Thejll: Earthshine
Earthshine: not just
for romantics
Peter Thejll, Chris Flynn, Hans Gleisner and Andrew Mattingly explain how
classic astronomical techniques, married to the latest technology, can be used to observe
earthshine, and learn about Earth’s climate, among many other things.
I
n about 1508, Leonardo da Vinci
Danjon built a “cats-eye” photometer,
drew, with pen and ink, the Moon lit
an ingenious device that produced a douby earthshine, with a note explainble image of the Moon, permitting the
ing that the normally dark side was visbrightnesses of two patches on the Moon
ible because sunlight was being reflected
(one on the bright part and one on the
off the Earth’s oceans and onto the
dark part) to be compared visually. The
Moon. Now, half a millennium later,
“cats-eye” allowed Danjon to stop down
a network of telescopes at sites around
the bright patch until the luminosities
the world is being planned with which
of the patches appeared identical (to the
to monitor earthshine with the goal of
human eye). Danjon was able to estimate
a much better understanding of Earth’s
the earthshine luminosity at a given time
albedo and climate.
with an internal precision (i.e. scatter)
The luminosity of earthshine depends
of about 5%. Earthshine measurements
on many of the basic properties of the
were converted into albedo estimates for
side of the Earth turned toward the 1: The Moon lit by earthshine, captured by Clementine in 1994. The the Earth, and ignoring for the moment
Moon, such as the amounts of landmass Sun’s glare is rising over the dark limb while the Earth illuminates some substantial systematic errors that
or ocean, how much is covered by ice, the right hemisphere. Saturn, Mars and Mercury are seen in the
were present in the method, the techbottom left of the image. (NASA)
desert and vegetation and, in particular,
nique demonstrated several interesting
the amount of cloud cover. Photometry of earthfeatures
of the albedo. Danjon showed it varies
Abstract
substantially from night to night, because difshine can thus be used to measure the Earth’s
This article presents a technique for
reflectivity – “albedo” – while spectroscopy
ferent parts of the Earth are presented to the
measuring Earth’s albedo by observing
allows the nature of the reflecting surface to be
Moon (as seen from the observing site) as the
earthshine on the Moon. Albedo governs
understood in more detail. For example, vegetaMoon–Sun–Earth angle changes, but mainly
the amount of energy entering the
tion absorbs optical wavelengths preferentially
because of changes in cloud-cover, although
climate system from the Sun and small
in the blue while being quite reflective in the
this latter effect wasn’t demonstrated rigorously
variations correspond to significant
near infrared, while ice, snow and soil have
until many decades later.
climate changes. This technique is based
other characteristic absorption features of their
What Danjon had demonstrated was that a
on relative photometry of the dark and
own. Oceans, if not heavily covered by clouds,
technique based on relative photometry works
bright side of the Moon, obtained with
will cast polarized light onto the Moon.
very well – that is, comparing the bright and
small rugged autonomous telescopes.
Albedo is of primary importance in determindark sides of the Moon simultaneously is an effiThe observations and the techniques
ing the amount of energy flowing into the climate
cient way to eliminate atmospheric and instrualso have wider applications.
system. At the simplest level, if the Earth’s albedo
mental effects; this remains the technique in use
increases, so that more sunlight is reflected,
today, at least for ground-based observations.
the climate will cool; conversely if the albedo
Danjon’s pioneering work started in 1926, and
decreases warmer climates will follow – in any It is most luminous just after or before a new was followed up by J Dubois, with measurements
case the consequence is climate change. But cli- Moon when the Earth is almost full.
continuing until 1960. A major issue was that
mate change may in turn result in changes to the
Earthshine can be measured by technically Danjon and Dubois were necessarily restricted
albedo; in other words they affect each other. It challenging but – in principle – straightforward in their inferences about the Earth’s overall
has long been recognized by climate scientists observations. The payoff is the opportunity to albedo because they were observing from sites
that we simply do not know enough about the monitor global changes in the Earth’s albedo over in France only. Their observations saw higher
interaction of albedo and the climate.
a range of timescales from minutes to years from than average earthshine intensity, because of
Despite being a rich source of information, the ground by relatively inexpensive means.
