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REVIEW 2 major objections 4 minor 78 references

Natural experiments from Earth Hour reveal urban night sky being drastically lit up by few decorative buildings

T0 review · 2 major / 4 minor · reviewed 2026-08-06 · deepseek-v4-flash

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Pith's one-line read A few dozen decorative building lights can dominate a city's night-sky glow, Earth Hour measurements in Hong Kong show.

desk verdict The qualitative conclusion—that a few dozen decorative lights dominate urban zenith skyglow in Hong Kong—is probably right, but the headline darkening percentages rest on an unvalidated baseline and should be treated as rough. read the letter →

arxiv 2507.02019 v1 pith:4MCUVIBF submitted 2025-07-02 astro-ph.IM physics.ao-ph

Chu Wing So , Chun Shing Jason Pun , Shengjie Liu , Sze Leung Cheung , Ho Keung Kenneith Hui , Kelly Blumenthal , Constance Elaine Walker This is my paper
classification astro-ph.IMphysics.ao-ph
keywords lightpollutionEarthHournightskybrightnessskyglowLEDbillboardsmetalhalidefloodlightsurbanlightingHongKong
verification ladder T0 review T1 audit T2 compute T3 formal

The pith

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The reading

This paper uses the annual Earth Hour lights-out event as a repeated natural experiment in downtown Hong Kong, combining photometric, spectroscopic, and imaging observations from 2011 to 2024. It reports that the zenith night sky darkened by up to about 50% during the lights-out hour, with clear-sky drops of 52.7%, 52.2%, and 25.5% in 2013, 2015, and 2021. The darkening is traced to roughly 120 decorative and advertising lights on commercial and office buildings, with spectral residuals matching LED billboard screens and metal-halide floodlights. If the interpretation is right, a small set of non-essential building lights, rather than street lighting or general urban illumination, is the main driver of urban skyglow at this site, so targeted regulation of decorative lighting could produce immediate reductions.

What carries the argument

The central mechanism is the before-and-after comparison of zenith sky brightness and spectra across the fixed one-hour Earth Hour window. The load-bearing measurement is the ratio of average sky brightness during lights-out to the average of the 30-minute periods before and after the event, applied to Sky Quality Meter photometry at a single urban rooftop; a parallel before-minus-after residual analysis of zenith spectra identifies which lamp types disappear. Because a large coordinated set of lights goes out and then back on at defined times, any brightness or spectral change tied to those times can be attributed to the switched-off fixtures. Supporting machinery includes all-sky and wide-field imagery to identify individual participating buildings, a 2021 street-level survey to count fixtures, and a moonlight model to remove lunar contribution in 2013, 2015, and 2021.

What would settle it

Run the same before/during/after brightness analysis on a clear Saturday night at the same site with no coordinated lights-out; if the 21:00 'darkening' relative to the 20:00 and 22:00 averages is near zero, the Earth Hour signal is real, but if it reproduces a large fraction of the 20-50% drop, the result is an artifact of the evening brightness trend.

Watch

Extended reading notes

Core claim

The central discovery is that the zenith night sky over a dense downtown area brightens and darkens in lockstep with a relatively small number of decorative and advertising lights. During Earth Hour, the lights-out of approximately 120 external light fixtures in the core business districts reduced zenith sky brightness by 23.4% to 52.7% relative to the average of the 30-minute periods before and after the event; the clear-sky years gave 52.7% (2013), 52.2% (2015), and 25.5% (2021). Spectroscopic before-minus-after residuals show that the dimmed light is concentrated in blue, green, and red LED emission bands and in the 585-595 nm band of metal-halide floodlights, while continuously operating street lights leave no residual. The paper concludes that a few decorative buildings with facade lighting, roof billboards, and LED video walls are the primary contributors to urban light pollution in this district, and that the surrounding weekday lighting pattern was otherwise unaffected by the event.

Load-bearing premise

The headline darkening percentages assume that the average sky brightness measured 30 minutes before and after the lights-out hour is the correct prediction of what the sky would have been during the hour without Earth Hour, and no non-event control nights are shown to check that assumption.

