Neptune, for instance. If you look at the excellent video below, you'll see that Neptune and Uranus have a similar spin period. The similarity of these spin periods implies that - as with Jupiter and Saturn - the two planets were born together. The probability of similar spin periods becomes much lower if you factor in one or more impacts large enough to tip Uranus sideways. Uranus' moons are a problem too. A sudden tilting resulting from impact would likely disrupt and destabilise its satellite system, yet the ice planet's moons are similar in relative size and spacing to the Galilean moons.
And those moons are icy, too. Impacts large enough to tip the planet should have generated enough heat to vaporise any ice on these moons, making them mostly rocky, yet all of the planet's major moons have at least equal parts rock and ice.
According to astronomers Zeeve Rogoszinski and Douglas Hamilton of the University of Maryland, these problems are solved if Uranus had a ring system large enough to cause it to wobble on its axis like a spinning top - a phenomenon called precession - and if that precession aligned with the planet's orbital precession , where the ellipse slowly shifts position around the Sun. Spin precession left and orbital precession right. This alignment of motion is called resonance, and it has occurred a few times in the Solar System - usually between the orbits of two or more bodies.
For example, Pluto and Neptune have a orbital resonance, which means that for every two of Pluto's orbits around the Sun, Neptune orbits three times. The resonance between a planet's precession and its orbital precession is known as a secular spin-orbital resonance, and it can generate a large axial tilt. The spring equinox in March marks the beginning of the transition from 24 hours of darkness to 24 hours of daylight at the north pole.
The fall equinox in September marks the shift into 24 hours of darkness at the north pole. During the equinoxes every location on Earth excluding the extreme poles experiences a hour day length. Other planets also experience these changes in day and night length because they too are tilted on their axes. Each planet's axis is tilted at a different angle. Jupiter is tilted only 3 degrees, so its change in day and night length as it moves around the Sun is less extreme than that of Earth.
Neptune's axis is tilted 30 degrees, so day and night changes would be more extreme on Neptune than on Earth. Uranus presents an interesting case because its axial tilt is even more extreme — 98 degrees! This means that the north pole of Uranus is pointed at the Sun during the north polar summer; the south pole is in total darkness. During the north polar winter, some 42 Earth years later, the south polar axis points at the Sun and the north polar region is in total darkness.
During the spring and fall, when its axis is perpendicular to the incoming rays of the Sun, Uranus experiences a hour day and night cycle as it spins on its axis. Good news for overachievers: Earth's days are getting longer! Researchers examining ancient corals noted that annual growth patterns suggested there were more days in a year in Earth's distant past.
Fossil corals, million years old, from the Devonian Period recorded daily cycles. About million years ago in the Pennsylvanian Period, there appear to have been daily cycles each year. Assuming that Earth's revolution around our Sun has not changed dramatically, this means that the number of hours per day has been increasing and that Earth's rotation has been slowing.
Today's day length is 24 hours. Earth's rotation appears to be slowing approximately 2 seconds every , years. Not Uranus. The planet's poles are tilted 98 degrees and it spins clockwise. Astronomers have long wondered why the seventh planet's strange orientation doesn't match up with its planetary neighbors.
One predominant theory believes that a giant object twice the size of Earth collided with the planet , knocking it off its vertical axis. The problem with this theory is that an impact of that magnitude would have vaporized the ice on Uranus' moons, leaving an orbit filled with rocky husks, for which there is no evidence. Additionally, Uranus and Neptune have similar spin periods, which implies that they formed at roughly the same time.
But astronomers Zeeve Rogoszinski and Douglas Hamilton of the University of Maryland have come up with another hypothesis. Basically, when a planet's orbital precession, or shifts in its orbit around the sun, matches up with its rotational precession, or how much the planet wobbles when rotating, it begins to tilt.
They propose that Uranus once had a larger, more robust ring system, which would have caused it to wobble even more. In fact, the planet still has rings today, but they are incredibly thin compared to Saturn's luminous rings.
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