IV.

Motion of Uranus

Uranus’s orbit varies from 2,740 million km (1,700 million mi ) to 3,000 million km (1,860 million mi) in distance from the Sun, with an average distance of 2,860 million km (1,780 million mi), or 19.10 astronomical units (AU). An AU is equal to the average distance between Earth and the Sun, or about 150 million km (93 million mi). The orbit of Uranus traces out a flat region of space called the planet’s orbital plane. The orbital plane of Uranus lies close to Earth’s orbital plane. As a result, Uranus always crosses the same region of Earth’s sky. Uranus takes 84 years to complete a single revolution around the Sun, so a year on Uranus is 84 times longer than a year on Earth.

Uranus spins in place around its axis (an imaginary line that runs down the middle of the planet) once every 17.25 hours (0.718 of an Earth day), just as Earth spins once every 24 hours. The ends of the axis mark the north and south poles of Uranus, just as Earth’s axis marks the North Pole and the South Pole on Earth. Uranus rotates about an axis (the way a plastic globe spins on a rod) that tilts 98° into its orbital plane (the plane created by Uranus’s orbit around the Sun). Another method is sometimes used to describe its rotation and its axis. If the North Pole is considered the pole that projects above the plane of its orbit, Uranus can be described as rotating in a retrograde (clockwise) direction in -0.718 Earth days tilted at an angle of 82.2° to the plane of its orbit.

Scientists do not know why Uranus’s axis of rotation is so strongly tilted. One theory is that the planet was struck by another large body early in the history of the solar system, tipping its axis from a more upright position. This cataclysmic event must have happened before its moons and rings formed since these objects orbit in the plane of the planet’s equator and in the same direction as the planet turns. Another theory suggests that gravitational interactions with the planet Saturn may have shifted Uranus’s axis. The giant planets may have formed nearer to the Sun and moved outward to their current orbits, affecting the orbits of other bodies in the solar system.

Because of this tilt, one pole of Uranus points almost directly toward the Sun during half of Uranus’s 84-year orbit, and the other pole points toward the Sun during the second half. This pattern creates 42-year-long seasons of lightness and darkness, alternately, on each end of Uranus. Despite these long seasons, the difference in temperature between the two poles is not great (the planet’s average temperature in its upper atmosphere is about -212°C/-350°F). This uniform temperature indicates that heat is conducted efficiently, or travels easily, throughout the planet.

As Uranus spins about its axis, material near the planet’s equator must travel farther to make one rotation than material near the poles must travel. This equatorial material must then move faster than material at the poles. All material has inertia (the tendency of a moving mass to continue moving in a straight line), and this property makes the fast-moving material near the equator want to fly off from the planet in a straight line. The rest of the planet’s mass gravitationally attracts the material and keeps it glued to the planet, but the material’s inertia makes the planet bulge out at the equator. The bulge around the equator of Uranus is about 2 percent of the radius, or about 500 km (about 300 mi).