Editorial extensions

If this is right

  • Regulating decorative and advertising lighting, especially facade lights, roof billboards, and LED video walls, can produce immediate and sizable reductions in urban skyglow without touching street lighting.
  • Cities with concentrated business districts could achieve most of the available darkening by applying curfews or permit limits to a small number of buildings rather than to the entire lighting stock.
  • Replacing blue-rich LED billboards and metal-halide floodlights with phosphor-converted amber LEDs would remove the specific spectral bands that dominate the measured skyglow while preserving energy efficiency.
  • The return of sky brightness to pre-event levels immediately after 21:30 shows that the effect is reversible and directly tied to lighting schedules, not to weather or moonlight.
  • Because Earth Hour recurs annually under varying lunar and cloud conditions, it provides a repeatable experiment for quantifying the skyglow contribution of specific light-source categories in other cities.

Reading between the lines

Editorial extensions of the paper, not claims the author makes directly.

  • Beyond the paper: the same before/during/after logic could be applied to other coordinated lighting events, such as holiday light displays or stadium blackouts, to estimate per-category skyglow contributions in cities that do not participate in Earth Hour.
  • Beyond the paper: the sensitivity of the 2022 result to baseline choice (23.4% darkening when both bracketing periods are used versus 6.3% when only the post-event period is used) suggests that the absolute percentages are less certain than the sign and timing of the effect, which are independently corroborated by imagery and spectra.
  • Beyond the paper: because the photometer pointed only at the zenith, a human observer looking horizontally toward harbour-facing billboards would likely perceive an even larger darkening than the measured 50%; directional luminance measurements during Earth Hour could test this directly.
  • Beyond the paper: if a few fixtures dominate skyglow in Hong Kong, similar few-fixture dominance may hold in other dense waterfront cities with large advertising displays, and replicating this survey elsewhere would show whether the pattern is general.
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Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, and a circularity audit.

Referee Report

2 major / 4 minor

Summary. The manuscript analyzes 14 years of Earth Hour observations in Hong Kong (2011-2024) using all-sky cameras, wide-field DSLR imagery, SQM photometry, and a spectrometer, all co-located at the Hong Kong Space Museum in Tsim Sha Tsui (TST). It reports that the zenith night sky darkened by 23.4% to 52.7% during the lights-out hour, with clear-sky darkening of 52.7%, 52.2%, and 25.5% in 2013, 2015, and 2021. The reduction is attributed to roughly 120 decorative and advertising lighting fixtures on commercial buildings in the core business districts, identified through a 2021 street-level survey and corroborated by spectral residuals corresponding to LED and metal halide sources. The paper concludes that a small number of decorative/advertising lights are the dominant contributors to urban skyglow at this site, and discusses policy implications such as turning off such lights after business hours and replacing them with less blue-rich alternatives.

Significance. If the quantitative claims hold, this is an important natural-experiment result: it demonstrates that a small, identifiable set of decorative and advertising lights can dominate zenith sky brightness in a dense urban environment, with direct implications for light-pollution policy and mitigation. The study's strengths are considerable: a 14-year observational baseline; the sharp 20:30/21:30 timing of the lights-out event, which provides strong causal identification; the multimodal corroboration (all-sky images, wide-field photography, a 122-fixture field survey, SQM photometry, and spectroscopy); and public data availability at figshare. The paper also transparently discloses the main caveats, including the sensitivity of the darkening percentages to the choice of baseline and the approximations in the moonlight model. These caveats, however, mean that the headline percentages, as presently stated, are not as firm as the abstract and introduction imply.

