As with the Earth, the rotational axis of Saturn, the second largest planet in the Solar System, is inclined with respect to the plane of its path around the Sun. This means that in the almost 29 (Earth) years and 164 days that Saturn requires to orbit the Sun once, there are also seasons on this celestial body. For half a 'Saturn year' – that is, almost 15 Earth years – the southern hemisphere receives the greater amount of sunlight and for 15 years the northern hemisphere is more brightly illuminated. However, the quantity of the Sun's energy that the planet receives at a distance of something over 1.4 billion kilometres from the Sun is only one ninetieth of that received on Earth.
In August 2009, Saturn passed through the vernal equinox – that is, for a virtual observer on the clouds of Saturn, the Sun appeared to cross the celestial equator. The southern summer has come to an end and after 11 August 2009, when day and night were of equal length – known as the equinox – spring started in the northern hemisphere. At the time of the equinoxes, the Sun shines precisely on the edge of Saturn's rings, which surround the planet at its equator. This rather rare astronomical event is something worth celebrating – not just for observers on Earth with their telescopes – because at that time the edge of the plane of the rings is seen as a small line. For the Saturn orbiter Cassini, too, the way that the light falls across the disc at a very shallow angle offers exceptional possibilities for observation, producing special scientific results regarding the structure and dynamics of the rings.
At a distance from Saturn of approximately 847,000 kilometres and a viewing angle of 20 degrees above the surface of the rings, Cassini’s wide-angle camera acquired a sequence of 75 images one-and-a-half days after the equinox in 2009, producing this image mosaic of Saturn, its rings and some of its moons. The scale of the picture is 50 kilometres per pixel. The unusual lighting geometry means that the rings appear very dark. In contrast, the structures outside the plane of the rings are unusually bright and throw long shadows across the rings. In addition, the shadows of Saturn's extensive rings appear at the equinox as a single, narrow band on the planet.
Credit: NASA/JPL/Space Science Institute.