Ring-plane crossing

The appearance of the Saturn rings changes constantly. Saturn and its orbit have the biggest influence on the changing appearance of the rings. Since Saturn's ring plane is inclined approximately 27° relative to its orbit the angle under which we see the ring will vary from 0° to +27° than through 0° to -27° and again back to 0°. This cycle takes one orbit of Saturn around the Sun, approximately 29.5 years (red line in the graph). Because Earth is also orbitting the Sun each year will cause minor fluctuations relative to the red line in the graph resulting in the blue line in the graph. The blue line is not a perfect sinus because of Earth's elliptical orbit.

Click on the graph for a full sized version.

The red line represents the height (inclination) of the Sun with respect to the ring-plane of Saturn. A positive inclination means that the Sun illuminates the northern side of the ring-plane, when the inclination is negative the Sun illuminates the southern side of the ring-plane. The blue line represents the height (inclination) of the Earth with respect to the ring-plane of Saturn. A positive inclination means that we look towards the northern side of the ring-plane, when the inclination is negative we see the southern side of the ring-plane.

The appearance of the rings during the orbit shown in above graph.

As we see often in astronomy also here special events occur. In order to clarify I want to describe two special events, the ring-plane crossings of 1979/1980 and 1995/1996.

The ring-plane crossing of 1979/1980

In 1979 on October 27 (point A1 in the graph) the Earth crossed the ring-plane of Saturn from South to North. After this crossing the observers on Earth should see a glimpse of the North side of the rings but because the Sun illuminated the South side of the rings the observers looked towards the not illuminated side of the rings. On March 12 1980 (point B1 in the graph) the Earth again crossed the ring-plane but now from North to South. Simultaneously also the Sun crossed the ring-plane. Because the Sun crossed the ring-plane from South to North, the rings remained invisible. On Juli 23 1980 (point C1 in the graph) the Earth crossed the ring-plane again but now from South to North. The rings of Saturn slowly became visible again because observers looked to the illuminated North side of these rings.

The ring-plane crossing of 1995/1996

In 1995 on May 21 (Point A2 in the graph) the Earth crossed the ring-plane of Saturn from North to South. After three months, on August 11 1995 (point B2 in the graph), the Earth crossed the ring-plane again but now from South to North. During tis three months the rings were invisible, first of all because they opened only 1 degree but also because the Sun illuminated the North side of the rings where observers on Earth looked to the South side. In a short period after August 11 1995 (point B2 in the graph) the rings opened slowly but this ended when the Sun crossed the ring-plane from North to South on November 18 1995 (point Z in the graph). Because the Earth also crossed the ring-plane from North to South on February 11 1996 (point C2 in the graph) and the Sun illuminated the South side of the rings observers on Earth could see the slowly opening rings again.

The used database for the above can be found here.