An Oddly Tilted Planet

The rotation period of Uranus’s atmosphere is about 16 hours.

Like Jupiter and Saturn, Uranus rotates differentially, so this period

depends on the latitude. This can be measured by tracking

the motions of clouds. To determine the rotation period for the

underlying body of the planet, scientists looked to Uranus’s magnetic

field, which is presumably anchored in the planet’s interior,

or at least in the deeper and denser layers of its atmosphere. Data

from Voyager 2 indicate that Uranus’s internal period of rotation

is 17.24 hours.

Voyager 2 also confirmed that Uranus’s rotation axis is tilted

in a unique and bizarre way. Herschel found the first evidence of

this in 1787, when he discovered two moons orbiting Uranus in

a plane that is almost perpendicular to the plane of the planet’s

orbit around the Sun. Because the large moons of Jupiter and Saturn

were known to orbit in the same plane as their planet’s equator

and in the same direction as their planet’s rotation, it was

thought that the same must be true for the moons of Uranus.

Thus, Uranus’s equator must be almost perpendicular to the plane

of its orbit, and its rotation axis must lie very nearly in that plane

(Figure 14-3).

Careful measurement shows that Uranus’s axis of rotation is

tilted by 98°, as compared to 231⁄2° for Earth (compare Figure

14-3 with Figure 2-12). A tilt angle greater than 90° means that Uranus exhibits retrograde (backward) rotation like that of Venus,

shown in Figure 11-7b. Astronomers suspect that Uranus might

have acquired its large tilt angle billions of years ago, when another

massive body collided with Uranus while the planet was still

forming. The radical tilt of its axis means that as Uranus moves along

its 84-year orbit, its north and south poles alternately point toward

or away from the Sun. This produces highly exaggerated

seasonal changes. For example, when Voyager 2 flew by in 1986,

Uranus’s south pole was pointed toward the Sun. Most of the

planet’s southern hemisphere was bathed in continuous sunlight,

while most of the northern hemisphere was subjected to a continuous,

frigid winter night. But over the following quarter of a

Uranian year later (21 of our years), sunlight has gradually been

returning to the northern hemisphere, triggering immense storms

there. Figure 14-2 shows some of these storms, which are much

more visible at infrared wavelengths than with visible light.

Also Watch this video:

Link: Tilt of planets

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