OBSERVATIONS OF VENUS MADE AT MOUNT WILSON IN THE WINTER OF Robert S. Richardson

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1 OBSERVATIONS OF VENUS MADE AT MOUNT WILSON IN THE WINTER OF Robert S. Richardson Mount Wilson and Palomar Observatories Carnegie Institution of Washington California Institute of Technology» Markings on Venus are notoriously difficult to observe. Al- though photographs of markings on Venus had been obtained by a few observers, notably W. H. Wright 1 at the Lick Observatory in 1924, F. E. Ross 2 in 1927 seems to have been the first to make systematic daily observations of the planet in ultraviolet light, which clearly revealed day-to-day changes in its atmosphere. After Ross s work, however, no photographic observations of Venus with large telescopes appear to have been made until those of G. P. Kuiper 3 in 1950 at the McDonald Observatory. The scarcity of photographic observations is particularly regrettable inasmuch as Ross s plates of 1927 have been unaccountably lost. After observations of Mars were finished in the summer of 1954, it seemed desirable to use the experience thus gained to photograph the more difficult subject, Venus. Accordingly, plans were made to observe Venus in the morning sky after the inferior conjunction of November 15, The photographs were to be taken with the motion-picture camera previously used on Jupiter 4 and Mars, 5 which has been described in a previous paper. 5 The film was Eastman Negative Background X, a fine-grain panchro- matic emulsion of moderate contrast. Thanks to the prompt action of W. A. Sykes of the Eastman Kodak Company at Rochester, N.Y., a Wratten 18A filter was obtained in time for the first ob- servations in December This filter is essentially the same as the one Ross used in It transmits from À 3100 to À 4000, with a flat maximum from À 3550 to À The details of the observations are given in Table I. Here k is the symbol used in the American Ephemeris for the fraction of the disk illuminated ; i denotes the phase angle at the planet between the sun and earth. Through the 18A filter the exposures at the coudé focus of the 100-inch telescope ranged from 0.3 sec. at the beginning of a run when Venus was near the horizon to 304

2 OBSERVATIONS OF VENUS sec. at the end near sunrise. At the Cassegrain focus of the 60-inch the exposure time through the filter was 0.9 sec. All ex- posures were taken with the 100-inch except those of January 9. As soon as the telescope could be set on the planet the image was focused on a ground glass in the plane of the film. Exposures were then taken at the rate of three or four a minute, or the camera was allowed to operate automatically at a much faster rate. After ex- posing for about five minutes the magazine containing the film was swung aside, and the image was centered and refocused on the ground glass. It was necessary to slow down the westward motion of the telescope by about the maximum amount possible with the rating adjustment to keep the image centered. The scale at the coudé focus of the 100-inch without enlarge- ment is 1 mm = 2'/62, found from exposures on 0 Virginis. At the 60-inch a negative lens was used to make the scale approxi- mately the same as at the 100-inch. The diameter of the images ranged from 14 mm on December 29,1954, to 6 mm on March 29, TABLE I Photographs of Venus Taken at Mount Wilson in Ultraviolet Light Date (UT) 1954 Dec. 29 Dec Jan. 9 Jan. 29 Feb. 15 Mar. 29 Begin End 13*24 14* Number of Exposures Quality 160 Fair- 150 Fair- 220 Fair 260 Good 225 Fair 260 Fair APPEARANCE OF THE PLANET i k Markings of a banded type similar to those described by Ross and Kuiper were always present on the disk. Dr. Ross has ex- amined our best images of January 29 and states that the mark- ings do not stand out as conspicuously as those on his photographs. This is probably due in part at least to the higher contrast of the Cramer Contrast plates on which his exposures were made. Photometry of the markings on our images shows that they have a range in intensity of 25 percent, about the same as Ross found on his images of June 6 and 26, 1927.

