[Spacetectonics (OP)]

Watching the marvelous transit videos, one of the video shows Venus crossing sun at the top half and another one at the bottom half? I understand tilt of the Venus but don’t get this one .does solar system planets changes their orbits or they are constant? Where the different come from?
2012 transit: http://www.wired.com/wiredscience/2012/06/video-photos-transit-of-venus/
                  http://www.wired.com/wiredscience/2012/06/ways-to-watch-transit-venus/

1882 transit: http://www.wired.com/wiredscience/2011/12/1882-transit-of-venus/

The other question I have: why Venus is orbiting on a tilted orbit compare to the other planets? Does it breach the rule of gravity symmetry (space-time curve)? should not they all orbit on  the same plane?

Thanks,

[evan_au]

Depending where on Earth you view the transit of Venus, you will see the planet cross a different part of the face of the Sun.

Traditional astronomical telescopes flip the image upside-down - it depends on whether the camera adapter flips the image again, or the person who took the video may have flipped it.

Also, I think that depending on whether you are viewing it from the Northern hemisphere or Southern hemisphere, or whether it was near dawn or sunset from your viewing point will affect which way you consider to be "up".

If the image has sufficient resolution, you should be able to compare which way is "up" from the position of the sunspots on the Sun during the 2012 transit (this was a time of moderately high sunspot activity).

[Spacetectonics (OP)]

Absolutely right!
http://en.wikipedia.org/wiki/Transit_of_Venus

Any idea about the second question anyone?!!

why Venus is orbiting on a tilted orbit compare to the other planets? Does it breach the rule of gravity symmetry (space-time curve)? should not they all orbit on  the same plane?

[evan_au]

There is no law of physics that forces all planets to orbit in the same plane - comets have orbits that can lie in any orientation, and ex-planet Pluto orbits somewhat away from the plane of the solar system.
Within our own galaxy, different stars (and their planets) are thought to spin around different axes (our Sun spins at an angle of 60 degrees to the plane of the galaxy).

A common theory is that a planetary system collapses from an initial cloud of dust and gas.
Initially, different parts of the cloud have different speeds and directions, but friction and collisions within this cloud reduce the different velocities over time, until what is left is the dominant rotation of the cloud as a whole*.

From this, the star and planets form, with the star spinning in one direction, and the planets orbiting in the same direction around the same general axis.

Once the dust has collapsed into planets, or been blown away by the newly ignited star, the friction effects are greatly reduced. A group of planets behave chaotically, and their orbital planes and eccentricity can vary over time, as they influence each other by gravitational attraction (see Milankovitch cycles). It is possibly that a close encounter with another star could also disturb planetary orbits. 

So the orbital plane of Venus may just reflect the way things are at the present time.

*Presumably, in the very unlikely event that all of the velocities in the initial dust cloud exactly cancelled out, there would be no net rotation, and the whole cloud would collapse to form a large star with no planets(?)

[CliffordK]

The orbital plane of all the planets varies somewhat.

The two inner planets, Mercury and Venus are closest to the sun's equatorial plane, but in opposite directions, at least now.

Earth may, in fact be the outlier. 

In each of Venus's orbits, it would cross Earth's orbital plane twice, on opposite sides of the sun. 

However, since Venus orbits quicker than Earth (has a shorter year), then passing each other at the exact same time Venus crosses Earth's orbital plane is rare.

[syhprum]

The 2004 one was much better as it came later in the morning here in the UK and I was able to get some good pictures, I did not try in 2012 as I had moved house and could not get a good view and the available period was very short.
I think I have been very lucky to see two transits and one solar eclipse albeit rather late in life. 

[Spacetectonics (OP)]

There is no law of physics that forces all planets to orbit in the same plane - comets have orbits that can lie in any orientation, and ex-planet Pluto orbits somewhat away from the plane of the solar system.

Thanks Evan_au

I may asked the wrong question !
it was better to ask "why most of the planets orbiting sun at the same plane?" natural selection or ..?!!