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Earthshine Observations of the Earths Reflectance

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Earthshine Observations of the Earths Reflectance GEOPHYSICAL RESEARCH LETTERS, VOL. 28, NO. 9, PAGES 1671-1674, MAY 1, 2001 Earthshine Observations of the Earth's Reflectance P. R. Goode,1 J. Qiu, V. Yurchyshyn,J. Hickey Big Bear Solar Observatory, New Jersey Institute of Technology,Newark, NJ 07102 M-C. Chu Department of Physics, The Chinese University of Hong Kong, Shatin N.T., Hong Kong E. Kolbe Departement ffir Physik and Astronomie, Universitgt Basel, Basel, Switzerland C. T. Brown, S. E. Koonin W. K. Kellogg Radiation Laboratory, California Institute of Technology, Pasadena, CA 91125 Abstract. Regular photometric observations of the moon's low-earth orbit view only a small fraction of the earth's sur- "ashenlight" (earthshine)from the Big Bear Solar Obser- face at once so that many observations must be combined vatory (BBSO) since December 1998 have quantified the to obtain global-scale information. It is also a major techni- earth's opticalreflectance. We find large (,,• 5%) daily vari- cal and logistical challenge to keep calibrated photometers ations in the reflectance due to large-scale weather changes continuously in orbit on decadal timescales. on the other side of the globe. Separately, we find compara- The earthshine, or "ashenlight", is the glow of the "dark" ble hourly variations during the courseof many nights as the part of the lunar disk visible to a nighttime observer. It is earth's rotation changes that portion of the earth in view. sunlight reflected from the earth and retroflected from the Our data imply an average terrestrial albedo of 0.297+0.005, lunar surface, and so offers an alternative route to studying which agrees with that from simulations based upon both the earth's reflectance. Earthshine data are complementary changing snow and ice cover and satellite-derived cloud cover to existing satellite data in that the coverage is instanta- (0.2965:0.002).However, we find seasonalvariations roughly neous and hemispheric in scale. Because the earth's phase twice those of the simulation, with the earth being bright- as seen from the moon is supplementary to that of the moon est in the spring. Our results suggest that long-term earth- seen from the earth (i.e., the earth is nearly "full" when shine observations are a useful monitor of the earth's albedo. the moon is a thin crescent),the instantaneousintensity of Comparison with more limited earthshine observations dur- the earthshine near the new moon samples almost half of ing 1994-1995 show a marginally higher albedo then. the earth. For over a quarter century, beginning in 1926, Danjon and his followers[Danjon, 1928, 1954;Dubois, 1947] performed regular earthshine observations from France. We A central parameter of the earth's climate is the planet's have reinvigorated and modernized this nearly forgotten way albedo, A, the fraction (roughly 0.3) of the incident sun- of measuring the earth's albedo. light that it reflects. Simple power balance considerations Our earthshine coronagraph is a 6-inch refractive tele- suggest the importance of this parameter: in the absence of scope with a cooled AP-7 512x512 pixel CCD camera; our feedbacks,the earth's blackbody temperature of some 255 K fiducial regions comprise about 100 pixels each. We mask would increase by roughly 1 K for a 0.01 decreasein A. Pre- the bright portion of the lunar disk (i.e., "moonshine")with cise measurement of the albedo can reveal variation with the a neutraldensity filter of transmission• 10-5, and correct earth's aspect (land is more reflectivethan water), weather the measured earthshine intensity for the effects of moon- (cloud patterns), and seasons(snow and ice coverand veg- shine scattered in the atmosphere and optical train. For de- etation). tails, seehttp://www. bbso.nj it. edu/Research/Earthshine. The albedo is determined most directly by satellite obser- The earthshine intensity is most conveniently determined vations[Gibson et al., 1990;Stephens, Campbell and Vonder by observingthe specificintensities Ia,b of two nearly dia- Haar, 1981;Rossow and Lacis, 1990]. However,satellites in metrically opposite lunar highland regions (each covering nearly 0.1% of the lunar disk) near the edge of the disk - •Also at W. K. KelloggRadiation Laboratory,California In- one (a) in the earthshine,the other (b) in the moonshine.If stitute of Technology, Pasadena, CA 91125 we define the earth's effectivealbedo, A*, as that of a Lam- bert (perfectlydiffusing) sphere having the samereflectance [Flattd, Koonin and MacDonald,1992], then Copyright2001 by the AmericanGeophysical Union. Papernumber 2000GL012580. 