the dominance of Eurasia to their east, casting
excess light onto the Moon (landmass appears
earthshine is in fact very feeble, with a brightconsiderably brighter than areas of ocean).
ness between a few thousand and a few tens of Earthshine pioneers
thousand times fainter than moonlight (i.e. the Efforts to measure the earthshine reach as far
A global network of telescopes is the key
light from the directly Sun-lit side of the Moon). back as the 1920s, when Frenchman André to getting around this issue and obtaining
A&G • June 2008 • Vol. 49 3.15
Thejll: Earthshine
3: Observer on the
dark side of the Earth
watches the Moon and
sees the sunlit bright
side, as well as the
faintly lit side of the
Moon illuminated by
earthshine.
2r
Moon
D
sunlight
er
erv
obs
Earth
2R
2: Oceans are dark, clouds and ice are bright and
landmass somewhere in between, so that the
luminosity (and spectrum) of earthshine depend
strongly on which areas of the sunlit Earth
illuminate the Moon. (NASA)
scientifically useful global albedo estimates.
The first efforts in this direction where made in
the 1950s by Fred Whipple and Gustav Bakos,
who set up six Danjon-type telescopes at the
Smithsonian Astrophysical Observatory’s satellite tracking station sites in the USA (Wisconsin,
New Mexico, Hawaii) and in Peru, Australia
and Iran (Bakos 1964).
Bakos used clever analogue methods to estimate the amount of continental and ocean surface as seen from the Moon at any given time, by
taking a series of photographs of a schoolroom
globe illuminated by a slide projector. He also
used cloud-cover data, leading to the first sustained attempt to measure correlations between
albedo and weather, although uncertainties in
the raw data, poorly sampled cloud-cover estimates around the Earth, and lack of snow-cover
information contributed to results less conclusive than he would have liked. Although un­able
to link earthshine and the weather clearly,
Bakos was able to show that earthshine could
be affected by specular reflection of the Sun off
Earth’s oceans, so that there is a “…bright spot
on the globe. Depending on the extent of cloud
cover, it produces daily variations of the earthshine, and at times of unusual meteorological
activity it brightens the dark side of the moon
considerably.”
It was clear to Bakos in 1960 that computers,
accurate weather data and electronic detectors
were the way forward, as all these were improving rapidly at the time. Nevertheless, interest in
earthshine remained sporadic for several decades. Others occasionally studied earthshine to
determine terrestrial albedo – Franklin (1967)
observed from South Africa and found values
near those Danjon had arrived at. But long-term,
ground-based monitoring of earthshine did not
take place until well after astronauts had actually seen it as earthlight from the Moon itself!
Real interest was revived in the 1990s. Flatté et
al. (1992) pointed out the need for a much better
3.16
understanding of the lunar surface albedo,
noting that lunar soil reflectivity increases
strongly at small angles (the “lunar opposition
effect”, which makes the full Moon particularly
bright) and that this hard-to-measure quantity
is very important if earthshine measurements
are to be converted to terrestrial albedo.
Earthshine is certainly seen at small angles
since the observer is standing on the reflector!
Philip Goode at Big Bear Solar Observatory
(BBSO) in California saw the great value of
earthshine observations and began long-term
monitoring. The BBSO is now the centre of such
observing activity, with a growing network
of dedicated telescopes – one in Tashkent and
one soon to start on Tenerife at the Instituto de
Astrofisica de Canarias. This effort is currently
the best one – the results have sparked important
debate in climate research circles, and the concepts used to develop instrumentation and the
growing network of telescopes are now the most
important in this area of observational climatology (Pallé et al. 2004).
Observing earthshine
Earthshine intensity at any given time is proportional to Earth’s albedo, but also to the
Sun–Earth–Moon geometry, the lunar albedo
itself, and the solar irradiance. Clearly, a single observation of the Moon will only give us
information on the part of the Earth that at that
moment reflects light onto the Moon. As the
night progresses new parts of the lit Earth are
presented to the Moon and the intensity changes.
Eventually the Moon sets and must now be
observed from an observatory further west. With
a network of well-placed telescopes around the
world, continuous coverage could be had.
