U R A N U S

THE seventh planet from the Sun is Uranus that was first discovered telescopically by Sir William Herschel in 1781. In Greek mythology, Uranus was one of the original gods whose off-spring included the twelve gods known as the Titans, being also the father to Cronos (Greek) or Saturn (Roman). [See the full discussion of these rrelated mythologies in Saturn Part 2.]

Lying 2.7 billion kilometres or 19AU from the Sun, Uranus’ diameter is around 51 000 kilometres, but appears just as a featureless disk even in large telescopes. Uranus has a diameter about four times that of Earth whose mass fourteen times heavier. From the faint band-like features that may appear from time to time, the planet has been found to rotate about once every 17h 24m, which differs significantly from the 15h 35m often quoted in pre-Voyager 2 publications.

For many planetary astronomers, Uranus and Neptune were seen as cousins, have many similarities in both size and nature. For many years Uranus was considered the larger and more massive, but it is now certain that Neptune is more significant planet. The main difference is because of the distances these planets lie from the Sun, where earth-based observations until very recently could reveal very little.

Uranus takes about eighty-four years to orbit the Sun, being in the order of the duration of an average human lifespan. Compared to all the other planets, Uranus has the peculiarity of having the pole spinning nearly perpendicular to its orbital plane at 97.86°. This means that any Uranian observers would have sunlight on one hemisphere for 40 years, then darkness for another 40 years, with dawn and twilight lasting about two years apiece. During 2007, the equatorial plane of Uranus was visible, with the plane of the satellites and rings being edgewise. It is unknown at the time of writing if there will be a series of occultation and transits, like we observe with Jupiter, and once every fourteen years with Saturn.

Not much was known of the planet until January 1986 when Voyager 2 made it close fly-by. Data revealed little about Uranus itself as it appears to be a rather inactive body, especially when compared to the planets of Jupiter and Saturn.

The Uranian atmosphere is composed of a high percentage of methane than the other giant gas planets, which accounts for its bluish, turquoise or cyan coloured appearance. The upper atmosphere of Uranus is some 28 kilometres in height where the winds can be up to about 200 metres per second, while the temperatures at the very edges of the atmosphere are about -220±2° (53K) warming slightly to -197±2° (76K). Atmospheric composition is approximately 82.5% Hydrogen, 15.2% Helium and 2.6% Methane (CH₄). Other gases and compounds exist, but are relatively rarer. These exist principally as aerosols comprising of ices like ammonia (NH₃), water (H₂O), and ammonia hydrosulfide (NH₄)SH - the main coloured gases seen in the visible atmosphere of Jupiter and Saturn. Although not as yet detected, methane may also exist in its icy state.

Images taken in 2003 and 2004 by the ground-based Hawaiian 10-metre Keck II telescope revealed something unexpected - that now shows clouds, spots and several belts. This suggests that the planet might be more dynamic than once thought, and long term studies into these atmospheric outbursts or some periodic behaviour may be revealed in the years to come. It seems likely that the possible high tilt of the rotational axis prbably has some significant effect. This is especially currently noted, as the equator in the beginning of 2005 was -15° and will be aligned to the ecliptic at 0° on the 21st-22nd February 2008.

Uranus has five main rings orbiting the planet that is much darker than the rings of Saturn and is thought to be more rocky. Each was originally discovered by an occultation of a faint star behind the rings in 1977. After the Voyager 2 fly-by, the number of rings increased to ten, then to twelve in 2003. The first seven are labelled in increasing distance from Uranus as alpha, beta, eta, gamma, delta, lambda and epsilon. The three other inner rings are labelled as the numerals; 6, 5 and 4; and are quite close together. All ring span only 10 000 kliometers or so, being some 40% in thickness compared to Uranus’ diameter, and lie some 20 000 kilometers above the surface. Individually, the widest and faintest is the epsilon ring, being some 20 to 100 kilometers across. This epsilon ring also varies in thickness as it surrounds the planet. Brightest of the rings is alpha and beta - lying 44 718 and 45 661 km. from Uranus. Together all the rings can be seen throughout the entire orbit - between edgewise to fully open - depending on the tilt of the planetary pole.

At opposition, Uranus reaches 5.8 magnitude, so that it is just visible to the naked eye on dark moonless nights. Telescopically, the disk’s diameter is a mere 4.4 arc seconds.

NEXT PAGE : The URANIAN MOONS

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Last Update : 2nd September 2008

Southern Astronomical Delights © (2008)

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