dipper@utastro.UUCP (Debbie Byrd) (04/24/85)
The change in Earth's seasons is fun -- but you might not like the seasons on Uranus. More -- after this. April 24 Nine Months to Uranus The Voyager spacecraft is now heading for Uranus -- with its closest approach due exactly nine months from today. When it gets there, it'll explore a world that has seasons -- but seasons far different from those on Earth. Earth's seasons are caused by our world's tilt on its axis. Earth's northern and southern hemispheres take turns in tilting most directly toward the sun. The planet Uranus also has seasons that result from that world's tilt on its axis. But the change is much more dramatic -- due to the strange orientation of Uranus in space. Uranus lies nearly sideways with respect to the plane of its orbit. It takes about 84 Earth-years to complete a single orbit around the sun. For one-quarter of that time -- about 21 years on Earth -- the southern hemisphere of Uranus has endless sunlight. Meanwhile, the equator has perpetual twilight -- and the northern hemisphere, nothing but darkness. After another quarter of the Uranian year, this world has changed its orientation to the sun -- and the sun rises and sets over the equator of Uranus. Then, a single day lasts a bit less than 24 hours -- as long as Uranus takes to rotate once on its axis. A quarter year later -- again, about 21 years in Earth-time -- the northern hemisphere faces directly into the sun. The equator is in twilight again -- and the southern hemisphere is sentenced to two decades of darkness. That's the situation on Uranus now. As Voyager approaches Uranus, the north pole of this world is pointing almost directly toward the sun. Script by Deborah Byrd. (c) Copyright 1984, 1985 McDonald Observatory, University of Texas at Austin
nyssa@abnji.UUCP (nyssa of traken) (04/29/85)
>The change in Earth's seasons is fun -- but you might not like the >seasons on Uranus. More -- after this. > >April 24 Nine Months to Uranus > >The Voyager spacecraft is now heading for Uranus -- with its closest >approach due exactly nine months from today. When it gets there, it'll >explore a world that has seasons -- but seasons far different from >those on Earth. > >Earth's seasons are caused by our world's tilt on its axis. Earth's >northern and southern hemispheres take turns in tilting most directly >toward the sun. > >The planet Uranus also has seasons that result from that world's tilt >on its axis. But the change is much more dramatic -- due to the >strange orientation of Uranus in space. > >Uranus lies nearly sideways with respect to the plane of its orbit. It >takes about 84 Earth-years to complete a single orbit around the sun. >For one-quarter of that time -- about 21 years on Earth -- the southern >hemisphere of Uranus has endless sunlight. Meanwhile, the equator has >perpetual twilight -- and the northern hemisphere, nothing but >darkness. > >After another quarter of the Uranian year, this world has changed its >orientation to the sun -- and the sun rises and sets over the equator >of Uranus. Then, a single day lasts a bit less than 24 hours -- as >long as Uranus takes to rotate once on its axis. A quarter year later >-- again, about 21 years in Earth-time -- the northern hemisphere faces >directly into the sun. The equator is in twilight again -- and the >southern hemisphere is sentenced to two decades of darkness. That's >the situation on Uranus now. As Voyager approaches Uranus, the north >pole of this world is pointing almost directly toward the sun. > > >Script by Deborah Byrd. > > > > >(c) Copyright 1984, 1985 McDonald Observatory, University of Texas at Austin When I read this, it seemed a bit misleading. As Uranus moves around its orbit, one hemisphere gradually lightens and one gradually darkens. In fact, most of the time, most of the planet will get sunlight in a given day. On earth, the seasons above the Arctic circle do not change in quantum, and only at the pole is there the 6 month day/6 month night. The farther south you get, the shorter the "endless" day/night, until, eventually, the Arctic circle, where there may be a single day when the sun doesn't set. In the case of Uranus, the Arctic circle is much farther south, so this effect is more pronounced. -- James C Armstrong, Jnr. ihnp4!abnji!nyssa Chap with wings there, five rounds rapid!
eugene@ames.UUCP (Eugene Miya) (05/07/85)
> When I read this, it seemed a bit misleading. As Uranus moves around its > orbit, one hemisphere gradually lightens and one gradually darkens. In > fact, most of the time, most of the planet will get sunlight in a given day. > Most? Maybe close to 50%? Half a sphere if we 'round' the oblate structure of the planet. > > In the case of Uranus, the Arctic circle is much farther south, so > this effect is more pronounced. > -- > James C Armstrong, Jnr. ihnp4!abnji!nyssa I think the difficulty came in describing the 72 degree axis tilt of Uranus [if my memory does not fail me]. One of the last things I did before leaving JPL was begin the preliminary planning for the simulation of Uranus encounter. There are several unknowns regarding the length of the Uranian day, I think we found four different lengths of a day. Seasons are not well understood. It would help immensly if it were possible to see planetary features thru ground-based telescopes (or space-based). Oh! to have digitized pictures of StarDate transmissions rather than the text. Debbie should illustrate using computer generated animation. Will Bill Skane of Science notes on PBS do the same? --eugene miya NASA Ames Research Center {hplabs,ihnp4,dual,hao,decwrl,allegra}!ames!aurora!eugene emiya@ames-vmsb.ARPA
nyssa@abnji.UUCP (nyssa of traken) (05/09/85)
>> When I read this, it seemed a bit misleading. As Uranus moves around its >> orbit, one hemisphere gradually lightens and one gradually darkens. In >> fact, most of the time, most of the planet will get sunlight in a given day. >> >Most? Maybe close to 50%? Half a sphere if we 'round' the oblate >structure of the planet. What I meant was that when Uranus was halfway between an equinox and a solstice that in one rotation of Uranus a sizeable amount of the planet will be illuminated at one time or another (even if it is only the surfaces of the clouds). At any given instant, around 50% will be illuminated, giving allowances for any irregularities. On a perfect sphere smaller than the sun, just over 50% will always be illuminated at any given instance. I am still looking forward to the encounter. Will PBS televise it live, as they did Saturn? -- James C Armstrong, Jnr. ihnp4!abnji!nyssa The Boss gave me one of these, ten seconds, he said. Let's see if it works...