Saturn plays an inspirational role in astronomy. The simple shape of a sphere with rings around it is the quintessential drawing of a planet. The experience of viewing Saturn and its rings through a telescope for the first time is often an awe-inducing revelation. The magnificent ring system has an allure and a grandeur that excites our imagination.
This simple montage of Saturn images from Hubble evokes a bit of that wonder. The series of images shows how Saturn appeared every year from 1996 (lower left) to 2000 (upper right). During that time, the ring plane opens up from a close-to-edge-on view to a tilted view, which displays the rings’ extent. This transition illustrates the slow change of seasons on Saturn.
Similar to Earth, Saturn’s rotational axis is tilted with respect to its orbital plane. Saturn’s axial tilt is 26.7 degrees, while Earth’s is 23.5 degrees. The ring system orbits in Saturn’s equatorial plane, and provides a distinct visual reference. In the series of images, one can see Saturn’s southern hemisphere getting more and more sunlight. That is the progression from spring to summer for the southern hemisphere (and likewise the change from fall to winter for the northern hemisphere). Due to its 29.5 year orbit around the Sun, each season on Saturn lasts over 7 years.
Hubble often takes images of Saturn at the times when Earth’s orbit brings the two planets closer. These shorter distances occur when Saturn is opposite the Sun on the sky, an event called “opposition.” Around the time of opposition, Saturn is closest, brightest, and most fully lit by the Sun from Earth’s point of view. Saturn opposition occurs every 378 days, providing astronomers with good photo opportunities roughly every year.
It is a bit eye-opening to note that since Hubble was launched in 1990, Saturn has made only one orbit around the Sun (the exact orbit completion date is Tuesday, October 8, 2019). The progression of the seasons throughout a full Saturnian year, along with some seasonal storms at various latitudes, have been documented across the decades by Hubble’s yearbook-like pictures of the ringed planet.