0094-8276/01/2000GL012580505.00 1671 1672 GOODE ET AL.' EARTHSHINE OBSERVATIONS OF THE EARTH'S REFLECTANCE Here,/• is the lunar phase(/• is nearly -•r at the new moon, snow and ice cover information, we use simulations from increasesto about/• - 0 at mid-month and is nearly/• - •r the Canadian Center for Climate Modeling and Analysis at the end of the lunar month), fb(/•) is the phasefunction (http'//w•-•. cccma.bc . ec . gc . ca), whichgives the monthly of regionb,/• • •r-/• is the earth'sphase angle, and fL (/•) = mean snow/ice cover for each computationalcell (2.5øx . [(•r- I/• [)cos/•+ sin [/• I]/•r is the Lambertphase function. 2.5ø). (All phasefunctions are definedso that f(0)-l.) The lunar Figure i showsthe observed(left panel) and simulated phasefunction, fa("7), arisesfrom the near retrofiectionof (right panel)effective albedo expressed in astronomicalmag- the earthshineby the moon,where -7• < 1ø is the anglebetween nitudes,[A*] - -2.5 logA*, as determinedfrom our earth- the incident and nearly retrofiected earthshine from region shine observations. This standard astronomical definition a. Rem, Re, Roa, Rob, Res and Rms are the earth-moon impliesthat the larger [A*] is, the smallerthe albedo.Note distance, the radius of the earth, the distance of the observer that [A*] -- 1.31 for A* -- 0.3 and that a 1% changein A* from patches a and b, the earth-sun distance and the moon- correspondsvery nearly to a 0.01 changein [A*]. The A*'s sun distance, respectively. from morning and evening observationsand simulations are, Our differential measurement of the earthshine removes on average, nearly indistinguishable. However, the spread many of the complications associated with varying atmo- about the mean of the observed[A*]'s is appreciablylarger spheric absorption and solar intensity. Lunar images around than that of the simulated ones (respectivevariances are a lunar eclipsedetermine the ratio of the geometrical albedos 0.14 and 0.02 about the fitted curves). While the nightly of the two fiducialregions, pb/p•, while photometricobserva- averagein both panels is relatively constant for larger phase tions during many lunations determine the phase functions angles, it decreasessharply toward the full moon, implying that the earth is relatively more reflective at grazing angles To fully account for the effect of atmosphericextinction, (glint). Note that we haveno reliableobservations for phase observations of both the earthshine and the moonshine are angles below 40ø or above 140ø becauseof scattered moon- made as the airmass varies through the night. The true shine and becauseof short observingperiods, respectively. intensitiesto be used in eqn. (1) are greater than the ob- The average albedo can be determined from our obser- servedintensities by a factor of e•'z. The local airmass, z, is vationsand simulations asA - 2 •r A* . the cosecantof the elevation angle for observationsnear the We haveextrapolated [A*] from 40ø •_ _<180 ø to the full zenith, while near the horizon the curvature of the earth re- range of the integration using a constrained least squares quiresthat we usetabulated values [Kondratev, 1973]. The fit in which [A*] is assumedto vary smoothlywith lunar atmospheric extinction coe•cient, c•, ranges between about phase. This method is used widely and successfullyin helio- 0.1 and 0.3, and on a typical night it is determined with a seismology[Goode, 1995]. We testedit by performingnearly relative precision of 1% for the earthshine and about 0.5% 300 additional simulations, for nights other than the 143 be- for the moonshineby fitting that night's observations.Typ- tween January 1999 and August 2000, so as to cover the en- ically, we find c,e- C,m"" 0.05. tire range from 0ø •_/• _• 180ø for the parts of the earth we To determine the earth's reflectancefrom the earthshine, "see"from BBSO. Fitting these and integrating A*, we find we must know the lunar phasefunctions, f•,b - the moon- shine intensity of the fiducial regions as a function of the lunar phase. However, raw moonshinedata near the quarter 2.0 0.16 moon show as much as a 5% variation from month-to-month 1.5 0.25 at the same lunar phase due to varying atmospheric absorp- tion, the lunar declination, and libration. We have addressed 1.0 0.40 this problem by making three successive,linear adjustments 0.5 0.63 to the observed moonshine intensities using the integrated crescent of the moon as a standard, and then correcting for 0.0 ,. 1.00 the declination and libration of the moon. After applying these three physically required corrections, the aforemen- -0.5 1.58 ß i . i . , . i . tioned 5% variation in f•,b is reducedto 0.5%. With these 40 80 120 160 40 80 120 160 adjustments, we estimate that we can measure the nightly Lunar Phase (degrees) Lunar Phase (degrees) effective albedo to a relative precisionof about 1% on clear nights. This precision is comparable to those quoted for Figure 1. Observed(left panel) [A*]'s plotted againstthe ab- satellitealbedo data [Gibsonet al., 1990; Stephens,Camp- solute value of the lunar phase (also indicated by the cartoons bell and VonderHaar, 1981;Rossow and Lacis, 1990]. along the x-axis) from 198 clear observing nights from Decem- Our results are best interpreted by comparing them to ber 1998 through the beginning of September 2000.
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