Uranus, the seventh planet in order of distance from the Sun, was the first to be discovered in telescopic times, by William Herschel, in 1781. It is a giant world, but it and the outermost giant, Neptune, are very different from Jupiter and Saturn, both in size and in constitution. It is probably appropriate to refer to Jupiter and Saturn as gas giants and to Uranus and Neptune as ice giants.

Data for Uranus are given in Table 11.1.

MOVEMENTS

Since Uranus' synodic period is less than five days longer than our year, Uranus comes to opposition every year. Opposition dates for 2010–2020 are given in Table 11.2. The opposition magnitude does not vary a great deal; under good conditions the planet can just be seen with the naked eye. The most recent aphelion passage was that of 27 February 2007; Uranus was at its minimum distance from the Earth (21.09 a.u.) on 13 March of that year. The last perihelion passage was on 20 May 1966; Uranus was closest to the Earth (17.29 a.u.) on 9 March of that year. The next perihelion will be that of 13 August 2050.

In June 1989, Uranus reached its greatest southerly declination (–23.7°). Greatest northern declination had been reached in March 1950.

Close planetary conjunctions involving Uranus are listed in Table 11.3. It is interesting to note that between January and March 1610 Uranus was within 3° of Jupiter.