Consider a ray of light that reaches the observer
after one reflection off Earth and one reflection
off the dark side of the Moon (figure 3). The
intensity of this ray is proportional to the solar
irradiance, the reflectivity of Earth, the reflectivity of the Moon, properties of the Earth’s atmosphere, some geometric factors and lastly of the
observing instrument. Consider now a ray that
reaches the observer after a reflection on the
Moon only: the intensity of this ray depends on
all these factors except the terrestrial albedo. In
fact, the ratio of the intensities depends only on
the terrestrial albedo – the other factors cancel
out, more or less. In particular the factors relating to atmospheric and instrumental stability
cancel. The ratio of the dark and bright sides of
the Moon allows one to measure the albedo of
the parts of the Earth producing the earthshine
at any particular time.
The major practical problem is the very large
ratio between the brightnesses of the bright
and dark sides on the Moon, which typically
exceeds 10 000:1. One of two methods is usually
adopted to “stop down” the bright side: in the
first approach the bright side intensity is reduced
by a well-calibrated neutral density filter; in a
second approach the bright side is imaged using
a very short, accurately known exposure time.
We have experimented over several years with
the former technique and plan to use it in a dedicated earthshine telescope now being designed
in a collaboration between the Danish Meteorological Institute and Lund Observatory (Sweden). The latter technique is successfully used
at Big Bear Solar Observatory in their Earthshine Project. We have also experimented with
a third technique, where large numbers of very
short exposures are co-added in order to prevent
saturation of the bright side on the CCD, but
still allowing the dark side counts to build up
to useful intensities.
In “serial mode”, lunar images are taken
sequentially: an exposure of short duration of
the bright side followed by a longer exposure
to capture the dark side (with the bright side
covered by an opaque occulter to hinder light
scattering into the dark side). In “parallel mode”
the optics of the telescope are arranged so that
the two halves of the lunar image are exposed
onto the same CCD frame simultaneously. The
bright side branch of the telescope has a filter
that reduces the brightness of the bright side;
the dark side branch occults the bright side and
captures just the dark side – the two image halves
are registered by the same CCD, at the same
time. The benefit of this procedure is, we hope,
A&G • June 2008 • Vol. 49
Thejll: Earthshine
4: The sunlit side of the Moon is to the right and
the earthlit side is to the left. It is possible to see
the earthshine side in single images, but the
signal-to-noise ratio is very low. By co-adding
350 images the noise is averaged out. There is
evidence of scattered light around the Moon –
from scattering in the atmosphere. There is also
an asymmetric feature at the top and bottom of
the image which may be related to reflections
in the instruments. The images were obtained
over half an hour, from Sydney, Australia. This
false-colour image was generated by co-adding
350 x 0.15-second CCD images of the Moon, taken
using a CCD camera (SBIG STL-1001E) with a high
dynamic range (~ 87 dB), attached to a 100 mm
APO refractor “stopped down” to 15 mm.
that any fast variations in instrumental sensitivity or sky transmission properties cancel out
more precisely if the exposures are simultaneous.
Such a system is not as simple as the sequential
one, however, because we need two objective
lenses and a device to splice the beams together.
Exchanging the tasks of the two branches ensures
that if their properties drift apart a stable mean
is still obtained; but this also adds complexity to
the device. An alternative would be to calibrate
carefully each branch against the same observing targets, such as stars with known brightness
or the same flat bright surface; these issues are
being studied in our development project. It is
essential that the observations are performed in
several independent ways so that an intercomparison is possible; therefore, we are considering
making our instrument able to operate both in
parallel mode and in serial mode, and to use the
third mode – co-addition of single images of the
whole Moon in the time after or before a new
Moon – to have several data sets on which to
base an internal consistency check. Comparison to BBSO observations at the same times and
from the same sites will also be undertaken.
Removing scattered light
Scattered light in any telescope pointed at the
Moon is a serious problem and must be carefully
removed if a useful dark-to-bright side ratio is to
be obtained. Scattering of light inside the instrument must be minimized by careful design in the
layout of optical components and the baffling
A&G • June 2008 • Vol. 49 of stray light. Ray-tracing calculations of the
chosen optics help in optimizing the design.
