Uranus, seventh planet from the Sun. It has the third-largest radius and fourth-largest mass in the Solar System. Uranus is similar in composition to Neptune. Along with Neptune, sometimes called an "ice giant" (prior post). It has the coldest planetary atmosphere in the Solar System, with a minimum temperature −224 °C. It has a complex, layered cloud structure, with water thought to make up the lowest clouds, and methane the uppermost layer of clouds. At 20 a.u. it has an 84 year orbit. Like other gas giants it spins rapidly, making one revolution in 17 hours.
Uranus has an axial tilt of 98°, so its axis of rotation is approximately parallel with the plane of the Solar System. This gives it bizarre seasonal changes. Combine this extreme tilt with it's 84 year orbit and you get a 21 year "night" at the pole during winter, compared to a 17 hour day at the equator during equinox.
Like Neptune, there's not much to see visually. Unlike Neptune, it can be seen with the unaided eye in dark locations. At high power it can be seen as a small disk rather than a dot. A slightly more green than Neptune. A monster telescope or camera might detect several moons (see below), and possibly faint cloud formations.
The Hubble or flyby space craft can detect faint rings.
Saturday, January 3, 2015
Neptune--eighth and farthest planet from the Sun (after Pluto’s demotion). Along with Jupiter, Saturn and Uranus, a gas giant. Smaller, but slightly more massive than its near-twin Uranus. Uranus and Neptune are sometimes referred to as "ice giants" as they contain a higher proportion of "ices" such as water, ammonia, and methane. Traces of methane in the outer atmosphere account for the planet's blue appearance. At 30 au its orbit around the sun takes 165 years, so it’s not moving much in the sky from year to year.
Discovered in 1846, Neptune was the first planet found by mathematical prediction rather than by empirical observation. Unexpected changes in the orbit of Uranus suggested gravitational perturbation by another planet. There’s been controversy over credit for the discovery:
Interestingly, Galileo made the first recorded observation of Neptune, but apparently did not recognize it as a planet, though some have suggested that he was aware that it had moved relative to fixed stars.
For visual astronomers, there’s not much to see except for the fact that at high power it can be seen as a small bluish disk rather than a dot. It generally cannot be seen with the unaided eye. Due to its distance, its apparent size is the smallest of the planets. With dark skies or a camera you may be able to pick up a few moons.
Here’s an image of Neptune, the white spot to the right is its large moon triton:
With the Hubble or flyby space craft, surface storms and very faint rings have been detected.