Hopefully, this subject hasn't been retread dozens of times here...What is the cause of a retrograde orbit, i.e. Venus. Or a retrograde planetary rotation.
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Hopefully, this subject hasn't been retread dozens of times here...What is the cause of a retrograde orbit, i.e. Venus. Or a retrograde planetary rotation.
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Everything I need to know I learned through Googling.
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Since when is Venus' orbit retrograde? The only objects in the Solar system that I am aware of with retrograde orbits are some commets, and some captured moons.
As to retrograde rotation, Venus, Uranus, and Pluto all share this characteristic. The prevailing theory is that these rotations started prograde, and were altered by one or more collisions.
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Venus's orbit (and Mercury's orbit) appears retrograde at times when viewed from Earth. This is because they are circling the Sun on a smaller circle than Earth. Being on a smaller circle (orbit), Venus is necessarily orbiting faster than Earth is. Thus the location in the sky changes.
Thus, when viewed from Earth, the location in the sky depends upon Venus's and Earth's position relative to the Sun. When tracking Venus in Earth's sky against the background stars, Venus will appear to move one direction, then at some point slow down and go the opposite direction, then change direction again. That is the retrograde motion of an orbit.
As for retrograde planetary rotation, the Nine Planets Website describes Venus's rotation as retrograde.
http://www.nineplanets.org/venus.htmlVenus' rotation is somewhat unusual in that it is both very slow (243 Earth days per Venus day, slightly longer than Venus' year) and retrograde.
They don't say anything else, other than define retrograde:
When looking at Earth from above the north pole, it rotates CCW. When looking at Venus from above the pole that matches Earth's, it rotates CW.rotation or orbital motion in a clockwise direction when viewed from above the north pole of the primary (i.e. in the opposite sense to most satellites); the opposite of direct. The north pole is the one on the same side of the ecliptic as the Earth's north pole.
However, there is further commentary at Uranus:
http://www.nineplanets.org/uranus.htmlActually, there's an ongoing battle over which of Uranus' poles is its north pole! Either its axial inclination is a bit over 90 degrees and its rotation is direct, or it's a bit less than 90 degrees and the rotation is retrograde. The problem is that you need to draw a dividing line *somewhere*, because in a case like Venus there is little dispute that the rotation is indeed retrograde (not a direct rotation with an inclination of nearly 180).
Either Venus has retrograde rotation or it is upside down, take your pick.
This NASA site also confirms retrograde rotation of Venus:
http://solarsystem.nasa.gov/planets/...m?Object=Venus
andVenus sluggishly rotates on its axis once every 243 Earth days, while it orbits the Sun every 225 days - its day is longer than its year! Besides that, Venus rotates retrograde, or "backwards," spinning in the opposite direction of its orbit around the Sun. From its surface, the Sun would seem to rise in the west and set in the east.
Orbital Inclination to Ecliptic 3.39 degrees
Equatorial Inclination to Orbit 177.3 degrees
By Comparison: 7.56 x Earth
Order of Kilopi
As viewed from Earth, other planets appear to move retrograde periodically.
Mars is the classic example, and I really like this web page that explains it:
http://www.lasalle.edu/~smithsc/Astr...retrograd.html
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Venus' rotation like Uranus and possibly other distant planet(oids) certainly must influence their orbital distance from the Sun in a significant way over a long period of time. Various computer models can generate some interesting results. Early collisions in the first million years of our Solar System probably remain the most probable explanation. There is nothing like real data to change this best guess in a hurry though, so good hunting!
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The general motion for the soalr system is prograde, which defines the north pole as the point "from above" where the planets appear to revolve counter-clockwise.
Presumably, from the north pole of the galaxy the revolution is counterclockwise.
is the north pole of the soalr system and the north pole of the galaxy co-incident?
I ask this because Astronomy magazine recently published a diagram showing that from the north polar perspective of the solar system the galaxy revolves clockwise.
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No, the galactic equator is tilted at about sixty degrees to the ecliptic. Let's see--this page says it's 86.5 degrees (63 + 23.5) but I'm pretty sure they're wrong.Originally Posted by ArgoNavis
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The galactic north pole, by convention, is the pole that lies in Earth's north celestial hemisphere. The galaxy revolves clockwise around that pole.
The declination of the north galactic pole is 27 degrees, which means the galactic equator is tilted 63 degrees to the Earth's equator.
But since the galactic north pole's right ascension is around 192 degrees, it's close to one of the nodes of the ecliptic on the equator, so it makes no sense to added the obliquity of the ecliptic to the obliquity of the galactic equator, as hhEb09'1's linked website does.
The true ecliptic latitude of the galactic north pole turns out to be 30 degrees, giving a tilt of 60 degrees relative to the ecliptic, just as hhEb09'1 says.
Grant Hutchison
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And that's from the Swinburne Astronomy Online encyclopedia.
Order of Kilopi
Oh dear. And to think I once considered taking one of their courses.
Grant Hutchison
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Sounds like you should be giving the course
PS: It can be a handy resource. I assume they got the NGP RA and Dec right, here
Order of Kilopi
I actually exchanged some e-mail with Larry Niven about the galactic north pole, once.
He writes that the Ringworld is 200 light years to the galactic north of Known Space, but also that the Puppeteer fleet were on their way to the Magellanic Clouds when they found it (south of the galactic plane).
I figured he therefore must have meant rotational north, rather than the current convention, but in four novels it's never clarified which "north" is intended.
(There's even a nice G star, HD 512, in exactly the right place in the real world, 200 light years galactic south from here.)
But he responded to my enquiry by saying that the Puppeteers were taking the "scenic route", and tagging on a winking smilie.
Grant Hutchison
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Actually the Puppeteers are taking the long way around in case anyone was following them, to throw them off the trail.I actually exchanged some e-mail with Larry Niven about the galactic north pole, once.
He writes that the Ringworld is 200 light years to the galactic north of Known Space, but also that the Puppeteer fleet were on their way to the Magellanic Clouds when they found it (south of the galactic plane).
I figured he therefore must have meant rotational north, rather than the current convention, but in four novels it's never clarified which "north" is intended.
(There's even a nice G star, HD 512, in exactly the right place in the real world, 200 light years galactic south from here.)
But he responded to my enquiry by saying that the Puppeteers were taking the "scenic route", and tagging on a winking smilie.
Grant Hutchison
Established Member
Convention in academia insists North [C,E,G] is toward Polaris, 6543, Coma Berenices respectively. Therefore counter-clock-wise [C and E] is a left spin. Galaxy (using Sabik thru Arcturus as a NGP visible marker post) unfortunately spins right (clockwise) from it's north pole. Cygnus is flying ahead of us on that merry-go-round, the dog star Sirius is on our tail hounding us. So the debate will be millenial. (A Northern Yankees Biased Opinion.)
Last edited by Fr. Wayne; 2006-Mar-22 at 02:28 AM.
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Mars retrograde in the Tychonian geocentric schema! (reproduced with permission)
http://tinyurl.com/omfno
http://tinyurl.com/ms9po
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