Focus on Uranus

By Riddle, Bob | Science Scope, October 2007 | Go to article overview

Focus on Uranus

Riddle, Bob, Science Scope

The solar system as we now know it is changing, with new objects and moons regularly being discovered. For many centuries the solar system was known to only contain the Sun, our Moon, and six planets: Mercury, Venus, Earth, Mars, Jupiter, and Saturn. It wasn't until the invention of the telescope that we started to realize that there was more to the solar system than first believed. However, it wasn't until 1781, when William Herschel aimed his six-inch reflecting telescope toward the skies, that our solar system quite literally doubled in size with his discovery of another planet. At first named after his patron King George III, this newly discovered planet was known as Georgium Sidus; it was also referred to as Herschel, but eventually was re-named after the Greek god Ouranos, or Uranus as it is now spelled.

William Herschel was born in Germany, but spent most of his life in England. As a young man, Herschel played the oboe for Hanoverian Guards, a military band that was transferred to England. In later years, as a band director, he composed many symphonies, but he also had an interest in mathematics, astronomy, and making telescopes. The latter soon became the focus of his life. Working with his sister, Caroline, he built more than 400 telescopes and spent much of his time searching the skies for comets and observing the known planets, nebula, galaxies, variable stars, and double stars. His accomplishments include discovering additional moons around Uranus and Saturn, determining the proper motion of several stars, concluding that the Sun was moving through space, and surveying enough stars to allow him to suggest the shape of our galaxy. But it was his observation of double stars that led him to the discovery of the planet Uranus.

During the spring of 1781, as Herschel was systematically plotting positions of double stars in the constellation Gemini, he observed a pair of stars. He described one as being somewhat like a comet in appearance. A few nights later, when he observed this same area, he noticed that the position of one of the objects had shifted. Over the next couple of months, as Herschel and other astronomers observed this moving object and its orbit was plotted, it was determined that the object did not follow a comet-like path and that it was twice as far from the Sun as Saturn was. William Herschel was given credit for discovering a new planet. Interestingly, Uranus had been observed and plotted several times by astronomers, including Galileo, but their telescopes were not as good as the ones made and used by Herschel and were unable to resolve Uranus into anything more then a nebulous blur.


Uranus is the seventh planet outward from the Sun at a distance of 19.2 A.U., and follows an 84-year orbit around the Sun that is essentially the same shape (eccentricity) as the other three Jovian planets. Uranus is currently known to have 27 moons of which two, Titania and Oberon, were discovered by Herschel. Eighteen of these moons are regular satellites of the planet, while the remaining nine are considered to be irregular satellites (those captured by the planet's gravitational field rather than forming with the planet). Within the moon system are 12 faint rings, nine of which were discovered in 1977 as astronomers observed an occultation of a distant star by Uranus. As the planet occulted (eclipsed) the star, the star's brightness decreased and returned to normal several times on either side of the planet. Several years later, Voyager 2's cameras revealed two additional rings. The 11th ring was discovered in 2004 by astronomers using a ground-based telescope equipped with an adaptive optics system.


Lying on its side with a tilt of 97.92[degrees], Uranus is one of two planets with an axial tilt dramatically different from the others. This sets up an interesting seasonal pattern as Uranus orbits the Sun. Both polar regions are directed toward the Sun at the solstices, while the sides of the planet are sunward on the equinox. …

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