic light pollution a=
mong
one-third of humans worldwide. [1] The loss of the night brings a diversity=
of
known and suspected hazards to wildlife ecology that, in concert with global
climate change, presents a new set of challenges to the relationship between
humans and the environment. [2] Steady growth of world population and the r=
apid
industrialization of emerging economies underscore the acute nature of both=
the
threat and the need to protect what natural darkness still exists. The
International Dark-Sky Association (IDA), a U.S.-based non-governmental
organization, offers accreditation through its International Dark Sky Places
(IDSP) Program1 to
protected public and private lands that proactively manage their use of out=
door
light at night and educate visitors on the value of natural nighttime darkn=
ess.
2.
The Program
2.1. Goals and Achievements
The
goals of the IDSP Program include encouraging good stewardship of the noctu=
rnal
environment in both cities and parks through the promotion of sustainable, =
night-sky-friendly
lighting practices, and raising public awareness of the issue of threatened
dark night skies around the world. Attendant to the IDA award is global exp=
osure
to the burgeoning astrotourism industry, which brings potential concrete
economic development benefits to places that embrace the principles of dark
skies conservation. [3,4]
Broadly
speaking, two kinds of IDSP awards are given. The first (¡°Dark Sky Communi=
ty¡±
and ¡°Dark Sky Friendly Development of Distinction¡±) recognizes the effort=
s of
municipalities and similar political entities in controlling the spread of
light pollution through the effective application of public policy and the =
building
of social and political coalitions. The second type of award (¡°Dark Sky Pa=
rk¡±,
¡°Dark Sky Reserve¡±, and ¡°Dark Sky Sanctuary¡±) highlights the active pro=
tection
of legally protected areas such as national parks that retain some degree of
natural nighttime darkness.
Both
types of award value the role of public outreach and education in raising
awareness of the issues of light pollution and dark skies; while Dark Sky
Communities focus efforts inward to educate their residents, Parks, Reserves
and Sanctuaries direct outreach to visitors and the world beyond the protec=
ted
area. In all cases, proper attention is paid to the qualities of outdoor ni=
ght
lighting at IDSP sites, with requirements for light warranting and shieldin=
g,
prevention of overlighting, and spectrum control to minimize the emission of
environmentally hazardous short-wavelength light. [5]
Program
designations are made on the basis of a written application to IDA
demonstrating adherence to a set of published guidelines. Applications unde=
rgo
a form of peer review from a standing committee of the IDA Board of Directo=
rs;
final approval is given at the discretion of the Board itself. Accredited I=
DSPs
assume the obligation to report to IDA on an annual basis regarding the sta=
te
of their night sky quality, ongoing outreach efforts, and changes or
improvements to site lighting. As of the start of 2017, 71 IDSPs have =
been
designated in 13 countries (see map, Figure 1), bringing some 62,655 k=
m2
of land under formal protection for the quality of its natural nighttime
environment.
Fig. 1. Wor=
ld
map showing the 71 designated IDA International Dark Sky Places as of the
beginning of 2017. The symbols are blue circles (Dark Sky Communities), pur=
ple
crosses (Dark Sky Friendly Developments of Distinction), red squares (Dark =
Sky
Parks), green diamonds (Dark Sky Reserves), and yellow stars (Dark Sky
Sanctuaries).
2.2. Night Sky Quality Tier Scheme
IDA
Dark Sky Parks, Reserves and Sanctuaries must establish their eligibility f=
or
participation in the program in part by submitting night sky luminance
measurements. Park and Reserve candidates are currently ranked according to
increasing mean zenith luminance in ¡®Gold¡¯, ¡®Silver¡¯ and ¡®Bronze¡¯ tie=
rs. These
values, typically obtained using the Unihedron Sky Quality Meter (SQM) [6],=
are
supplemented with horizon photography documenting artificial light domes,
narrative descriptions of observed faint night-sky phenomena, and naked-eye
limiting magnitude and Bortle Scale [7] estimates. Criteria defining the th=
ree
tiers are listed in Table 1, adapted from the IDSP Program guidelines.
Table 1. Summary of the subjective and objective qualities
characterizing the three IDA International Dark Sky Places sky quality tier=
s.
