PUBLICATIONS OF THE ASTRONOMICAL SOCIETY OF THE PACIFIC. Vol. XLm San Francisco, California, August, 1931 No. 254

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1 PUBLICATIONS OF THE ASTRONOMICAL SOCIETY OF THE PACIFIC Vol. XLm San Francisco, California, August, 1931 No. 254 RECENT PHOTOGRAPHIC OBSERVATIONS OF THE PLANETS* By E. C. Slipher This note deals with recent photographs of Mars, Jupiter, and Saturn made at Flagstaff with various color filters. The discussion here relates to changes occurring on the planets themselves and to differences arising between photographs made at the same time but with different filters. The original negatives were taken with the 24-inch refractor and a negative amplifying lens to secure the desired magnification of the images. The original yellow-light photographs were taken with a magnification of about 215, and the blue ones, about 175 times. Two cameras were used in the observations, the one adjusted for photographing in violet light and the other in yellow or red light. For the photographs made with light from the blue end of the spectrum the spectrographic correcting lens was used. Examples of the planet photographs and of experimental ones were shown in lantern slides to exhibit the various influences discussed in the paper. It should be pointed out that these positive copies contain some unavoidable photographic effects but care was taken to preserve the truth of the original photographs. In the first slide were shown prismatic spectra of daylight sky taken on a panchromatic plate to display the spectral transmissions of some of the color filters used in the observations. Still other filters were employed for deeper red and more restricted regions of the violet, with suitable plates. The second slide showed nearly simultaneous yellow and * Read at the Pasadena meeting of the Society, June,

2 242 PUBLICATIONS OF THE blue photographs of the Great Red Spot of Jupiter taken September 14, The marked difference in the intensity of the Red Spot in the two sets of photographs is produced by its reddish color, which, for obvious reasons, will cause it to photograph darker in blue than in yellow light. The equatorial and the south tropical belts are of the same order of brightness in the yellow photographs, but in the blue images the south tropical belt is decidedly brighter. Visible color differences in the two belts doubtless altered the photographs, the Equator being yellowish while the tropical belt was bluish white. The third slide (Plate X, 3) showed the Red Spot region photographed about one year later, February 11, Here the Red Spot is bright in both yellow and blue light, evidencing that it was colorless. The belts appear about the same in the two sets of photographs and they show to the very edge of the blue images. This seems to indicate that the belts are high in the planet s atmosphere. The fourth slide (Plate X, 4) showed yellow and blue photographs of the Red Spot region one year later, February 17, Here the Red Spot appears darker than in It is darker in the blue than in the yellow photographs for the reasons pointed out in the second slide. The fifth slide (Plate X, 5) showed blue- and red-filter photographs taken at the same time, January 30, The south tropical belt is broad and dark in the blu^ photographs, but in the red images the upper component of the belt is scarcely visible. Visually, the upper component was very red. Thus the disappearance of this part of the belt in red light was doubtless due to its red color, which caused it to photograph in red light as bright as the light region above it. The polar diameters of the blue and red images here appear about equal. The defect of illumination due to phase is marked in the red images but not appreciable in the blue ones. It has been found that when red and blue images are taken on a soft gradation plate and longer exposures given the difference in size of blue and red images of Jupiter is much reduced. This suggests that the higher contrast factor of slow red plates, combined with the greater brightness of the center of the disk in red light, has much to

3 ASTRONOMICAL SOCIETY OF THE PACIFIC 243 do with the smaller size of red images as compared to blue ones. The sixth slide (Plate X, 6) showed yellow photographs of Jupiter in 1914 and in These illustrated the effect of exposure upon the size of image. The unequally exposed images in the upper row were produced by the unequal action of the camera shutter. The photographs in the lower row were accidentally so spaced on the original plate that the outermost parts of the planet overlap, and by this overlapping, and the consequent doubling of the exposure there, parts of the planet s disk are revealed which otherwise would not have registered in these apparently normal photographs. These show the dependence of size of image on exposure and display how much the diameter of the planet s disk is increased by doubling the exposure. The original photographs were made on the most rapid Cramer s Iso-Medium plates, such as have been used here for more than a quarter of a century in photographing the planets. The seventh slide showed red, violet, and yellow photographs of Saturn in 1927 and 1928, which revealed that the ball of the planet was then much darker in the violet images than in the red and yellow, especially across the Equator. This condition was temporary and probably due to redness of the ball, as will be seen from the following slides. The eighth slide showed yellow and violet photographs of Saturn taken in The equatorial region was again darker in the violet photographs. The ninth slide showed yellow and violet photographs of Saturn taken in 1930, showing that the equatorial region was then about as bright in the violet as in the yellow photographs, and that the whole ball showed about as bright in violet as in yellow light. Contemporaneous visual observations indicated that these changes were due to changes in the color of the ball. The tenth slide showed blue and yellow photographs of Saturn in 1928, compared with blue and yellow images of a colored photograph of the planet. These indicate that color changes over the planet s ball are sufficient to explain the differences between violet and red photographs of the planet in recent years.