The atmosphere is another source of scattered
light. Experience shows that its modelling and
removal in post-processing of earthshine images
requires care, but is not impossible. Currently,
the BBSO effort removes scattered light from
the lunar disc image by simply extrapolating
linearly the intensity of the sky background just
off the limb of the disc and onto the earthshine
side. BBSO has demonstrated that this works
very well. Another approach, which we are testing, is the forward modelling of the bright side
illumination of the Moon, assuming a point
spread function (PSF) for light scattered over
quite large angles (a few tenths of a degree). The
method iterates until the modelled sky intensities
match the observations, and then the model of
the scattered light is subtracted from the whole
image. The linear method is fast, of course,
but uses a limited amount of the information
available, and assumes that scattered light falls
off linearly in intensity with distance from the
lunar limb; the forward method uses all information available in the sky background to build
a model, but requires an “ideal” Moon image
to convolve, and makes assumptions about the
PSF. The ideal image can be obtained by using
only the brightest pixels in a real image of the
Moon, since the scattered component is small
everywhere, or one can use an idealized image
of the Moon generated synthetically using the
DMI lunar simulator.
The PSF of the atmospheric scattering over
these large angles is challenging to measure, but
has been studied on Palomar Observatory Sky
Survey plates by King (1971), who found that
the PSF intensity falls off quadratically beyond
a Gaussian core of about 10 arcsec width, while
work based on the solar aureole profile in “photo­
metric condition” images of the Sun (i.e. when
you can place your thumb over the Sun and see
blue sky all the way up to the thumb; as opposed
to days when the sky is bright and milky-white
near the Sun) have shown that the profile fall­
off is close to linear and can be well-modelled
beyond the solar limb (Barducci et al. 1990).
At present, the optimal method to model and
remove scattered light off the lunar limb remains
open, with testing proceeding.
Obtaining terrestrial albedo
Following measurement of intensities in the
dark and the bright side of the lunar image, correcting for filters or different exposure times
used, and applying corrections due to the angledependent lunar albedo, a terrestrial albedo for
that moment can be determined.
Terrestrial albedo also depends on the angles
of incident and reflected light, so a method has
to be chosen for how to present the observationally determined albedo. One way is to calculate
carefully the mean albedo of the Earth on the
basis of satellite data. The Earth is observed at
all times from space and it is possible to generate global maps of the cloud cover and surface
reflectivity, typically at three-hourly intervals.
Such data can be transformed into the geometry
of the lunar observing situation, so that a direct
comparison of the earthshine-based albedo and
that from satellite data can be performed. This
has been done in the BBSO effort.
How well must this be done?
In order for monitoring of terrestrial albedo to be
useful, we must be able to deliver precisions and
accuracies that are relevant to research issues.
The albedo of the Earth is a composite value,
which depends on the distribution, occurrence
and reflectivity of various components, all varying with seasonal timescales (months), as well as
at meteorological timescales (hours and days).
The atmosphere itself also reflects light thanks
to scattering on aerosol and dust particles as
well as Rayleigh and Mie scattering.
The climate system is in almost perfect balance at all times – large imbalances in the energy
budget are quickly smoothed out by cooling or
warming until approximate radiative balance
is re-established. The sunlight reflected back
into space, without absorption in the air, ocean
or land surfaces, amounts to about 100 W m–2 ,
averaged over the whole globe over a 24-hour
period. About 2.5 times more is absorbed into
the climate system – 240 W m –2 . To put this in
perspective, the IPCC report on climate change
has estimated that the increase in warming
expected from a doubling of the amount of CO2
in the atmosphere (we have so far added about
30% CO2 since pre-industrial times) is between
2 and 4 W m –2 .
