Photointerpretation of the Lunar Surface* (with fold-in maf) supplementt) ]{OBERT J. HACKMAN, u. S. Geological Survey, Washinglon 25, D.C. ABSTRACT: PhotoKraphs of the .Iroo'/l taken at different libration positiolls hili'e an lIlIglllar d~/ference of perspectil'e, and hence stereoscopic photo inter­ pretil'e methods may be applied to studies of the lunar su'rface, The common in­ terpretation elements of shape, size, textllre, pat/ern. position and shadow are important criteria in the stlldy of the "Moon, just as they are in the study of the Earth's surface, a,lthough the photo iuterpreter must adapt to the nOl'elty of ex­ tremely sm(Lll-swle photography and must modify certain of the procedures and instmments usedfor cOn1'entional aerial photographs. The dominant featllres of the lunar landswpe are nearly circlliar craters, t!lOlIf!.ht to be the resllit of meteoroid impact, and great lowland Meas now flooded with dark material beliel'f(! by many to be lam fields. Relatil'e ages of the lunar featllres may be determined from the degree of dissection of crater rims, and from the relative positions of craters u.:ith respect to the lowland and highland areas. In places the lun(Lr surface is cut by conspicuous faults more than a h1/.1ulred miles long. 1'.1any of these features, which bear on an understanding of the Moon's surface and its development. have been compiled on generalized geo­ logic and physiographic maps at a scale of 1 :3,800,000. Afore detailed studies ([ n being made and maps are being compiled al a scale of 1:1,000,000 for lar­ get areas for futare lunar probes. I NTRODUCTIO:\ tures may be obtained from the dimensions of the hroad lowland Mare Imbrium, which is HOTO interpretation of the lunar surface approximately 700 miles across. The ray is the first step in geological exploration of P crater Copernicus is about 56 miles across, the Moon. Many photographs from different and the crater Abulfeda is 39 miles in diame­ observatories have been examined stereo­ ter (Figure 2). All of metropolitan Washing­ scopically to obtain physiographic and geo­ ton, D. C. and its suburbs could be placed logic data. Before discussing the application on the floor of Abulfeda, and there would still of photo interpretation to the lunar surface, be some room left over. The crater Ptole­ however, brief consideration will be given to maeus (Figure 2) is 90 miles across; the state the general features of the Moon, the scale of Connecticut would just about fit inside it. and resol Ll tion of lu nar images as seen The smallest object that can be seen on the through the telescope and on lunar photo­ best lunar photograph is about half-a-milc graphs, the phases of the Moon and how the across. A building the size of the Pentagon Moon's mo\'ement around t he earth permits \\'ould not be \'isible on the best telescori" the taking of stereoscopic pairs of photo­ photography of the :\100n. In contrast. th· graphs. smallest object that can be seen through the telescope under optimum \'iewing conditions GE'\'ERAL FEATURES OF THE ,,100:\ is about an eighth of a mile across or abollt Some of the most stri~ing features on the one-fourth the size of the smallest object re­ 1\1 (jon are the dark-colored lowlands, the solved on the best lu nar photographs. ligh ter-colored cI'ater-scarred highlands, and the light-colored rays associated with certain PHASE AND FACE OF THE ,100;\ craters (Figure 1). The phase of the 1\'Ioon, "'hich is deter- An appreciation uf the scale of lunar fea- mined by the direction of illumination, in- • Reproduced e~~entially a~ pre~ented a~ the 27th :\nl~ual i\leeting ~f the Suciet)',.Shore.ham Hotel, \\"ashington, D. c., March 19-22, 1961. l'uulicalloll authOrized by the Dlre,·tor. U. S. GeologIcal Survey. t This map nny be purchased from the Americall Society of Photograllll11etry, 1515 Massachusetts Ave., N. IN., Washin:;tol 5, D. C. $1.00 postpaid ill the United States. Foreign mailillg extra. 