Venus, rotational period and orbital period...

[R1Geezer]

Venus has a rotational period of 243 Earth days and an orbital period of 224.7 Earth days, ie it's day is longer than it's year. Can anyone explain how the day is measured, is this simply a question of measuring from sunrise to sunrise from an arbitrary postion? Take the moon orbiting the earth for example, the same side always faces us so we say it rotates once per orbit, simple, but how does that apply to Venus? Now I know that Venus orbits in the same direction as all the other planets but rotates in the opposite direction so is this producing this "long day" effect? ie if I was on the surface of Venus would I have to wait 243 Earth days between sunrises? There is potential for BGB's here!

[Clanad]

The "day" is measured (time wise) in two different ways, or more accurately, from two differing perspectives. One method results in your example of nearly 243 Earth days to equal one Venus day. This "Sidereal" time frame is the result of the stars appearing to move around a projection of the planet's axis into space. However, as the planet moves around the Sun it changes the apparent position of the Sun among the stars and causes an appearance of it not moving around the sky in quite the same period of time that the stars do. This "Synodic" period is actually shorter, in Earth reference, than that of Mercury, being -116.75 days. (The negative factor is introduced by the "retrograde" rotation as you have noted)...
Additionally, the relatively slowness is also caused by the planet being nearly tidally-locked with the Sun, similar to our own Moon being tidally-locked with the Earth. Finally, additional relative slowness is thought to be caused by the thick atmosphere present of Venus...

We need to be clear about length of days (the time for the Sun (apparently) to go around the planet) and rotational periods (the time for the stars to go around the planet)... they are often intermingled. Most planets have very similar times for each. However, as noted, Venus and Mercury (and our own Moon) have very large differences between the defined times. (Source: Voyages Through the Universe, Andrew Fraknoi).

[R1Geezer]

Thanks Clanad, yes I see, so just clarify, If I was on the surface of Venus, how long would I percieve it to be between sunsets?

[Clanad]

116.9 days, measured noon to noon on Earth... formula provided on request...

[mibn2cweus]

http://en.wikibooks.org/wiki/Wikijunior:Solar_System/Venus#How_long_is_a_day_on_Venus.3F

[mibn2cweus]

http://en.wikibooks.org/wiki/Wikijunior:Solar_System/Venus#How_long_is_a_day_on_Venus.3F

AB broke the link. You'll have to copy/paste the whole thing in your address bar to get to this:

"One day on Venus, from noon to noon . . . is about 117 earth days."

[R1Geezer]

no need mibn, as you can see above Clanad hs already furnished the answer. Thanks anyway