An order-of-magnitude estimate of very significant albedo change is therefore 1%, and in
order to say anything scientifically useful about
changes in albedo we need to be able to monitor it to precisions of within a few tenths of a
percent. From ERBE and CERES data – instruments on space platforms that measure the
shortwave and longwave radiation from Earth
– we know that the intrinsic, daily variability of
the global mean albedo is on the order of 1.6%,
and this limits the accuracy with which a yearly
averaged albedo can be determined. At a good
site, one might obtain 200 daily observations
of earthshine a year, and taking into account
the approximately three-day serial correlation
timescale (large cloud systems tend to cause the
cloud variations and these last from days to
weeks but do not totally dominate the albedo
variability) we can expect to measure the yearly
averaged albedo to about 0.2%. This limit, due
to the intrinsic, daily variability of albedo,
sets the level of precision we should hope to
approach with our instrumental design choices:
doing better than this is wasted effort unless we
wait for decades for data to accumulate; doing
3.17
Thejll: Earthshine
5: Simulated image of the Moon. The left half has
been multiplied by a factor of 1000 to bring out
the faint details. The intensity of the earthshine
has been calculated by assuming a particular
albedo and reflectance property of the Earth,
and a particular observatory on Earth, and
propagating light from the Sun to the Earth and
onto the Moon. Images like these can be used to
generate realistic scenarios of what the Moon
should look like at a given time of month, and
this can be compared to observations and by this
modelling we can adjust our preconceptions of
the Earth’s reflective properties until they match
reality. The image is calculated using the DMI
lunar simulator software.
worse is adding potential biases and unnecessary scatter to our observations.
Why bother – aren’t satellites better?
Satellites in space – such as weather satellites
and special Earth observing platforms – already
observe the Earth around the clock; why not just
use the data they provide, instead of setting up
ground-based programmes around the world?
The first and most obvious reason is that
ground-based programmes are cheap compared
to satellites. A ground-based monitoring system
of six to eight telescopes can be built and robotically operated for a longer time and for a budget
several orders of magnitude smaller than any
similarly capable space-based system.
Space platforms observing the Earth commonly have optical and microwave instruments,
all of which need calibration on the ground
before launch and repeatedly in space as sensitivities degrade due to their harsh environment.
Most microwave instruments can be calibrated
in space by use of on-board reference sources,
but visual-light instrument calibration was commonly not provided for in the first generation
of weather satellites. The platforms were never
designed to generate climate data, but instead
were optimized to deliver daily pictures of the
Earth for weather forecasters. The next generation of meteorological satellites – the MSG
system – promises 0.5% accuracies in the visual
band; but what will the precision and accuracy
be once the platform is in orbit, and after several
years of operation?
3.18
So-called “vicarious calibration” is possible
from space – where calibration targets such
as the Moon, or flat white areas on Earth are
observed. Through the ROLO project (http://
www.moon-cal.org) a method was developed
to enable vicarious calibration on instruments
by occasionally observing the Moon. Precisions between 1 and 2% are possible with
this method. Observing large white reference
surfaces, such as salt deserts on Earth, gives
similar results.
In any case, satellites are in orbit already, and
are being used to make estimates of Earth’s
albedo. Satellites typically provide a stream
of high spatial resolution data: careful work
is done to combine large numbers of highresolution scans (“swaths”) of small areas of the
Earth’s surface into global albedo averages (e.g.
from low-flying platforms, such as the CERES
instrument). In contrast, earthshine monitoring
of the Moon provides already averaged data as
the core material.
Ground-based monitoring of Earth’s albedo
has another major advantage: satellites are
unlikely to be able to provide calibration stability over very long timescales, whereas earthshine
makes use of a reflective monitoring screen, the
reflectance of which is likely to be extremely
stable and will still be there 100 years from now.
Each time a new satellite is launched (satellites
last years, not decades) a fresh start is made on
various calibration issues. Earthshine data from
a ground-based programme would be of high
utility for many satellite calibration issues.
The Intergovernmental Panel on Climate
Change report of 2007 estimates timescales for
significant climate change stretching to 2100.
Such timescales cannot be covered continuously
by one space-platform system, but there is good
reason to believe, based on historical evidence
from the astronomical community, that groundbased telescopic systems offer the potential to
cover a large part of this century with essentially
unchanged techniques.
|Sun (Wm–2)
|model Earth (Wm–2)
0.08
1500
6: Simulation of how
Sun
earthshine intensity
would vary through
0.06
model
a month (blue line)
Earth
1000
compared to the
0.04
intensity of the sunshine
(constant yellow line).
