Uranus


 

Uranus and its ring system, seen by the Hubble Space Telescope in 2003

Uranus and its ring system, seen by the Hubble Space Telescope in 2005     [Source]

In 1781, Uranus became the first of the non-classical planets to be discovered. Just bright enough to be spotted with the unaided eye, it was inadvertently included on star charts no less than 17 times before its true planetary status became known. Its characteristic color comes from the presence of methane (CH4), the third most abundant component in its atmosphere (after hydrogen and helium).

From Earth, its angular diameter spans a mere 3.4-3.7 arcseconds. While it is an easy target for binoculars or a small telescope, even large telescopes will not reveal much more than a pale cyan disk with a darkened limb.

With an unusually high axial tilt of 98 degrees, Uranus rotates on its side. As a result, it features extreme seasons, with each pole receiving 42 years of continuous sunlight followed by 42 years of continuous darkness.

Uranus-Keck infrared

Uranus imaged in the infrared by the Keck Observatory     [Source]

The atmosphere of Uranus is the calmest of the gas giants. Near the 1986 Uranian solstice, Voyager 2—the only spacecraft to visit Uranus to date—was presented with a nearly featureless disk (as shown below before the end of the text). However, the years approaching the 2007 equinox presented a different appearance (as shown above).

Unlike any other planet in the Solar System, Uranus is not named after a Roman deity, but rather a Greek one instead. Its moons also break the naming convention, in that they are named for characters in the literary works of English poets William Shakespeare (1564 – 1616) and Alexander Pope (1688 – 1744).

Uranus seen up-close by Voyager 2 in 1986

Uranus seen up-close by Voyager 2 in 1986

Six years after the discovery of Uranus, the brightest and largest two moons—Titania and Oberon—were also realized by the planet’s discoverer: none other than Sir William Herschel (1738 – 1822), a legendary observer later also known for his compilation of a 2500 object deep sky catalog (as discussed on the “Herschel 400 objects” page) and the discovery of infrared radiation in 1800. Titania and Oberon both exhibit an apparent brightness around 14.0 magnitude. Though certainly a challenge, it is in fact possible for telescopes with apertures as small as 8 inches to resolve either of these moons from Uranus, particularly when either of them are near their maximum apparent separation from the planet (about 30 and 40 arcseconds respectively).

 


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