The Rings of Uranus

by on February 5th, 2011
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Discovery of the Ring System

William Herschel claimed that Uranus had a ring, and his description matches what is now called the ε ring. Rightly or wrongly, his contribution to the discovery of the Uranian rings is not recognized – at least not yet.

In 1877, James Elliot, Edwin Dunham, and Douglas Mink were studying the atmosphere of Uranus as it passed in front of a star, Unexpectedly, the light of the star dimmed several times as it draw near the planet. After passing behind Uranus, the star again dimmed several times as it receded from the planet. They concluded that Uranus had rings. They counted five rings, but the NASA website claims that they must have missed some on account of the excitement of the discovery. (When I speak of the star passing behind the planet, I am using phenomenal language, of course. It was actually the planet that moved, not the star.)

Eventually it was discovered that Uranus had at least 13 rings. Voyager 2 and the Hubble Space Telescope detected some of these additional Uranian rings.

Names of the Rings

Astronomers have used Greek letters as designations of most of the rings of Uranus. A few are identified by numerals.

The closest ring to Uranus is called ζ (the Greek letter zeta). Next come three rings designated by the Arabic numerals 6, 5, and 4. Proceeding outward, we arrive at rings called α (alpha), β (beta), η (eta), γ (gamma), δ (delta), λ (lambda), ε (epsilon), ν (nu), and finally μ (mu).

Composition of Rings

The rings are composed of small bodies that revolve around the planet Uranus. Their orbits are nearly circular but more eccentric than the rings of Jupiter and Saturn. The planes of their orbits lie close to the plane of the equator of Uranus. The outer ν and μ rings are thought to contain tiny particles in the micrometer range. The other rings also contain a little dust, but their particles are usually more than a meter in diameter, according to the Oxford Dictionary of Astronomy.

The materials in the rings are darker than the corresponding particles in the rings of Saturn. Their chemical components are not yet known.

Orientation of the Rings in Space

Because there is a 98˚ degree angle between the plane of the equator of Uranus and the plane of its orbit around the Sun, the plane of the rings have a vertical orientation when viewed from the Sun.

Width of Rings

The rings of Uranus are not very wide. For example, the widest parts of the γ ring are 4.7 kilometers, according to Wikipedia. In contrast, the B ring of Saturn is 25,500 km wide, according to the same source.

Distances from Saturn

The innermost ring (ζ) is less than 40,000 km from the center of Saturn. This ring is wider than most of the others and might be composed of two or more distinct rings. My own suspicion is that it may be material that is gradually being pulled into the mother planet, but this is unsupported speculation.

The two outermost rings (ν and μ), which were discovered by the Hubble Space Telescope, are also wider than most of the others. According to Wikipedia, the outer edge of the μ ring is 113,000 km from the center of Saturn.

ε Ring

The ε ring is the densest and most visible of the rings of Uranus, but it is very narrow. The orbit of the satellite Cordelia lies close to the inner edge of the ring, and the satellite Ophelia revolves around Uranus beyond the outer edge of the ε ring. According to current scientific thinking, these two satellites act as shepherds that keep ring particles from straying from their appointed place.

The other rings appear to be sheep without a shepherd, but perhaps other shepherd moons might be detected in the future.

References

NASA: Uranus – Rings

http://solarsystem.nasa.gov/planets/profile.cfm?Object=Uranus&Display=Rings

Wikipedia: The Rings of Uranus

http://en.wikipedia.org/wiki/Rings_of_Uranus

Oxford Dictionary of Astronomy; Ian Ridpath, editor; 2007


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