500
The up-and-down
0.02
motion of the earthshine
intensity comes from
0
the rotation of dark
0.00
0
5
10
15
20
25
30
oceans and brighter
days
continents so that
surfaces of different albedo reflect onto the Moon. The overall dip in earthshine intensity at day 15
is due to the phase of the Earth as seen from the Moon – the Earth is near full in the beginning and
end of the month while it is almost “new” near day 15. The change in the appearance of the peaks
in earthshine through the month is due to the motion of the Moon with respect to the equator of the
Earth so that the reflecting surface that illuminates the Moon is centred over different latitudes
on Earth. The simulation here does not include clouds – clouds will modulate the blue curve and
make the excursions smaller due to the mottled averaging effect of clouds over land and ocean.
In the end, the earthshine method of deriving albedo for use in climate research should be
seen as complementary to satellite data – not in
competition with it.
Increasing earthshine telescopes
A small network of semi-automatic earthshine
telescopes has been installed by the BBSO earthshine team, with instruments sited at BBSO, in
Asia and one soon to start on the Canary Islands
(in summer 2008).
A global system is mandatory for global albedo
monitoring. Simulations suggest that six to eight
observing sites around the globe, as uniformly
scattered in longitude as possible, optimizes
resource use. The system would give sufficient
redundancy to handle inevitable outages and
clouds at individual sites, while allowing enough
sites to observe the terrestrial albedo continually for all longitudes and to cross-calibrate the
different telescopes.
At middle-latitude observing sites, the Moon
is typically above the horizon and accessible to
observations for only a few hours before setting.
Following the Moon down to the horizon is not a
good idea from such sites – refraction and atmospheric absorption of light can lead to differential
effects across the lunar disc. At polar latitudes
the Moon can be seen above the horizon for
quite long periods, which provides opportunities for better cross-calibration of several instruments sited at lower latitudes, and because the
air is very dry at polar latitudes fewer atmospheric problems, even close to the horizon, are
expected. Should a global network of earthshine
telescopes include telescopes at both poles?
Apart from logistical problems with instrument operation and maintenance in polar
regions, there is the problem that the Moon is
never very high in the sky from high latitudes.
Under normal circumstances, high photometric
quality observations are rarely attempted at an
“airmass” above 2, i.e. at less than 30° over the
horizon. At the right polar sites, however, the
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Thejll: Earthshine
50
40
% absorption in visual band
7: For two or three
years following
the July 1991
eruption of Mt
Pinatubo on the
Philippines, the
atmosphere
globally was less
transparent. Here,
observations from
the observatory
on La Silla in Chile
show the amount
of extinction. Even
on clear nights
the atmosphere
absorbs about 10%
of the light.
30
20
10
0
0
500
air may be so dry (and free of dust) that the
Moon could be imaged quite low to the horizon. At a high site, like Antarctica’s Dome C
at 3.2 km altitude, a further advantage is that
there is less atmosphere to look through, and
airmass 2 is reached at larger zenith angles, giving better opportunities for long sequences of
observations. Dome A is even better from this
point of view, at 4 km altitude. Dome A is presently being investigated by the PLATO project
(http://mcba11.phys.unsw.edu.au/~plato) for
robotic astronomical observations, and experience there will guide decision-making about
how useful polar sites could be for a global
earthshine network.
We are aiming at a network of telescopes operated like autonomous meteorological instruments, which are rugged, autonomous, feature
access via the internet, and operate for very
long periods without servicing. A single control centre oversees routine operations remotely,
but the instruments operate by themselves and
know when to observe, when to shut down due
to bad weather, and so on.
The system now under development at the
Danish Meteorological Institute and at Lund
Observatory will be suitable for deployment
even in areas that have minimal science infrastructure, the main site-requirements being
power and internet access (possibly based on
mobile communications technology). Placement
at sophisticated scientific sites will not be necessary, so that opportunities exist to site telescopes
in nations developing their scientific traditions.
A long-term earthshine monitoring programme of this type will also make available
observations with applications in other fields.
Atmospheric transparency
One step in the reduction of observed data to
a form from which terrestrial albedo can be
determined is the measurement and removal of
the nightly varying atmospheric transparency.
This is routinely performed by astronomers by
A&G • June 2008 • Vol. 49 1000
days since 1 January 1991
1500
2000
observations of “standard stars” of known,
constant brightness, and is termed “extinction”. Following these stars through the night,
between lunar observations, provides the necessary information to remove from each lunar
image the effects of extinction, and we will also
obtain as a side product a database of a very
interesting property of the atmosphere, namely
changes in its transmission over long timescales.
