Sky & Telescope December 1979 [antikvár]

Disappearances of Saturn's Rings, 1600-2100Richard E. Schmidt, U. S. Naval ObservatoryFOR THE LAST SEVERAL YEARS, the rings of Saturn have appeared increasingly foreshortened until finally, on October 27th, they were turned edgewise to the earth. On this date, the earth passed through the plane of Saturn's equator, beginning a new cycle in the periodic disappearances of the ring system. In the following months, the northern face of the rings, after having been hidden since April, 1966, will be exposed again to Earth. Only after the sun passes through the ring plane, and the earth makes two more passages, will the rings of Saturn begin to open up for us, in the second half of 1980.The conditions for an edgewise disappearance were explained in detail in the September, 1965, issue, page 128, and need be only briefly summarized here. Throughout Saturn's travel around the sun, its north pole remains directed toward right ascension 2h 34m, declination -J-83°.3 (1950 coordinates). The ring plane coincides very precisely with the equatorial plane of Saturn, which is inclined 28° to the ecliptic. Therefore, twice during each 29V2-year revolution of Saturn, the sun passes through the ring plane (at alternating intervals of about 13H and 15^/4 years, due to the motion of the planet being faster near perihelion).Since the earth, as seen from Saturn, is never more than 6° from the sun, the earth must pass through the ring planeThe latitudes of the sun and earth as seen from the center of Saturn. During the interval shown, except for a brief period in March, the sun and earth will be situated on opposite sides of the ring plane. Positive values are north of the ring plane, negative values south. This U. S. Naval Observatory diagram was provided by the author.near the times when the sun does. In actual fact, the earth can intersect the ring plane either once or three times in each disappearance cycle, depending on the relative positions of the earth, sun, and Saturn.If the earth makes contact with the ring plane while on the far side of the sun from Saturn, it may not catch up with the plane again before Saturn moves outsideVSaturn as it might appear from Rhea, the ringed planet's fifth most distant bright satellite, which circles its primary at some 527,000 km. From suchNaval Observatory. 500 Sky and Telescope, December, 1979of that part of its orbit in which disappearances can occur. Such a single crossing of the plane last occurred in 1950. More commonly, the earth is in position to cross the plane three times during the disappearance cycle, as in 1966 and in 1979-80. When Saturn is near perihelion (as in 1974) we see the southern face of the rings; when near aphelion (as in 1988), the northern face.In 1979-80, the earth goes three times through the ring plane, on October 27,1979,and on March 12 and July 23, 1980. The sun's passage occurs on March 3,1980.There are three distinct geometric configurations that cause the rings to vanish in small telescopes: (1) the earth's passage through the plane, when the rings are too thin to be observed; (2) the sun's passage through the plane, at which time neither face of the rings receives direct sunlight; and (3), when the earth and sun lie on opposite sides of the plane so that the ring face we see is only indirectly illuminated.The case of passage by the earth through the ring plane presents the most difficuh observing situation. Only in 1966 were the rings successfully observed at or very near the time of closure. Present estimates of the rings' thickness range downward from a few kilometers, corresponding to an angular thickness of lO"* second of arc at Saturn's distance from Earth. The rings at that time are considerably less bright than the atmospheric halo of Saturn's disk. The best chance of detecting the edge-on ring is at a high-altitude observatory such as Pic du Midi.