URANUS

URANUS

THE SATELLITES OF URANUS

THE DISCOVERY OF URANUS

(NASA, Jet Propulsion Laboratory, "Voyager at Uranus: 1986," JPL 400-268, U.S. Government Printing Office, Washington, D.C., July 1985.)

William Herschel and the Discovery of Uranus

A professional interest in musical theory, which broadened to encompass a general study of mathematics, preceded Sir William Herschel's passionate interest in astronomy and led to his discovery of the seventh planet.

He was born Frederich Wilhelm Herschel in Hanover in 1738. His father, a bandmaster in the Hanoverian Guards, encouraged him toward a musical career, and Herschel joined the Hanoverian Guards as a musician in his midteens. He moved to England in his early twenties after service in the Seven Years' War. With the goal of becoming a composer, he traveled throughout England working as a freelance musician, music copier, and organist. In 1766, Herschel won appointment as the organist for the new Octagon Chapel in Bath. He was later named director of public concerts for the city.

Herschel read widely on the subjects of harmonics, mathematics, and philosophy. Historians believe the first book he read on astronomy was James Ferguson's "Astronomy Explained Upon Sir Isaac Newton's Principles, and Made Easy to Those Who Have Not Studied Mathematics."

By his late thirties, his dabbling in astronomy had become a more consuming hobby. He put rudi- mentary telescopes together from scraps and used parts. The more of the sky he saw, the more he wanted to see. By late 1773, when he couldn't afford to buy the most powerful telescope available, he determined to build his own.

Herschel and his brother Alexander and sister Caroline, also musicians, shared a house in Bath. Building William's telescopes became a family affair.

Alexander helped with the construction. "It was to my sorrow," wrote Caroline in her memoirs, "that I saw almost every room turned into a workshop ...Alex putting up a huge turning machine in a bedroom for turning patterns, grinding glasses and turning eyepieces."

But Caroline, who would also become a talented astronomer, pitched in as well, even feeding Herschel his meals while he spent hours grinding or polishing by hand a metal speculum, or reflector, for his telescope.

With the best of his telescopes (a 7-foot focal-length instrument with a 6.2-inch reflector), he began what he called "reviews of the heavens" from his garden. Over months of observations, he believed he'd spotted forests of trees on the moon and noted them in his meticulously kept log. He spent many of his observing hours studying double stars and decided to study generally the distribution of the stars and to try to calculate their distances.

While studying stars in the constellation Gemini the night of March 13, 1781, he found a disk-like object moving slowly across the starfield. He believed it to be a comet and reported his observation as such to the British Astronomer Royal.

Within days, however, the object's orbit was calculated as one no comet would likely follow. In addition, it was so distant that, if it had been a comet, it would have been too small to be seen with the instruments of the day.

News of the sighting spread quickly throughout the scientific community. Astronomers and mathematicians across Europe computed the object's approximate size and orbit and, by May 1781, concluded that 42-year-old amateur astronomer William Herschel of Bath had discovered a new planet as far beyond Saturn as Saturn is from the Sun.

Many names were proposed for the new body: "Hypercronius" ("above Saturn"), "Minerva" (the Roman goddess of wisdom), and "Herschel" were candidates. Herschel offered "Georgium Sidus" ("The Georgian Planet") to flatter King George III of England and Herschel's native Hanover. (Royal patronage would later support Herschel's work during his distinguished scientific career.) But, astronomy being an international concern and George being an unpopular monarch outside of England and Hanover, variations on his name were vetoed. Astronomers finally agreed upon "Uranus" ‹personification of the heavens in Greek mythology, son of Gaea (Earth) and, by her, father of Saturn and grandfather to Jupiter.

Herschel, continually building bigger and better telescopes throughout his career, also discovered the Uranian moons Titania and Oberon in 1787. English astronomer William Lassell found Ariel and Umbriel in 1851. Herschel's son, John, named all four. They are the only moons in the solar system not called after figures in Greek and Roman mythology. Instead, they are named for characters in English literature: Oberon and Titania are the king and queen of the fairies in Shakespeare's "A Midsummer Night's Dream"; Ariel and Umbriel appear in Alexander Pope's "Rape of the Lock"; Ariel also appears as a spirit in Shakespeare's "The Tempest." Miranda, discovered by the late American astronomer Gerald Kuiper in 1948, is named after Prospero's daughter in "The Tempest."

FACTS ABOUT URANUS

(Bevan M. French and Stephen P. Maran, eds., "A Meeting with the Universe," NASA EP-177, U.S. Government Printing Office, 1981.)

*The ancients knew of five planets in the sky. In less than a generation, all five have been brought close to us by spacecraft. Further out in space, three more planets await our inspection. Uranus, the first outward beyond Saturn, is nearly 3 billion kilometers (nearly 2 billion miles) from the Sun and takes 84 years to make one orbit around it.

* Pioneer 10 passed the orbit of Uranus, but did not come near the planet. Uranus is another gas giant, smaller than Saturn, a cold world that is a distant greenish disk even in the largest telescopes. From Earth, we can see five moons circling the planet.

* Uranus has one unique property: its axis of rotation lies in the plane of its orbit rather than nearly vertical to it as is the case with the other planets. Because of this curious orientation, Uranus moves around the Sun, not so much like a top spinning on its end, but like a barrel rolling along on its side.