While extinction monitoring is routine at astronomical observatories around the world, and
sufficiently accurate to see the global impact
on atmospheric transmission of major volcanic
eruptions (such as that of Mount Pinatubo in
1991), longer term changes are much more difficult to pin down (figure 7). The best readily
available records of nightly extinction from
astronomical observatories are the La Silla
and La Palma records (in Chile and the Canary
Islands respectively), which give a coverage from
the 1980s up to the present time – however,
neither are suitable for detailed climate change
studies because there is no explicit separation
of the instrumental sensitivity changes and the
atmospheric transmission changes.
Observing earthshine from several stations
globally would therefore give a picture of the
extent of variations in atmospheric transparency – the key is that the telescopes will be
distributed in both hemispheres, at different
latitudes and longitudes.
Lunar studies
The lunar surface is impacted by meteoroids,
which can be seen from Earth as visible light
“flashes”, and typically have peak brightnesses
in visual magnitude range of 5 to 9 (MontañésRodriguez et al. 2007). This should be well
within reach of even a small imaging telescope
such as our earthshine instrument. Monitoring
the lunar surface for flashes would yield data on
meteor­oid impact rates and the flash luminosities used to estimate the kinetic energy released
in such events.
Reports of lunar surface luminescence and
various transient luminous phenomena have
occurred since systematic observations of the
Moon began in the telescopic era. The presence
of lunar surface luminescence (Grainger and
Ring 1962, Chanin et al. 1982) could influence
our ability to use the lunar surface as a screen
to reflect earthshine, but we note that a series
of reports of lunar surface luminescence (e.g.
Sekiguchi 1980) has cast doubt that optical
surface luminescence is a real phenomenon:
luminescence of higher energy radiation does
exist; Lee and Wilson (2008) calculated that
luminescence due to impacting cosmic rays
would be X-rays or gamma-rays.
Studies of transient events, such as the meteoroid impacts, but also longer-lasting brightness
enhancements of certain areas of the Moon, will
be a useful side product of long-term earthshine
monitoring (see e.g. Crotts 2007). Moon observers have for hundreds of years reported that some
areas of the Moon occasionally brighten, and
speculations that this is due to venting of gases
from the Moon exist. As there are problems with
such an interpretation (the Moon is not thought
to be geologically active any more) it is important
to gather proof of such events, and the earthshine
telescopes will gather huge amounts of data that
can be searched for such events.
Astrobiology
If our network of earthshine instruments had
a spectroscopic capability – or even a spectrophotometric one – we could offer long-term
monitoring of the spectrum of the Earth on
hemispheric scales (Montañéz-Rodríguez et
al. 2006). These data are of interest to Earth
observers who wish to model the properties of
the Earth due to seasonal, climate- and humaninduced changes in the reflectance properties,
but also to astronomers looking for exoplanets
– planets orbiting other stars. When spectroscopic – or broad-band photometric – observations of Earth-like exoplanets became possible,
the astrobiology community had a special interest in looking for “fingerprints” of various land
covers that hint at the possibility (or absence) of
life. Spectra of an exoplanet – a gas giant known
as HD 189733b – have already been obtained
and signs of methane have been reported (http://
www.planetary.org/news/2008/0320_Methane_Found_in_Exoplanet_Atmosphere.html).
The longer time spent by the Moon in the sky
at polar latitudes has allowed Danielle Briot
and Jean Schneider to obtain a series of spectroscopic observations of the earthshine from the
French Antarctic base Dumont d’Urville using
the LUCAS instrument. These observations
have detected biological and other features in
Earth’s reflected spectrum (e.g. Arnold 2007),
as shown in figure 8.
While exobiology may or may not be
chlorophyll-based, observing such a signature
3.19
Thejll: Earthshine
8: Vegetation in Tivoli Gardens in
Copenhagen. An ordinary, visual-light
colour image (upper left); a black and
white visual-light image (lower left), and
an infrared (IR) image (lower right). The
greenery is very bright in IR, compared
to visual light (compare the dark-leaved
trees in the backgrounds of the colour
and B&W images with the IR image).