3 306 ROBERT S. RICHARDSON Instead of trying to reproduce the delicate markings on the disk photographically, we have asked Chesley Bonestell, well-known astronomical artist, to make drawings of the planet from an examination of our best negatives (Plate XX). We believe that Mr. Bonestell s long experience as an artist and draftsman has enabled him to give a much better representation of the markings on Venus than would have been possible from the difficult process of trying to transfer an enlarged negative to a print. Inspection of the drawings shows that on December 30, January 9, and February 15, the principal marking consisted of a broad central belt yffiich met the terminator at an angle of slightly less than 90. On March 29 there were two broad belts which merged near the terminator with an angle of about 50 between them. The image of January 29 shows three belts in the southern portion of the disk and two fainter belts in the north. These belts are much narrower and better defined than those on the other dates. Simultaneous visual and photographic observations of markings on Venus are so rare as to be worthy of special mention. Shortly after the first of this year an amateur astronomer, Henry P. Squyres of El Monte, California, came to the Observatory offices to see Thomas Cragg, who has the office next to mine. Squyres and I had never met before. He had drawings of Venus made December 30 and January 9 with a six-inch reflector through a blue filter. The markings on his drawings were readily identified with those on our photographs of the planet. In particular, the images of December 30 show a narrow streak in the southern portion of the disk running nearly perpendicular to the terminator. A marking of this general nature has been frequently reported adjacent to the south polar cap by Schiaparelli, Trouvelot, Lowell, and Quénisset. 6 Ross remarks that such a marking is well seen on his photograph, a, of June 6 (Plate VI), 2 but the broad area shown there and on Quénisset s drawing does not resemble the narrow wedge-shaped feature on Squyres drawing or on our images. POSITION OF THE POLE OF VENUS Kuiper from his observations believes that normally Venus presents a banded structure, usually with three bright and three

4 OBSERVATIONS OF VENUS 307 dark bands. These show changes from day to day, and at times the band system is quite irregular. The orientation of the axis of Venus was found on the assumption that the bands as they nor- mally appear are parallel to the equator of the planet. A white globe with a co-ordinate grid was used as a model of the planet and illuminated from the proper direction to simulate the phase. The orientation of the globe was then changed by an assistant until the ruled circles seen from a fixed position agreed best with the orientation and shape of the bands as observed. For con- venience of reference Kuiperis values for the position of the pole of Venus are given in Table II. TABLE II Determinations of Positions of Pole of Venus (Kuiper) Date (UT) April 9.6 June 16.0 June 26.0 June 27.0 July 19.0 Year R.A Decl. 82? Weight 2 2 * * 1 Our observations indicate that the only time Venus presented its normal appearance was on January 29, when there were three narrow, well-defined bands in the southern half of the disk and two fainter bands in the northern half. The orientation of the axis of Venus was determined on the assumption that the three bands in the southern portion of the disk were parallel to the equator of the planet. The method adopted was one devised originally to determine the position of the solar axis. 7 It uses the condition that the latitude of a marking carried across the disk of the sun or of a planet by rotation remains unchanged. From three observations of the position angle and distance of a marking from the center of the disk, the inclination of the equator of the planet and the longitude of its ascending node relative to some fundamental reference system can be calculated. In the case of Venus, instead of measuring the position of a marking at three different times, three positions were measured along a band assumed parallel to the equator. The bands are of such low contrast that they practically van-

5 308 ROBERT S. RICHARDSON ish when viewed even under a low-power eyepiece on the measur- ing machine. It was tantalizing to be able to see these markings on the film without being able to measure them. The following method was finally adopted and proved satisfactory. First the film was laid on a screen illuminated from below, which rendered the bands on an image of Venus readily visible. A straight edge was laid across one of the bands and a pencil line drawn along it over the film. The image thus marked was then put on the measur- ing machine and enough points were measured to define the disk of the planet and the pencil lines. These points were plotted on co-ordinate paper, and the pencil lines were continued on across the disk. The positions of three points along each of the bands designated A, B, C, counting from north to south, respectively were read off the diagram. These yielded three independent values for the inclination of the equator and longitude of the node, and the latitudes of the bands. Lines defining the positions of the bands were drawn by Pettit, Cragg, and Richardson. Pettit marked only the two stronger bands, A and B. One determination was also made from three microphotometer tracings made across the disk par- allel to the terminator. This might seem to be the best way of measuring the positions of the bands, but they are so shallow that measures could be made with assurance only on A. The re- sults are given in Table III. TABLE III Direction of North Pole of Venus from Images of 1955 Jan UT Band R.A. Decl. Pettit A Pettit B Pettit A Pettit B Cragg A Cragg B Cragg C Richardson A Richardson B Richardson C Micro. A Lat

