The Voyager 2 spacecraft has determined that Uranus has a 16.8-hour "day," but because it rotates on its side, these days are not like Earth's.
And in a surprise, scientists have determined that the coldest part of the planet is at its equator and that the pole on what is now the dark side of Uranus is warmer than the side facing the sun. They said they do not have an explanation.
"We haven't any idea why the part of the pole in the dark is warmer than the pole in the sun," said Barney J. Conrath of the Goddard Space Flight Center. "It may be that the planet's dynamics move heat from the interior to whichever pole is in the dark."
Analyzing the Voyager's measurements of the planet's unexpectedly strong magnetic field, scientists estimated that the day on Uranus is no shorter than 16.5 hours and no longer than 17.06 hours. This was the first time since Uranus was discovered 205 years ago that scientists had any estimate of the planet's period of rotation.
Uranus is covered with such a thick methane haze that astronomers are unable to follow clouds or surface features as the planet rotates. But by taking numerous measurements of the magnetic field, scientists have been able to determine how long it takes for the radioactive core of the planet to revolve.
Because Uranus takes 84 Earth years to travel around the sun, one pole is in sunlight 42 years and then in darkness 42 years. Only twice during this 84-year period would there be "days" as on Earth, with the sun rising and setting once during each rotation of the planet.
With one of the poles now directly facing the sun, the sun would appear to be circling about the sky on the sunny side, never setting or rising.
"Because of the way the planet is tipped, the sun would go round and around the horizon without ever rising or setting," said Andrew P. Ingersoll of the California Institute of Technology. "The only way to tell day from night on Uranus is to watch the stars rise and set."
Voyager is now more than 2 million miles from Uranus and moving deeper into the outermost reaches of the solar system. Its next and last encounter with a planet is scheduled to take place Aug. 25, 1989, when it flies by Neptune.
Meanwhile, the silver-and-black spacecraft continues to unravel the mysteries that Uranus had kept hidden for so long.
By following as many as five clouds circling the planet, scientists have measured high-altitude winds up to 220 mph. This is more than twice the speed of the Earth's upper-altitude jet stream, the equivalent of the high-altitude winds on Uranus.
The Voyager instruments today found an aurora on the dark side of the planet roughly near the magnetic pole of the planet's dark half.
Looking back through the planet's 10 known rings, now backlit by the sun, Voyager found that the particles in most of the rings were as large as one meter across, making Uranus' ring particles much larger than those in Saturn's rings