The upper right panel shows reflection
spectra from optical to IR of soil, dry
vegetation and green vegetation.
The green vegetation is particularly
reflective longward of about 720 nm
and is known as the “vegetation red
edge”, and has been detected in spectra
of earthshine. It is a simple method to
reveal the presence of green plants on
Earth, and is the type of feature that
may have applications when spectra of
exoplanets become available. (Photos
are by Harry Lehto, graph is from Arnold
et al. 2002)
from exoplanets would be highly significant
for astrobiology. So far studies of the reflected
Earth spectrum in earthshine have revealed
such properties as the variability of the vegetation red edge (figure 8) as a function of cloudcover. Clouds over Africa and the Amazon
modulate the signal strongly. Ordinary photometric time-series studies of the earthshine also
reveal the rotation of the Earth, as dark oceans
and brighter continents alternatively dominate
the reflecting surface facing the Moon – such a
signal could potentially be observed from exoplanets too. Polarimetric studies of the earthshine have revealed a strong signal when the
“sun glint” moves from an ocean onto an adjacent land mass – reflections off the ocean also
polarize the light quite strongly. The sun-glint
is a small area on the reflecting oceans in which
most of the reflected light is present, thus giving
a probe-like look at terrestrial properties during
the glint. When there is no glint – when clouds
are covering the reflecting hemisphere, or when
the reflection point is over land – the reflected
light gives a more diffuse average picture of the
reflectivity at that moment.
Astronomical spin-off benefits
Further spin-off interests for our telescope system could include observations of any suitably
bright source in the sky. Earthshine observations are not possible near full or new Moon
and so the telescope system could be used for
other studies. Nova and variable-star studies, or
fast-response servicing of the Gamma-ray Burst
Network, are conceivable projects, but these
are jobs that are already handled well by existing dedicated systems – for instance, the “Pi of
the Sky” project (http://grb.fuw.edu.pl/index.
html). We could focus rather on contributing
3.20
reflectance
0.6
green
vegetation
dry
vegetation
0.4
0.2
0.0
to automated long-term planetary photometric
observations (such as that carried out at Flagstaff, Arizona) with the science interest in this
area centred on the reported variability of the
brightness of the large gas giants Uranus and
Neptune in time with the solar cycle (Lockwood
and Thompson 1986, but see also Lockwood
and Thompson 1991). It has been suggested that
solar variability induces cloud-phase changes on
these planets which in turn change the albedo.
Monitoring the bright planets for decades in a
well-calibrated automatic system might be an
attractive spin-off use.
Other applications
Finally, we envisage using terrestrial albedo
measurements for monitoring of albedo changes
induced by possible future attempts at geo-engineering, as well as studies of the suggested influence of the Sun on the climate, via its effect on
cloud cover. The main problem for studying
short-term changes in albedo is that the natural
daily variability is high and must be averaged
before conclusions about episodic changes can be
drawn. For longer-term changes, an earthshine
telescope system would have advantages over
satellites because the suspected changes in cloud
cover are small and long-term satellite data rarely
offer continuity, making small offsets in the data
– as satellites go in and out of service – hard to
distinguish from real changes in cloud cover.
Evidently there are many things you can do
with observations of the Moon – in a few years,
when a global chain of automatic telescopes is
ready, we will provide several types of data for
the climate research community, for satellite calibration, and for astronomical studies. Hopefully,
the Moon will continue to inspire future artists
and scientists as it did for Leonardo da Vinci. ●
soil
0.5
1.0
1.5
wavelength (µm)
2.0
2.5
Peter Thejll, Danish Meteorological Institute;
Chris Flynn, Tuorla Observatory, Finland; Hans
Gleisner, Danish Meteorological Institute; Andrew
Mattingly, Grove Creek Observatory, Australia.
Acknowledgments. The UK embassies in
Denmark, Austria and Switzerland provided
funding for a workshop on earthshine, held
in Lund, Sweden 2008. VINNOVA, Sweden
has provided major funding for a research and
development project for automatic earthshine
telescopes. In Denmark, the research funding
agencies SNF and OFR supported a pilot project.
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