377 371:> l'IIOTOGHA~I~IETHIC ENGINEERING Earth, the angular difference in view would be about 1¥ degrees, much too small for good stereoscopic viewing. Fortunately the Moon has apparent oscillations which enable us to see a little more around the edge at one time than at another. These oscillations, called libration, provide the means of ob­ taining stereoscopic pairs of lunar photo­ graphs. At times it is possible to see as much as 6! degrees beyond the north pole of the Moon and at other ti mes 6! degrees beyond the south pole. This latitudirial change in view of the Moon is called latitudinal libration. This is observed because the Moon's axis is inclined 1! degrees to the plane of its orbit, which in turn is inclined 5 degrees to the plane of the Earth's orbit (the ecli ptic). There is also a longitudinal change in view of the Moon; this is called longitudinal libra­ tion. I t is caused by acceleration of the Moon in its elliptical orbit according to Kepler's second lall· while the rate of rotation remains nearly constant. Longitudinal libration al­ lows viell·i ng about 8 degrees more of longi­ tude on either side of the Moon than may be seen at mean libration. Photographs taken at different librations appeal' the same as though they lI·ere taken from different points in FIG. J. Photograph of the quarter moon showing space. \\'ith a sufficiently long base line general scale of lunar features. Distance A to B, (Figure 3) lunar photographs can be viewed across Mare Imbrium, is about 700 miles. Ray stereoscopically with sufficient depth percep­ cra ter Copernicus is 56 miles across. tion for stereoscopy to be of use in photo Ruences the appearance of the Moon and interpretation. stereoscopic study of lunar features. A com­ plete cycle of phases includes, in succession, PREPARI:-iG LU:\AR PHOTOGRAPHS FOR new moon, crescent, 1st quarter, gibbous, STEREOSCOPIC VtEWING full, gibbous, 3rd quarter, crescent, and !vIany lunar photographs can be prepared linall)' new moon again. Regardless of phase, for stereoscopic viewing in a manner some- the Moon always shows essentially the same side to the Earth. This is due to the fact that the Moon rotates on its axis while revolving in its orbit around the Earth, and because the period of rotation is equal to the period of revolution. This movement of the Moon around the Earth may be likened to a man \\'alking around a globe that is in the center of a room and always keeping his face to\\'ard the globe. In moving around the globe once (one revolution) he would have made one complete rotation \\·ith respect to the room. Viewed from the globe, only his face would be visible, ne\'er the back of his head. LlBRAT10~ AXD STEREOSCOPIC I'HOTOGRAPHS It may be asked how stereoscopic pairs of FIG. 2. Enlargcd pholo"raph of an arca near the photographs arc o!Jtaincrl if the \Ioon shows central part of the Innar disk. The crater Abulfeda only one face. rr telescopic photographs \\·ere is 39 miles across. The crater Ptolemaeus is 90 taken of the l\'Ioon from opposite sides of the miles across. i i L PHOTOINTERPRET.\TlON OF THE lXN.\R Sl·RF.\CE 379 template (puints .. 1 and H,Figure 4). The cunjugate image-puint of these centers are located on the respective photographs, and the four points are alined in the x-direction for stereoscopic yiewing. Enlarged lunar photographs can be cu tinto sections, and by use of reference points indicating the shift of the geometric center to the mean libration center, the sections can be properly oriented for stereoscopic viewing. THE STEREOSCOPIC MODEL Because of different base-height ratios in steoreoscopic pairs of lunar photographs taken at different libration positions, the ap­ parent surface relief will vary from one model B to another. The effect of different base-height FIG. 3Ao Diagram showing relation of the Earth ratios is so marked that even the curvature to the Moon at two clifferent librations; 3B. Dia­ of the Moon's surface is noticeably different gram showing how photographs of the Moon at two from one model to another. Yet the relati\Oe librations are the same as though taken from two different points in space. spatial relationship of one feature to another within the vertical dimensions of the model what similar to the method used for prepar­ generally can be ascertained. \\·ith photo­ ing terrestrial vertical aerial photographs. graphs having 12 degrees or more difference The geometric center (If the lunar disk for of libration, relief of 1,000 feet or more can be each pair of photographs is located by geo­ discerned in normal stereoscopic models. Re­ metric construction or by use of a cilTtdar lief of features only 100 to 200 feet high FIG. -to Two Moml photographs at different librations orientecl for stereoscopic ,oiewing with a lens or pocket stereoscope. 380 PHOTOGRAMMETRIC ENGINEERING FIG. 5. Photograph showing wrinkle ridges in maria. Although not visible in the stereoscopic model, such features, 100 to 200 feet in relief, are visihle under "ery low sun because of the shadows they cast on the lunar landscape. cannot be discerned stereoscopically with ex­ emanating from certain craters are con­ isting photographs, but the presence of fea­ spicuous features of the full moon but details tures with much less relief is clearly shown on of topography are generally obscure.