Uranus' Rings
Occasionally a planet will pass between the earth and a star. Such an event is called an occultation (from the Latin word meaning "hiding"). In recent years astronomers have carefully monitored these occultations since the time and place on the earth at which the occultation will be visible can be calculated. It requires a precise knowledge of the planet's orbit to make such a calculation, and the precision with which the prediction is confirmed by the observation in turn tells us how well we really know the orbit.
As the planet begins to occult the star, its atmosphere, which is partially transparent, covers the star first so that there is a gradual dimming of the star. If there were no atmosphere, the star's brightness would remain constant until the opaque body of the planet cut off all light; the change would be sudden, not gradual.
In this manner astronomers aboard the Kuiper Airborne Observatory, an airplane fitted with an infrared telescope, flying high over the Indian Ocean discovered a ring system around Uranus on March 10, 1977. About a half hour before the occultation was to take place, the star's light dimmed unexpectedly for a few seconds, followed by four other dips in brightness minutes later. The sequence was repeated in reverse as the star passed beyond the disk of Uranus on the other side. Since the original discovery of five rings four less prominent rings have been identified; making a total of nine rings.
The rings appear to be very narrow, not more than 10 to 100 kilometers in width, and they lie close to the planet's equatorial plane. Hence the origin of the rings is closely related to that of Uranus since the planet's equatorial plane is almost perpendicular to its orbital plane. Six of the rings appear to be slightly elliptical, with the radii for all the rings lying between 1.6 and 1.95 planetary radii. All the rings are dark and have sharp edges. Since the ring particles are poor reflectors, it is hard to believe that they are coated with water (or ammonia or methane) ice. More likely they are a silicate- or carbon-bearing material.