6 OBSERVATIONS OF VENUS 309 When equal weight is given to each determination, the mean position for the direction of the north pole of Venus is : R.A. = 311 = 20 h 44 m, Decl. = -f-64. The latitudes of the bands A, B, and C are +2, 14, and 33, respectively. When referred to the ecliptic, the position of the north pole is: Long. = 11, Lat. = +73. The inclination of the equator of Venus to the plane of its orbit is 14. These results obtained from calculation were checked by draw- ing three bands on a ball parallel to its equator, and viewing it from the angle at which Venus was seen from the earth on Jan- uary 29, The bands on the ball should have appeared paral- lel to the pencil lines drawn on the image of the planet, and they did. An obvious objection to this method is that the bands are made to cross the disk in straight lines, a situation that can occur only when the earth is in the line of nodes. On the images the bands show some indication of curvature, but it is far too slight to measure. Since our results would carry greater weight if our method of measurement worked for some other planet the orien- tation of whose axis was already accurately known, we tried such a test on Jupiter. This could be done readily because Jupiter had been photographed on January 29 on the same roll of film with Venus. Measures were made on three Jovian belts designated here as I, II, and III, and reduced in the same way as those for Venus. The results are given in Table IV. Belt I II III TABLE IV Direction of North Pole of Jupiter from Images of 1955 Jan. 29 R.A Decl. Lat The position of the north pole of Jupiter determined in this way is : R.A. = 249, Decl. = +65. The position adopted in the American Ephemeris is R.A. = 268, Decl. = +65. From the observations listed in Table I, Kuiper finds for the position of the north pole of Venus : R.A. == 53, Decl. = +81 ;

7 310 ROBERT S. RICHARDSON or when referred to the ecliptic : Long. = 80, Lat. = +59. The inclination of the equator of Venus to its orbit is about 32. Al- though these values differ considerably from ours, the differences are probably no larger than might be expected from the uncertain nature of the data upon which they are based. The difference in direction of the two positions of the axis upon the celestial sphere is only 29. The assumption that the bands are parallel to the equator may be questioned. If observations could be taken over a considerable period, say ten years, the positions found for the pole would probably cluster uniformly around some mean value, while isolated observations might depart considerably from the mean. Thus Kuiper s values for the R.A. of the pole show a difference of 52 between June 26 and July 19, The assumption that the bands are parallel to the equator can be examined by using Kuiper s position of the pole to calculate the latitude of the three points measured along the bands A, B, C. If the bands are parallel to the equator as assumed, then the three points measured along any one band should come out with the same latitude. The results of these calculations are given in Table V. Attention is directed to the diagrams below each of the draw- ings in Plate XX, indicating the orientation of the disk of Venus. The direction of the north celestial pole is vertically upward. In TABLE V Latitude of Points on Bands A, B, C Found from Kuiper s Data; Images of Jan , 1955 Latitude of Points Pettit Pettit Pettit Pettit Band (1) (2) A B A B ? (3) Cragg A Cragg B Cragg C Richardson A Richardson B Richardson C

