MOONLIGHT
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1 Despite its importance to biologists
studying physiology and behaviour,
there is no common reference value
for the illuminance of the full Moon.
(Ricardoreitmeyer, Dreamstime.com)
How bright is
moonlight?
Christopher Kyba, Andrej 2016), especially in tidal environments (e.g. bright moonlight – and to stress that this is
Brady et al. 2016). Despite its importance for not representative of moonlight in general.
Mohar and Thomas Posch seek many organisms, the scientific literature We report observations of the supermoon
a standard figure for moonlight lacks a commonly accepted reference value of 14 November 2016, made in an open field
illuminance. for the photopic illuminance of full moon- 49 km from the centre of Vienna, Austria
light. As a result, many existing publica- (47.7956°N, 16.1366°E) at 23:25 local time.
tions report incorrect values of full Moon The Moon was at 55.5° elevation, 8 hours
R
ecent years have seen increasing illuminance, for example 2.2 lux (Marcum and 33 minutes after full Moon (99.8%
recognition and study of the effect et al. 2004), 2 lux (Yorzinski et al. 2015), and full), and 11 hours 2 minutes after perigee
of moonlight on plants and animals. 0.5–1 lux (Bruce-White & (Espenak 2016). The lunar
The typical brightness of moonlight is Shardlow 2011). Even the “Exceptionally bright distances and elevations
therefore an important parameter for Wikipedia page for “Lux” moonlight is not at are based on Meeus (1991),
biological laboratory studies, yet incorrect currently reports an incorrect all representative of calculated in Perl using
values for the moonlight illuminance are range of up to 1 lux for full ‘typical’ moonlight” Astro::Coord::ECI::Moon by
frequently used and cited. We have used moonlight, based on an old Tom Wyant. Illuminance was
the brightness of the “supermoon” of 14 citation (Bünning & Moser 1969), which in measured using a Minolta T-10 illuminance
November 2016 to demonstrate that typical fact bases the value on an even older Ger- meter. The horizontal photopic illuminance
lunar illuminance is around 0.05 to 0.1 lux man text (Sauberer & Härtel 1959). was 0.26 lux. In a second observation, the
at temperate latitudes during the summer. illuminance meter was tilted in order to
The amount of visible light on Earth’s Maximum value measure the illuminance in the plane per-
surface at night varies by about three orders Biologists have a particular interest in the pendicular to the Moon, yielding 0.30 lux.
of magnitude over the course of a month, maximum possible values of moonlight, i.e. The background from airglow, stars and
as a result of the lunar cycle. This change for a full Moon at zenith (directly overhead) the Milky Way can be excluded, as it is in
has profound effects on the physiology in a clear atmosphere. Our intentions here the millilux range (Hänel et al. in prep.). The
and behaviour of many plants and animals are to provide an easy-to-find reference influence from the artificial skyglow from
(Kronfeld-Schor et al. 2013, Reinberg et al. containing a measurement of exceptionally Vienna and surrounding communities was
A&G • February 2017 • Vol. 58 • aandg.org 1.31
MOONLIGHT
negligible for this observation: Falchi et 0.3
al. (2016a,b) estimated the zenith skyglow 100
to be 0.265 mcd m–2, which corresponds to 0.25
illuminance (lux)
approximately 0.001 lux (using the approxi-
80
total minutes
mation I = πL, where I is illuminance and 0.2
L is zenith luminance, which is probably
60
accurate to within about a factor of 2 [see 0.15
Kocifaj et al. 2015]). For comparison, in
40
central Vienna, Falchi et al. (2016a) predict 0.1
4.59 mcd m–2 (~0.014 lux), and Puschnig et
0.05 20
al. (2014) reported best sky luminance of
2.15 mcd m–2 (~0.007 lux) at 3.5 km from cen-
tral Vienna. While moonlight dominates 0 0
-5 0 5 0 0.1 0.2 0.3
over skyglow under clear conditions, this
hour illuminance (lux)
is not the case under overcast conditions.
The typical overcast skyglow observed by 2 Comparison of photopic illuminance from the full Moon according to equation 1 for 14 November
Puschnig et al. (2014) before midnight was 2016 (red) and 7 August 2017 (blue). The left panel shows illuminance against time relative to local
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about 45 mcd m–2, approximately 0.14 lux, of midnight, and the histograms at right show the total number of minutes under each illuminance
similar magnitude to full moonlight. Other condition. Note that these graphs do not account for atmospheric scattering and absorption, which
areas are considerably brighter than Vienna would further reduce the illluminance when the Moon is at low elevation angles.
(Pun et al. 2014, Kyba et al. 2015).