major comments (2)
  1. [Sky darkening as seen from NSB observations; Methods, Eq. (2)] The central quantitative claim—the 23.4%-52.7% darkening range—rests on the baseline defined in Eq. (2), which compares the lights-out period with the mean of the 30-minute intervals before and after the event. No non-Earth-Hour control nights at TST are presented to test the implied flat-counterfactual assumption. The manuscript itself provides a sensitivity test in Methods: using only the post-event baseline changes the 2022 value from 23.4% to 6.3% and the 2013 clear-sky value from 52.7% to 45.9%. Because the abstract and Introduction present these percentages as the main quantitative result, the paper must either justify the adopted baseline with control data or report the darkening as a range/with uncertainty that reflects this sensitivity. As written, the precision implied by the single-number percentages is not supported.
  2. [Removing moonlight from NSB observed] For 2013, the Moon transits at 21:25, inside the lights-out window, and the predicted moonlight contribution to the SQM reading increases from 38% at 20:00 to 51% near transit. The 2013 and 2015 clear-sky darkening figures (52.7% and 52.2%) are corrected with a moonlight model whose stated accuracy is 8%-23% and which does not account for cloud shielding. A residual temporal trend in the moonlight correction would directly bias the computed darkening. The paper shows pre- and post-correction curves in Supplementary figure S7 but does not provide a quantitative sensitivity test (for example, using a different moonlight model or perturbing the model within its quoted accuracy). Without such a test, the reliability of the two clear-sky percentages that anchor the 'up to 50%' claim is not fully established.
minor comments (4)
  1. [Man-made emissions darkening as seen from spectroscopic observations] The spectral attribution would be strengthened by showing the 'normal Saturdays' control data mentioned in the last paragraph of this section; the statement that no significant residuals were detected is not quantified or plotted, so the reader cannot assess the size of the null result.
  2. [Methods, NSB observations] The sentence listing alternative darkening percentages (55.8%, 45.9%, 53.3%, etc.) is a sensitivity result that would be more readable if presented as a table or with explicit mapping to each year; as written, the reader must count through the list to match years.
  3. [Reference 37] Reference 37 contains garbled characters ('´Sci˛ e˙zor'); the author name should be corrected (likely 'Šciężor').
  4. [Figure 6 caption and Results text] The caption states that 2021 pre-20:50 data are shown 'for visualization purposes' in one place and 'for completeness only' in another; this wording is inconsistent and should be clarified to indicate whether these data are used in any quantitative analysis.

Circularity Check

0 steps flagged · score 2.0 of 10

No significant circularity: the Earth Hour darkening is a direct measurement, and the only self-cited input is a non-load-bearing moonlight model.

full rationale

Eq. (2) is a measurement definition, not a fitted prediction: the darkening percentage is the ratio of the average sky brightness during the lights-out period to the average of the 30-minute intervals before and after. The paper does not fit any parameter to the Earth Hour dip and then rename that fit as a result. The dip is visible in the raw data even before lunar correction, as the paper explicitly states: "the influence of the Earth Hour event is still discernible even before removing the lunar contribution." The only self-cited element in the derivation chain is the moonlight model from the first author's prior thesis (ref. 35), used to correct the 2013, 2015, and 2021 light curves. This model is not fitted to the Earth Hour data, its formula and 8-23% accuracy range are disclosed, and the qualitative event is independent of it. The spectral attribution relies on standard emission features (LED bands at 445-500, 500-540, 615-650 nm; metal halide at 585-595 nm), with verification against normal Saturdays and the 2022 case. The sensitivity table showing different darkening values if only the post-event baseline is used (e.g., 2022 changing from 23.4% to 6.3%) is an honest robustness caveat about the flat-baseline counterfactual; that is a correctness/assumption concern, not a circular reduction of a claim into its own inputs. No equation in the paper reduces a target result to a fitted quantity, a self-citation chain, or a definitional identity.

Assumptions & free parameters 1 free parameters · 5 assumptions · 0 invented entities

The central claim rests on a handful of domain assumptions about the measurement setup and the counterfactual baseline, plus a moonlight model carried over from prior work by the same group. No new physical entities are introduced.

free parameters (1)
  • Moonlight model calibration parameters (from ref. 35) = Not reported in this paper
    The modified Krisciunas and Schaefer moonlight model used to correct NSB in 2013, 2015 and 2021 was fit to SQM observations in Hong Kong in the first author's PhD thesis. The model's accuracy is stated as 8-23%, and it does not account for cloud shielding, so errors propagate into the reported darkening percentages.
assumptions (5)
  • domain assumption The SQM zenith measurement is representative of the ALAN-affected sky brightness relevant to the study.
    The SQM has a 20-degree cone and spectral sensitivity different from the human eye; the paper acknowledges this limitation in the Discussion.
  • domain assumption No other simultaneous changes (traffic, business closures, weather) caused the observed NSB dip beyond the Earth Hour lights-out.
    The all-sky images and the exact timing support this, but no control nights at TST without Earth Hour are provided.
  • domain assumption The 'before-and-after' average (or the post-event period for 2021) is a valid counterfactual for the non-Earth-Hour sky brightness.
    This is the weakest assumption; the alternative calculations using only the post-event baseline yield very different darkening percentages (e.g., 6.3% vs 23.4% for 2022).
  • domain assumption The modified moonlight model (ref. 35) accurately predicts the moonlight contribution to SQM readings.
    The model has 8-23% accuracy and ignores cloud shielding; predicted moonlight contribution reaches 51% of NSB in 2013.
  • domain assumption The spectral normalization and median subtraction isolate the spectral signature of the lights that were turned off.
    The normalization procedure is not fully specified in the main text, and no error bars are given for the residuals.