4 244 PUBLICATIONS OF THE The eleventh slide (Plate X, 11) showed yellow photographs of Mars made February 1, 1931, showing graduated exposures (1, 2, 4, 8, and 16 seconds) and overlapping images. These illustrated the increase in size of image with increase of exposure. They suggest some difficulties in the way of determining planet diameters from photographic images. The twelfth slide (Plate X, 12) showed yellow and blue photographs of the same face of Mars taken February 8, The characteristic differences between yellow- and blue-light photographs of the planet were here displayed, such as the numerous bright areas present in the blue images and the absence there of the well-known dark markings. The thirteenth slide (Plate X, 13) showed yellow and blue photographs taken at the same hour on December 28, It is remarkable here how clearly the chief surface markings of the planet showed in the blue photographs. The well-known dark markings are not as sharply contrasted in the blue as in thé yellow images but they are recognizable over almost the whole disk. The writer has secured, on various occasions, other blue photographs which have shown the dark markings of the body of the planet quite clearly. Since these changes occur in photographs made on the same kind of plates and with the same color filter it seems that they can only be accounted for by some widespread change on the planet. The fourteenth slide (Plate X, 14) showed a blue and a red photograph of an oil painting of Mars by Mr. Howard Russell Butler, N.A. The photographs were taken with filters and plates used in making the planetary photographs. The photographs are strikingly similar to the results obtained in blue and red photographs of the planet itself. In these photographs the dark, bluegreen markings disappear in the blue photograph while they are highly contrasted in the red. Obviously the result here is due to color. The dark-blue markings and rose-ochre desert region become of the same luminosity in monochromatic blue light. In the red photograph the colors act in the opposite way and increase the contrast between desert and dark markings. We are not concerned, here, with how faithfully the colors in the paintings represent those existing on Mars, but to let

5 ASTRONOMICAL SOCIETY OF THE PACIFIC 245 it appear how the colors of the surface of Mars could account for some of the difference between the red and blue photographs, much as has been seen in the photographs of Jupiter. The fifteenth slide (Plate X, 15) showed a red and a blue photograph of a group of various colored rocks seen against snow. Some of these are as bright in red light as is the snow. The small white spot on some of the specimens is magnesium carbonate applied for the experiment because of its nonselective properties. The sixteenth slide showed red-filter photographs of Mars, January 14, It was noted that the northern snow cap appeared here scarcely brighter than the surrounding light regions. The explanation of this seems clear from the behavior of the snow and the red rocks as pictured in red light in the preceding slide. The conclusion to these observations must be, for want of time, very briefly stated. However, many of the inferences which should be drawn from the foregoing photographs have been touched upon or suggested already. In the course of our long-continued observations on the planets it has become increasingly apparent that several factors are more or less involved in the matter of the differences occurring between the color-filter photographs. The factors which have contributed to the differences pointed out in the foregoing photographs are : 1. The different color luminosities over the planet s disk, most evident in Jupiter. 2. The different photographic characteristics of various emulsions, especially the gamma factor of the plates. 3. Irradiation and the effect of exposure upon size of image. As this material indicates and as is evident to those who deal with photographic photometry, very careful evaluation of the disturbing factors of photographic action (already complex enough in the simplest case but greatly multiplied in comparing images from light of widely separated wave-lengths) must be made in our dealings with planetary photographs for planet diameters. And we must weigh carefully the factors influencing photographic action in interpreting differences in

6 246 PUBLICATIONS OF THE the aspect of a planet when photographed in light of different colors in order to decide which are due to the body of the planet and which to its atmosphere. However, one can conclude : That seasonal, diurnal, and secular, change is constantly taking place on Mars as has been evidenced by the visual and photographic observations continued here for more than a quarter-century. That in case of Jupiter and Saturn the changes are less systematic, in fact extremely chaotic. That color of their markings is a prime factor contributing to differences in their showing in the color-filter photographs. That the markings on Jupiter and Saturn originate in their atmospheres, and that the markings are high in the atmospheres. The first is evidenced by their mobility, and the second by their similar showing in light of different colors. Finally, the observations of Jupiter and Saturn portray them as planets possessing physical conditions far removed from earthly ones; but Mars displays markings and changes which denote that its physical state greatly resembles that of the Earth.