391 000 km. This will reduce the expected effect of atmospheric conditions. The
Celestial illuminance illuminance by nearly 20%. More impor- observations reported here were for a clean,
Neglecting atmospheric scattering and tantly, summer full Moons do not rise as clear sky. Increased air pollution can reduce
absorption, the horizontal illuminance high; on 7 August 2017 the Moon will only lunar illumination, and overcast skies
from a celestial source goes as I = Io sinqe, rise to 26.5° above the horizon near Vienna. dramatically reduce it. Moonlit landscapes
where qe is the elevation angle and Io is the The horizontal photopic illuminance esti- free of artificial light pollution experienced
illuminance from the source when it is at mated by equation 1 for 14 November 2016 by wild nocturnal animals will in general
zenith. The observation of 0.26 lux for a (red) and 7 August 2017 (blue) is shown be far less bright than the maximum values
Moon at 55.5° elevation (34.5° from zenith) in figure 2. The time of the observation is reported here.
suggests that had the Moon been directly shown by the black circle in the left panel. We have shown that in the extremely
at zenith, the illuminance would have been The effect of the higher elevation angle in unusual case of a near-perigee full Moon,
near 0.32 lux. It is critical to note that 0.3 lux November is reflected both near-zenith, under near-ideal
is not “typical” for moonlight, but rather in the maximum illuminance “A definitive publication atmospheric conditions, the
a value that could occur under optimal and in the length of time the is needed of long- maximum possible horizon-
atmospheric conditions, for a few hours Moon is up. Note that the term ‘typical’ values of tal photopic illuminance is
each year, in the tropics. The Moon never blue curve in the left panel moonlit nights” approximately 0.3 lux. This
reaches zenith at latitudes greater than 28° is shifted by one hour due to value does not represent a
(all of Europe, most of the USA), and the full daylight saving time. typical value of moonlight, but rather the
Moon rises to lower elevations in summer. Equation 1 and figure 2 neglect atmo most extreme case possible: a supermoon,
The Earth surface to Moon distance spheric scattering and absorption. When near zenith, near perigee, nearly fully illu-
(henceforth Earth–Moon distance) has a the Moon is at an elevation of less than minated and experiencing the opposition
minor effect on illuminance, because the 45°, these processes also reduce moonlight surge. These conditions are not representa-
Moon is slightly smaller in the sky when illuminance. Even when the Moon reaches tive of moonlit nights as a whole, and we
it is further away. The Moon illuminance its highest elevation, most full Moons at urge biologists studying the influence of
decreases with the square of the Earth– mid-latitudes will produce only 0.05– moonlight illumination levels on physiol-
Moon distance, so we can approximately 0.2 lux. Furthermore, the Moon experiences ogy and behaviour to study lower light
compare full Moon illuminance using a dramatic increase in brightness when it levels (e.g. 0.05–0.10 lux). There is a need
I = Io sinqe (do/d)2(1) is completely full (the so-called “opposi- for a definitive publication, written with
where do is the Earth–Moon distance when tion surge”, see e.g. Buratti et al. 1996). This biologists in mind, that reports long-term
Io was estimated. means that on the nights before or after the typical values of moonlit nights. These
On 14 November 2016, the Earth–Moon night of full Moon, direct illuminance can- observations must be taken from a location
distance was about 351 000 km. In contrast, not reach 0.32 lux, regardless of the Moon’s without light pollution, and should cover at
during the full Moon of 7 August 2017, position and atmospheric conditions. least one year of natural variations in lunar
the Earth–Moon distance will be about Finally, it is important to return to the elevation and cloud cover. ●
AUTHORS Anne Aulsebrook for commenting on a draft of the moon/fullperigee2001.html (USGS Publications) p284 http://pubs.er.usgs.gov/
Christopher C M Kyba, German Research Centre manuscript. Falchi F et al. 2016a Supplement to The New World publication/sir20055293
for Geosciences, Potsdam, and Leibniz-Institute Atlas of Artificial Night Sky Brightness (GFZ Data Meeus J H 1991 Astronomical Algorithms
of Freshwater Ecology and Inland Fisheries, Berlin, references Services) http://doi.org/10.5880/GFZ.1.4.2016.001 (Willmann-Bell, Richmond, Virginia, USA)
Germany. Andrej Mohar, Dark-Sky Slovenia, Brady A K et al. 2016 The Biological Bulletin 230 130 Falchi F et al. 2016b Science Advances 2 e1600377 Pun C et al. 2014 J. Quant. Spectrosc. Ra. 139 90
Ljubljana, Slovenia. Thomas Posch, Institut für Bruce-White C & Shardlow M 2011 A Review of Hänel A et al. 2017 J. Quant Spectrosc. Ra in prep. Puschnig J et al. 2014 J. Quant. Spectrosc. Ra. 139 64
Astrophysik, Universität Wien, Austria, and German the Impact of Artificial Light on Invertebrates (Buglife, Kocifaj M et al. 2015 Mon. Not. Roy. Astron. Soc. Reinberg A et al. 2016 Chronobiology International
Research Centre for Geosciences. The Invertebrate Conservation Trust) 446 2895 33 465
Bünning E & Moser I 1969 Proc. Nat. Acad. Sci. Kronfeld-Schor N et al. 2013 Proc. Roy. Soc. B-Biol. Sauberer F & Härtel O 1959 Pflanze und Strahlung
ACKNOWLEDGMENTs 62 1018 Sci. 280 20123088 (Akad. Verl. Ges. Leipzig)
This article is based upon work from COST Action Buratti B J et al. 1996 Icarus 124 490 Kyba C C et al. 2015 Sci. Rep. 5 8409 Yorzinski J L et al. 2015 Peer J. 3 e1174
ES1204 LoNNe, supported by COST (European Espenak F 2016 Full Moon at Perigee (Super Moon): Marcum S N et al. 2004 Recovery of the Black-
Cooperation in Science and Technology). We thank 2001 to 2100 http://astropixels.com/ephemeris/ footed Ferret: Progress and Continuing Challenges
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