(cache) Astroprof’s Page » Uranus’ Rings Edge-On

« The orange, orange grass of home …Walter Marty Schirra, Jr. (Mar. 12, 1923 - May 3, 2007) »

Uranus’ Rings Edge-On

Published on May 2, 2007 at 4:52 pm. 1 Comment.
Filed under planets.

All of the gas giant planets have rings. Saturn’s rings are the most famous, of course, and are easy to see from Earth, even in small telescopes. But, Jupiter, Uranus, and Neptune also have rings. However, these ring systems are much smaller and dimmer than Saturn’s rings, and so are difficult to see from Earth, even with the best telescopes available.

All of the ring systems share several characteristics. One of these is that they all orbit in their planet’s equatorial plane. All of the major planets orbit the Sun with orbits tilted only within a few degrees of one another. All of the planets also rotate about an axis inclined with respect to their orbital axis. This gives rise to seasons on the planets. But, it also means that the equatorial plane of one planet will occasionally line up with another planet. For the planets having rings, this means that their rings will appear edge-on to any observer on any planet lined up with the first planet’s equatorial plane. This is happening today with Earth lining up with the equatorial plane of Uranus.

The planet Uranus orbits the Sun at an average distance of about 19.2 Astronomical Units (that means 19.2 times farther from the Sun than Earth’s average orbital distance). At this distance, Uranus takes 84.1 years to complete one orbit. It also means that being so far from the Sun, Earth would always appear very near the Sun as seen from Uranus. Thus, Earth can only line up with Uranus’ equator very near in time to that planet’s equinoxes. Uranus’ equinox will be December 7, 2007. But, remember, Earth itself doesn’t orbit in exactly the same plane as Uranus, so as Earth moves around the Sun, it will cross back and forth past the ring planes more than once. In this case, Earth is lined up with Uranus’ equator three times: first today, May 2, 2007, and then again on August 16, 2007, and finally on February 16, 2008. With an orbit of 84.1 years, equinoxes on Uranus occur at intervals of about 42 years. So, the next time that Earth is lined up with the rings will be in 2049. The last time was in the mid 1960’s, about a dozen years before astronomers even discovered the rings of Uranus. So, this will be the first time that we will get a chance to observe Uranus with its rings edge on using telescopes.

Astronomers actually can learn a lot about a planet’s rings by observing them edge on. When Saturn’s rings are observed edge on they seem to disappear. This tells us that they are very thin. This last occurred with Saturn in 1996, and will not occur again until 2008. But, during the last Saturn Ring crossing event, the Hubble Space Telescope was able to discover small moons orbiting Saturn within the ring system. So, one may imagine that there may be discoveries of moons embedded within Uranus’ rings during an edge-on viewing as well. Already, we know of several moons embedded in the ring system, as shown in the diagram at the top of this posting, so it is conceivable that others may yet be found. There are at this time no plans for another spacecraft to visit Uranus up close, so this will be about the best chance to find these moons, and perhaps to learn more about Uranus’ rings. We’ve never actually observed them edge-on, so who knows just what we’ll find?