* The greatest Space Age discovery about Uranus was made in 1977, not from a spacecraft but rather with a telescope mounted in a high-altitude jet aircraft, the Kuiper Airborne Observatory. Like many great discoveries, it was entirely unexpected, and the scientists were looking at Uranus for something entirely different. They intended to study Uranus' atmosphere by measuring the light from a distant star as the planet passed slowly in front of it. What happened was that the star seemed to flicker on and off several times, long before and long after the planet had passed in front. The only explanation was that Uranus like Saturn and now Jupiter has rings! The planet is surrounded by at least five and perhaps nine rings. The rings are dark, thin, narrow, and invisible from Earth, but they were thick enough to block out the light from the star.

* Uranus had its first visitor from Earth. The spacecraft Voyager 2 followed a path that took it past Uranus in 1986.

* The dark rings of Uranus were covered by telescopic observations from a NASA aircraft in 1977. The thin narrow rings, composed of dark particles, are invisible from Earth. They were discovered when they blocked the light of a distant star on a night when Uranus passed in front of it. The rotation axis of Uranus lies almost in the plane of its orbit around the Sun, so the ring system (in the plane of the planet's equator), appears like a gigantic bullseye as Uranus rolls around the Sun.

(The following is from "The Solar System," NASA/ASEP, 1989, p. 10.)

Uranus

* Uranus is named for the father of Saturn and grandfather of Jupiter.

* Uranus' symbol is the sign used for the metal platinum.

* This is the seventh planet from the Sun.

* Uranus is the third largest planet in our solar system.

* William Herschel discovered Uranus in 1781.

* Uranus circles the Sun every 84.01 Earth years.

* One day on Uranus is 17 hours, 14 minutes, and 24 seconds long.

* The gravity of Uranus is .86 of Earth's gravity.

* Uranus is one-fifth denser than Earth.

* The diameter of Uranus is 32,116 miles.

* Uranus is 19 times Earth's distance from the Sun.

* Uranus' bluish-green color is caused by methane gas (natural gas).

* Uranus' atmosphere is made up of helium, hydrogen, and methane.

* Uranus has 15 moons (which are named for characters from famous plays). A. Cordelia F. Rosalind K. Miranda B. Ophelia G. Portia L. Ariel C. Bianca H. Cressida M. Umbriel D. Juliet I. Belinda N. Titania E. Desdemona J. Puck O. Oberon

* Uranus is tipped-over on its side.

* Communication to Uranus takes about 2 hours and 45 minutes.

* Voyager 2's flyby of Uranus took place in January 1986.

JPL SUMMARY OF URANUS

(NASA, Jet Propulsion Laboratory, "Our Solar System at a Glance," NASA Information Summaries, PMS 010-A (JPL), June 1991.)

Uranus

In January 1986, four and a half years after visiting Saturn, Voyager 2 completed the first close-up survey of the uranian system. The brief flyby revealed more information about Uranus and its retinue of icy moons than had been gleaned from ground observations since the planet's discovery over two centuries ago by the English astronomer William Herschel.

Uranus, third largest of the planets, is an oddball of the solar system. Unlike the other planets (with the exception of Pluto), this giant lies tipped on its side with its north and south poles alternately facing the Sun during an 84-year swing around the solar system. During Voyager 2's flyby, the south pole faced the Sun. Uranus might have been knocked over when an Earth-sized object collided with it early in the life of the solar system.

Voyager 2 discovered that Uranus' magnetic field does not follow the usual north-south axis found on the other planets. Instead, the field is tilted 60 degrees and offset from the planet's center, a phenomenon that on Earth would be like having one magnetic pole in New York City and the other in the city of Djakarta, on the island of Java in Indonesia.

Uranus' atmosphere consists mainly of hydrogen, with some 12 percent helium and small amounts of ammonia, methane and water vapor. The planet's blue color occurs because methane in its atmosphere absorbs all other colors. Wind speeds range up to 580 kilometers (360 miles) per hour, and temperatures near the cloud tops average -221 degrees Celsius (-366 degrees Fahrenheit).

Uranus' sunlit south pole is shrouded in a kind of photochemical "smog" believed to be a combination of acetylene, ethane and other sunlight-generated chemicals. Surrounding the planet's atmosphere and extending thousands of kilometers into space is a mysterious ultraviolet sheen known as "electroglow."

Approximately 8,000 kilometers (5,000 miles) below Uranus' cloud tops, there is thought to be a scalding ocean of water and dissolved ammonia some 10,000 kilometers (6,200 miles) deep. Beneath this ocean is an Earth-sized core of heavier materials.

Voyager 2 discovered 10 new moons, 16-169 kilometers (10-105 miles) in diameter, orbiting Uranus. The five previously known‹Miranda, Ariel, Umbriel, Titania and Oberon‹range in size from 520 to 1,610 kilometers (323 to 1,000 miles) across. Representing a geological showcase, these five moons are half-ice, half-rock spheres that are cold and dark and show evidence of past activity, including faulting and ice flows.

The most remarkable of Uranus' moons is Miranda. Its surface features high cliffs as well as canyons, craterpocked plains and winding valleys. The sharp variations in terrain suggest that, after the moon formed, it was smashed apart by a collision with another body‹an event not unusual in our solar system, which contains many objects that have impact craters or are fragments from large impacts. What is extraordinary is that Miranda apparently reformed with some of the material that had been in its interior exposed on its surface.

Uranus was thought to have nine dark rings; Voyager 2 imaged eleven. In contrast to Saturn's rings, which are composed of bright particles, Uranus' rings are primarily made up of dark, boulder-sized chunks.

RETURN TO THE ASTRONOMY HOME PAGE


INDEX

SPACE EDUCATORS' HANDBOOK HOME PAGE

SCIENCE FICTION SPACE TECHNOLOGY HOME PAGE

SPACE MOVIES CINEMA HOME PAGE

SPACE COMICS

SPACE CALENDAR