¡®mpsa¡¯ stands for magnitudes per square arcsecond, a unit of luminance us=
ed
mainly by astronomers.
|
Indicator |
Gold |
Silver |
Bronze |
|
Philosophy |
Nighttime
environments that have negligible to minor impacts from light pollution a=
nd
other artificial light disturbance, yet still display outstanding quality
night skies and have superior nighttime lightscapes. |
Nighttime
environments that have minor impacts from light pollution and other
artificial light disturbance, yet still display good quality night skies =
and
have exemplary nighttime lightscapes. |
Areas
not meeting the requirements of Silver, yet still offering people, plants,
and animals a respite from a degraded nocturnal environment and suitable =
for
communicating the issue of light pollution and connecting people with the
many aspects of the night sky. |
|
Artificial Light and Skyglow=
|
Typical observer is not =
distracted
by glary light sources. Light domes are only dim and restricted to sky cl=
ose
to horizon. |
Point light sources and
glary lights do not dominate nighttime scene. Light domes present around
horizon but do not stretch to zenith. |
Areas with greater artif=
icial
light and skyglow than Silver, but where aspects of the natural sky are s=
till
visible. |
|
Observable Night Sky Phenome=
na |
The
full array of visible sky phenomena can be viewed— e.g. aurora, air=
glow,
Milky Way, zodiacal light, and faint meteors. |
Brighter sky phenomena c=
an
be regularly viewed, with fainter ones sometimes visible. Milky Way is
visible in summer and winter. |
Many sky phenomena canno=
t be
seen. Milky Way is seen when pointed out to the average person, as is the
Andromeda Galaxy. |
|
Nocturnal Environment |
Area is devoid of obvious
lights that can cause wildlife disorientation. Artificial light levels are
thought to be below the threshold for plant and animal impact. Ecological
processes related to nocturnality are unaltered. No lighting atop towers =
or
buildings within Park boundary. |
Areas that have minor to
moderate ground illumination from artificial skyglow. Lights that may cau=
se
disorientation to wildlife are distant. Disruption of ecological processe=
s is
minor with no impairment to plants or wildlife. |
Areas with greater noctu=
rnal
impact than Silver, but where ecosystems are still functional. |
|
Naked Eye Limiting Magnitude=
|
Equal or greater than +6=
.8
under clear skies and good seeing conditions |
+6.0 to +6.7 under clear
skies and good conditions |
+5.0 to +5.9 under clear
skies and good seeing conditions |
|
Bortle Class |
1-3 |
3-5 |
5-6 |
|
Zenith Luminance |
>21.75 mpsa |
21.74-21.00 mpsa |
20.99-20.00 mpsa |
The
number of designated Dark Sky Parks and Reserves is not evenly distributed
among the tiers. Of the 42 Parks and 11 Reserves named to date, 25 (47.2%) =
are
designated Gold, 26 (49.1%) Silver, and 2 (3.8%) Bronze. A histogram of ove=
r 30,000
individual SQM-L (narrow acceptance cone) measurements obtained from 33 IDS=
Ps
around the world, shown in Figure 2, illustrates the range of night sky
luminances encountered over various protected lands considered ¡®dark¡¯ to =
some
degree.
3.
The Problem
To
properly assess the eligibility of candidate Dark Sky Parks, Reserves and S=
anctuaries
for IDA accreditation we must gather sufficiently objective night sky
brightness information in order to make meaningful comparisons among differ=
ent
sites. Further, for Parks and Reserves, we must establish the proper sky
quality tier within the existing scheme. This has proven to be difficult in
practice. Reliance on qualitative descriptions of night sky quality and
quantitative measurements obtained solely at the zenith anecdotally undercu=
ts
the integrity of the Gold/Silver/Bronze tier scheme, which itself has been
criticized on the basis of visibility studies. [8] Even the names of the ti=
er
labels are problematic, evoking a psychology of winners and losers and crea=
ting
the potential for misrepresentation of sky conditions by IDSP applicants wh=
en
site reputation and future tourism revenue are considered to be at stake. A=
ny
relative rating scheme for IDSPs involves subjective demarcations of the
labels, which results naturally when we attempt to impose arbitrary limits =
or
ranges on empirical observations. As an example, Figure 2 shows the particu=
lar
difficulty in deciding between awarding the Silver and Gold tiers.
It
is increasingly clear that while SQM measurements reliably characterize sky=
luminances
in the presence of significant skyglow from anthropogenic light pollution,
their reliability breaks down in situations where there is little or no
artificial skyglow. [9] In those circumstances the temporally variable
intensity of airglow often dominates, and in all cases the zenith luminance=
is
at best a quasi-periodic function of time. Furthermore, although zenith
luminance measurements offer better inter-comparability of different sites,
they fail to account for light domes along the horizon that may significant=
ly impact
visibility over a large fraction of the sky. Thus, while the SQM is valuabl=
e in
assessing light-polluted locations, its value is compromised in places that=
are
naturally very dark.
We
are left with remarkably fundamental questions unanswered in the context of
natural nighttime landscape protection: What is a ¡®dark sky,¡¯ and how do =
we
describe the quality of ¡®darkness¡¯? And two specific programmatic needs h=
ave
emerged: first, to amend or replace the current tier system according to our
best understanding of human visual perception of the night sky and the vari=
ous
natural and artificial light sources that influence it; and second, to deve=
lop
low-cost, readily deployable best-practice methods and data collection
protocols for characterizing and monitoring night sky quality.
