National Geographic News
Like parade floats gliding down Main Street, four moons—and their shadows—seem to saunter above Saturn's rings in rare Hubble Space Telescope footage released today.
The video shows Titan, Mimas, Dione, and Enceladus crossing, or transiting, above Saturn's rings, as seen from Earth-orbiting Hubble on February 24.
Saturn moon transits are visible only when the planet's rings are roughly edge-on to Earth, which happens about every 15 years. Hubble last witnessed a similar ring-plane crossing in 1996.
According to astronomers, January and February 2009 were good months for people with backyard telescopes to glimpse Saturn's moon crossings, as the rings were favorably tilted. The rings have been tilting upward since then and will reach a less ideal four-degree angle by May.
Saturn's rings will then tilt back down to be perfectly edge-on to us on August 10 and September 4. Unfortunately for sky-watchers, in September the ringed planet will appear in the sky too close to the sun to be viewed from Earth.
Video courtesy NASA, ESA, G. Bacon and M. Estacion (STScI)
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