8 OBSERVATIONS OF VENUS 311 all cases east is counterclockwise to the left. The positions of the pole and equator of Venus have been drawn according to the results derived from our images of January 29 only. The reader is hereby warned that although this helps in making a nice-looking diagram, it should not be taken too seriously. From Table V it is seen that Kuiper s values make the latitude of the point nearest the terminator consistently about 13 north of the point nearest the limb. Although the bands may not be strictly parallel to the equator, there would seem to be no other means at present of attacking the problem except on such an assumption. Since Kuiper s results are based upon more observations than ours, they should be accorded much greater weight. If we simply ignore the large differences between them in longitude and latitude, however, we find that the inclination of the equator of Venus to its orbit is 23, indicating an axial tilt for this planet about the same as that of Mars and of the earth. PHOTOMETRY OF THE DISK The film was calibrated by exposures with a tube photometer taken through an ultraviolet filter with approximately the same transmission curve as the 18A Wratten filter. Tracings were made only on the images of January 29. Figure 1 shows tracings made parallel to the terminator at five distances from the center of the disk. Figure 2 shows two tracings made through the center of the disk perpendicular to the terminator. The highest intensity found on the disk was for a point on Figure 2 at sin 0 = 0.75, where 0 is the angle at the center of the planet measured from the center of the disk in the plane of the earth, planet, and sun to the point of observation. The intensity of this point was taken as 1.00, and the intensities on the other tracings were made to correspond to this scale. The following method was used for determining the geometrical position of the limb. The midpoint on the tracings made parallel to the terminator was chosen by inspection. Knowing the size of the image from direct measurement on the film, and the magnification (40) used on the microphotometer, we calculated the distance of the limb from the center of the tracing. The limb usually fell at a point where the measured intensity was about 0.1.

9 VENUS PARALLEL TO TERMINATOR NORTH SOUTH parallel to the terminator at different distances ([r/r) from the center. The abscissae are fractions of a radius measured from a line through the center of the disk in the plane of the sun, earth, and planet. The tracings were made on an image of Venus photographed January 29, 1955, in ultraviolet light, when the disk was half illuminated. 312

10 OBSERVATIONS OF VENUS 313 VENUS PERPENDICULAR TO TERMINATOR Fig. 2. Intensity curves across the disk of Venus taken perpendicular to the terminator. The tracings were made on an image of Venus photographed January 29, 1955, in ultraviolet light when the disk was half illuminated. In Figure 2 the position of the limb was arbitrarily taken as the point at which the intensity was 0.1. Figure 2 shows that the brightness of the disk increases almost directly with distance from the terminator to about sin 0 = 0.2, beyond which the brightness increases more rapidly with distance than the linear rate, to a maximum at sin 6 = Ross found a similar effect on his ultraviolet images of Venus taken when k = On his images of Venus taken in red light, however, the intensity curve is practically a straight line from the termina-

11 314 ROBERT S. RICHARDSON tor to the point of maximum brightness. He interprets this as indicating a billowy structure for the upper cloud layer of Venus. Since the ultraviolet light is scattered more than the red, the shadows will be less pronounced on the ultraviolet images than on those taken in light of longer wave length. The tracings made parallel to the terminator out to r/r = 0.56 give dish-shaped intensity curves, shallow in the middle and high- est toward the limb. These become less shallow with increasing distance from the terminator, until at r/r = 0.59 the intensity curve is nearly flat. At r/r = 0.8 the intensity curve shows only a rapid rise toward the bright central point. The southern cusp was considerably the brighter, as usually seems to be the case. My thanks are due to F. E. Ross, Thomas Cragg, Henry P. Squyres, and Edison Pettit for assistance in preparing this paper. I am especially grateful to Chesley Bonestell for the drawings of Venus in Plate XX. 1 Pub. A.S.P., 39,220, Mt. W. Contr., No. 363 ; Ap. J68,57, Ap. J., 120, 603, 1954; Contr. from the McDonald Observatory, Univ. of Texas, No Edison Pettit and Robert S. Richardson, Pub. A.S.P., 65, 91, Pettit and Richardson, Pub. A.S.P., 67, 62, M. F. Quénisset, Comptes rendus, 172, 1645, A. C. Petersen, Astron. Nach., No. 419,18,161,1841.

12 PLATE XX Drawings of Venus by Chesley Bonestell from study of original photographs taken in ultraviolet light. The orientation of the pole was derived on the assumption that the three bands in the southern half of the disk on January 29, 1955, were parallel to the equator of the planet.