Figure 13.5 shows Uranus and Neptune to scale, along with Earth for comparison. The two giant planets are quite similar in their bulk properties. The radius of Uranus is 4.0 times that of Earth; that of Neptune, 3.9 Earth radii. Their masses (first determined from terrestrial observations of their larger moons and later refined by Voyager 2) are 14.5 Earth masses for Uranus and 17.1 Earth masses for Neptune. Uranus's average density is 1300 kg/m3, and Neptune's is 1600 kg/m3. These densities imply that large rocky cores constitute a greater fraction of the planets' masses than do the cores of either Jupiter or Saturn. The cores themselves are probably comparable in size, mass, and composition to those of the two larger giants.
Figure 13.5 Jupiter, Saturn, Uranus, Neptune, and Earth, drawn to scale. Uranus and Neptune are quite similar in their bulk properties. Each probably contains a core about 10 times more massive than Earth. Jupiter and Saturn are each substantially larger, but their rocky cores are probably comparable in mass to those of Uranus and Neptune.
Like the other jovian planets, Uranus has a short rotation period. Earth-based observations of the Doppler shifts in spectral lines first indicated that Uranus's "day" was between 10 and 20 hours long. The precise value of the planet's rotation period—accurately determined when Voyager 2 timed radio signals associated with its magnetosphere—is now known to be 17.2 hours. Again as with Jupiter and Saturn, the planet's atmosphere rotates differentially, but Uranus's atmosphere actually rotates faster at the poles (where the period is 14.2 hours) than near the equator (where the period is 16.5 hours).
Each planet in our solar system seems to have some outstanding peculiarity, and Uranus is no exception. Unlike all the other planets, whose spin axes are roughly perpendicular to the ecliptic plane, the rotation of Uranus axis lies almost within that plane—98° from the perpendicular, to be precise. (Note that because the north pole lies below the ecliptic plane, the rotation of Uranus, like that of Venus, is classified as retrograde.) Relative to the other planets, we might say that Uranus lies tipped over on its side. As a result, the "north" (spin) pole of Uranus, at some time in its orbit, points almost directly toward the Sun. As in Chapter 9, we adopt the convention that a planet's rotation is always counterclockwise as seen from above the north pole (that is, planets always rotate from west to east). 
(Sec. 9.2.)
Half a "year" later, its "south" pole faces the Sun, as illustrated in Figure 13.6. When Voyager 2 encountered the planet in 1986, the north pole happened to be pointing nearly at the Sun, so it was midsummer in the northern hemisphere.
Figure 13.6 Uranus's 98° axial tilt places its equator almost perpendicular to the ecliptic. As a result, the planet experiences the most extreme seasons known in the solar system. The equatorial regions experience two warm seasons (around the two equinoxes) and two cold seasons (at the solstices) each year; the poles are alternately plunged into darkness for 42 years at a time.
The strange orientation of Uranus's rotation axis produces some extreme seasonal effects. Starting at the height of northern summer, when the north pole points closest to the Sun, an observer near that pole would see the Sun move in gradually increasing circles in the sky, completing one circuit (counterclockwise) every 17 hours and dipping slightly lower in the sky each day. Eventually the Sun would begin to set and rise again in a daily cycle, and the nights would grow progressively longer with each passing day. Twenty-one years after the summer solstice, the autumnal equinox would occur, with day and night each 8.5 hours long.
The days would continue to shorten until one day the Sun would fail to rise at all. The ensuing period of total darkness would be equal in length to the earlier period of constant daylight, plunging the northern hemisphere into the depths of winter. Eventually, the Sun would rise again; the days would lengthen through the vernal equinox and beyond, and in time the observer would again experience a long summer of uninterrupted (though dim) sunshine.
From the point of view of an observer on the equator, by contrast, summer and winter would be almost equally cold seasons, with the Sun never rising far above the horizon. Spring and fall would be the warmest times of year, with the Sun passing almost overhead each day.
No one knows why Uranus is tilted in this way. Some scientists have speculated that a catastrophic event, such as a grazing collision between the planet and another planet-sized body, might have altered the planet's spin axis. There is no direct evidence for such an occurrence, however, and no theory to tell us how we should seek to confirm it.
Neptune's clouds show more variety and contrast than do those of Uranus, and Earth-based astronomers studying them determined a rotation rate for Neptune even before Voyager 2's flyby in 1989. The average rotation period of Neptune's atmosphere is 17.3 hours (quite similar to that of Uranus). Measurements of Neptune's radio emission by Voyager 2 showed that the magnetic field of the planet, and presumably also its interior, rotates once every 16.1 hours. Thus, Neptune is unique among the jovian worlds in that its atmosphere rotates more slowly than its interior. Neptune's rotation axis is inclined 29.6° to a line perpendicular to its orbital plane, quite similar to the 27° tilt of Saturn.