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Cite this review

Pith. "Pith review of Natural experiments from Earth Hour reveal urban night sky being drastically lit up by few decorative buildings." pith.science (2026). https://pith.science/paper/4MCUVIBF

@misc{pith2026250702019,
  author       = {Pith},
  title        = {Pith review of: Natural experiments from Earth Hour reveal urban night sky being drastically lit up by few decorative buildings},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/4MCUVIBF}},
  note         = {Machine review of arXiv:2507.02019}
}
read the original abstract

Light pollution, a typically underrecognized environmental issue, has gained attention in recent years. While controlling light pollution requires sustained efforts, Earth Hour offers a unique natural experimental setting to assess temporary lights-out measures. Using photometric and spectroscopic sensors, we observed up to 50% night sky darkening during Earth Hour from 2011 to 2024 in Hong Kong, primarily as a result of a small but critical number of lights-out instances in central business districts, as evidenced by crowd-sourced photography records. Weekend lighting pattern in the city during Earth Hour remained unaffected. The emission reductions mostly occurred in the 445-500, 500-540, and 615-650 nm spectral ranges-corresponding to peak emissions from LED billboard screens- and in the 585-595 nm range, associated with metal halide floodlights used for facades and billboards. Our study identifies these sources as major contributors to urban light pollution. By combining multimodal observations, we offer a comprehensive assessment of light pollution sources and the potential benefits of sustainable lighting practices in urban environments. This research highlights the importance of targeted light pollution mitigation efforts and provides critical insights for policymakers to enhance urban sustainability and human well-being.

Figures

Figures reproduced from arXiv: 2507.02019 by the authors.

Figure 1
Figure 1. (a) The nighttime image of Hong Kong taken from the International Space Station at 00:58, 20 January 2015. (b) The 2021 map of downtown Hong Kong, highlighting five CBDs in Kowloon and northern Hong Kong Island. Different symbols represent the locations of observation methods used in this study: a yellow star indicates the same position of the all-sky camera, SQM and spectrometer (at the Hong Kong Space Museum that … view at source ↗
Figure 2
Figure 2. All-sky images of the lights-out in 2015-2019 are arranged from top to bottom. Images were taken at 20:00 (before the lights-out, left column), 21:00 (during the lights-out, middle column) and 22:00 (after the lights-out, right column). Timestamps in YYYY MM DD HHMM are repeated above the images. The Moon, if presented and was not blocked by clouds, appeared as a bright spot in the images. North is oriented at the t… view at source ↗
Figure 3
Figure 3. Same as [PITH_FULL_IMAGE:figures/full_fig_p005_3.png] view at source ↗
Figures from the paper (4 more)
Figure 4
Figure 4. Figure 4: This figure shows a sample all-sky image taken at 20:02 on 28 March 2020 (a half hour before the start of lights-out), with north oriented up and east on the left. Labels are provided clockwise from north, identifying groups of light sources being studied. The side pho…
Figure 5
Figure 5. Figure 5: Cropped wide-field images of CBDs taken at TST during Earth Hour 2013: (a) 20:00 (before the lights-out). (b) 21:00 (during the lights-out). (c) 22:00 (after the lights-out). A booth canopy blocked a part of Wan Chai in the foreground. See [PITH_FULL_IMAGE:figures/ful…
Figure 6
Figure 6. Figure 6: (a) Relative sky brightness in term of NSB at TST in Hong Kong during clear Earth Hour evenings. (b) Same for cloudy evenings. Each NSB measurement was converted to a linear quantity with Equation 1 in Methods then presented as a ratio with reference to the average bri…
Figure 7
Figure 7. Figure 7: (a) Zenith sky spectra, plotted in logarithmic scales, acquired at TST (same location as the all-sky and NSB observations) in Hong Kong during the 2021 Earth Hour evenings. (b) Same for 2022. The red curves represent the median "before lights-out" normalized spectra, w…

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