General Features of

Uranus is the 3rd of the Gas Giant planets, and the first planet discovered in "modern" times (1781). It is barely visible from the Earth without a telescope, which explains why it was not known as a planet to the ancients, and why it had been observed various times after the telescope had been invented without the observers realizing that it was a planet and not a star. Documented sightings go back to at least 1690 when Flamsteed catalogued it as a star.

The density is about 1.2 g/cc, implying that it is mostly hydrogen and helium. The mass is about 15 times that of the Earth, which makes it the 4th most massive planet. But its radius of about 4 times that of the Earth makes it the 3rd largest planet, since Neptune has a smaller radius but larger mass (because Neptune's density is higher). The rotation axis is unusual in that it lies only 8 degrees out of the plane of the orbit. Thus, at times the rings and the orbits of the moons appear like a "bulls-eye" when viewed from the Earth. The average rotational period is a little over 17 hours, and its orbital period is 84 years, at a mean separation of 19.2 A. U. from the Sun.

The rings are less extensive than those of Saturn, and may be rock rather than ice. There are 5 large moons and 10 small ones.