4.
Potential Solutions
4.1. Improved Methods to Measure Night Sky Luminan=
ce
The
problem we face is both one of measurement and of deciding which things to
measure. It seems clear that we can no longer rely solely on point measurem=
ents
made with the SQM(-L) at the zenith as our main source of information on the
conditions of nighttime darkness in Dark Sky Places. Several potential
solutions to the problem exist, all of which lead toward a holistic approac=
h to
measuring the brightness of the entire night sky.
Grids
of SQM-L measurements taken around the sky may be interpolated to produce c=
rude
all-sky luminance maps [10], but they lack spatial resolution of individual
light domes. Progress has been made in developing calibrated all-sky camera
systems that provide information on the spatial distribution of both natural
and artificial sources of light and the gradient of sky luminance
[11,12,13,14,15,16,17]; however, these systems are generally expensive to
procure and often require expertise to properly operate. Another option is =
to
combine zenith SQM-L measurements with uncalibrated horizon photography; gi=
ven
that most of the artificial sky luminance observed toward the zenith origin=
ates
from ground sources within ~25 km of a given location [18], this method can
help identify the nearest problematic sources to target for mitigation. Tab=
le 2
summarizes the different approaches and the benefits and drawbacks of each:=
Table
2. Summary of the subjective and objective qualities characterizing the thr=
ee
IDA International Dark Sky Places sky quality tiers.
|
Appro=
ach |
Advan=
tage |
Disad=
vantage |
Refer=
ence |
|
Makes use of existing, inexpensive and widely-available
SQM devices |
Lacks good spatial resolution of individual light dome=
s on
the local horizon |
10 |
|
|
Gives more precise information on the brightness and
extent of light domes impacting nighttime environment |
Systems are generally expensive to procure and often
require expertise to properly operate |
11-17 |
|
|
Involves readily available instruments not requiring
special calibration |
Uncalibrated imagery is of limited use for tracking
quantitative changes to light domes |
18 |
There
is presently an increasing demand for an affordable, off-the-shelf imagery
system for obtaining simultaneous all-sky measurements of night sky luminan=
ce
for both initial site evaluation and ongoing monitoring. Duriscoe [19] sugg=
ests
a number of useful metrics that can be extracted from such spatially-resolv=
ed sky
luminance data; combined with visibility studies, these may lead toward a m=
ore
robust and objective means of deciding which candidates are best suited for=
IDA
Dark Sky Place recognition.
4.2. A New Aesthetic Approach
Another
means of solving the problem is to develop new metrics of the human visual
experience of dark-sky sites, recognizing that this experience is both
quantitative and qualitative in nature. Characterizing IDA Dark Sky Places =
in
terms of their night sky brightness has scientific value, but the program is
equally oriented toward aesthetic appreciation of night skies by visitors. =
One
possible improvement on the current tier scheme is to cite a range of
statistically likely values on one or more scales. These scales may be
intuitive and user friendly. For example, the Sky Quality Index (SQI; see
Figure 3) is an experimental 1-100 index developed by the U.S. National
Park
Service that features units of equal aesthetic value. [20]
Fig. 3.
Even
in the absence of anthropogenic light pollution, a metric such as the SQI is
subject to time variability on scales spanning hours to years due to change=
s in
the intensity of airglow and aurorae, atmospheric aerosol optical depth, and
other natural factors. It may not be possible to completely capture the
¡®typical¡¯ conditions at a dark sky site through point measurements of a s=
ingle
characteristic. Rather, it may be best to quote a range of typical values on
one scale or another, centered on the most probable value, and in which the
statistical scatter of long-term measurements indicates the width of the ra=
nge.
Combining a dark-sky quality range with an expression of local historical
climate information drives a new type of index that better equips travelers=
to
Dark Sky Parks and Reserves to choose which sites to visit based on the
greatest probability of observing the darkest (and clearest) night skies.
5.
Summary
While
the IDA International Dark Sky Places Program has been highly successful in
securing protections for the dark night skies over parks and protected areas
around the world, a serious deficiency exists in the methods of characteriz=
ing
the quality of those skies. We have established here the nature of the prob=
lem
and explained why it is important to solve. We have also briefly examined a
variety of approaches to an improved understanding of night sky quality
indicators. From this analysis, we conclude that broad metrics such as the
existing IDSP quality tiers do not adequately characterize the complexity of
night sky conditions from a given location over various timescales, and that
further reliance on them may be detrimental to achieving the program¡¯s goa=
ls.
We find there are several ways forward that lead toward alternatives to the
existing Gold/Silver/Bronze tier system. It is expected that IDA will annou=
nce
a new framework for ranking IDSPs by sky brightness along with updated prog=
ram
guidelines in 2017.
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