1) "The large number of craters on Mercury suggests that Mercury has changed little since the formation of the solar system."

Why? I can't see how these 2 things are related!



2) "1 Venus day is longer than 1 Venus year."

Is the above statement true?
http://www.planetary.org/explore/topics/our_solar_system/compare_the_planets/terrestrial.html
From this web site, I found that 1 Venus solar day is 117 Earth days (although one rotation takes 243 days), while its orbit period is 225 days.............




3) "The direction of rotation of Venus is opposite to that of the other planets."
Then, is it true that 'the direction of rotation of Venus is opposite to that of ALL the other planets IN THE SOLAR SYSTEM?', or in other words, Venus' reverse rotation is unique?




4) "Most planets, including the earth, rotate with their axes perpendicular to their orbital planes as they revolve around the sun. The axis of Uranus, however, is almost horizontal to the plane of tis orbit."
How come the earth rotate with its axis perpendicular to its orbital plane around the sun? I was long told that the earth's axis is tilted at 23.5 degrees now......confused......




5) "The following gives the rate of rotation of each planet:
Mercury: 59 days
Venus:-243 days
Earth:23 hr 56 min
...............
Uranus: -17 hr 14 min
Neptune: 16 hr 07 min
Pluto: -6.4 days"

The rates of rotation for each planet is positive except for Venus, Uranus, and Pluto. Venus I can understand, because from the quote in Question 3, I know that Venus has a rotation with direction opposite to the other planets and therefore negative.......but what I can't understand is why Uranus and Pluto also have negative signs?

Can somebody explain? Thanks a million! :)

1) Early on in the Solar System's history, the planets were pummelled by still-accreting debris. The fact that Mercury still shows the scars of these impacts suggests that not much has happened geologically in the intervening 4.5 billion years or so.

2) This depends on whether you use the sidereal day or the solar day.

3) It's not quite unique because, as you mentioned, Uranus and Pluto also display retrograde rotation.

4) They do. 23.5 degrees is fairly close to perpendicular.

5) Uranus might have suffered a major collision early on, which caused its tilted axis. KBOs probably show a wide range of axis orientations, so it should be no surprise that Pluto follows the general rule of the planets. I have no idea why Venus has a retrograde rotation. Maybe more impacts?

2) "1 Venus day is longer than 1 Venus year."

Is the above statement true?
http://www.planetary.org/explore/to...errestrial.html
From this web site, I found that 1 Venus solar day is 117 Earth days (although one rotation takes 243 days), while its orbit period is 225 days.............
If I may, Venus takes 243 earth days to complete one rotation and only 225 earth days to orbit the sun. So yes, the venus 'day' is longer than the venus year by 17 earth days. dou you mind if I ask what grade level you are studing at Kingwinner?

3) 5) Prehaps I can answer Q3 and Q5 by myself:)
"Venus, Uranus, and Pluto have a retrograde rotation, or a rotation that is in the opposite direction from the other planets."
http://64.233.161.104/search?q=cache:AokZfCiI9TgJ:www.solarviews.com/eng/solarsys.htm+venus+rotates+in+opposite+direction+t o+all+other+planets&hl=en

So Venus, Uranus, and Pluto all rotates in opposite direction to the other 5 planets...and thus Venus is not the only one...

To snake river rufus:
But I thought 1 day is defined by 1 solar day, i.e. from noon to noon, so 1 solar day on Venus is 117 solar days on Earth, and 1 revolution is 225 days......the strange thing is I was told by my teacher that 1 Venus day is longer than 1 Venus year..........

Grade 12, High School

You can measure a day with respect to stars (sidereal day) or the Sun (solar day). Since the planet moves around the Sun as it rotates, the different definitions will have different periods.

1 day on uranus = 30,588.697 earth days!!!!!

Dwayne D.L.Rabon

30,687 days.

Noon to noon counts as 24hrs. But you measure it from 12:00am to 12:00am... Thus you have one complete day. I takes 243 days for venus to rotate a complete rotation. and it takes Venus 225 days for it to complete one revolution. Therefore, 1 venus day is longer than 1 venus year. Its pretty straight forward. I dont know what you could possibly be confused on.

I takes 243 days for venus to rotate a complete rotation. and it takes Venus 225 days for it to complete one revolution. Therefore, 1 venus day is longer than 1 venus year.
The period of rotation is not the same as a day. (See solar day vs sidereal day. Like Laika said. Twice.)
Earth takes 24 hours and 4 minutes to complete one rotation,but an Earth day is only 24 hours.


1 Venus day is less a Venus year
1 Venus sidereal day is longer than 1 Venus year.

Earth takes 23 hours and 56 minutes to complete one rotation. The movement of the Earth in its orbit means it takes longer to point so the sun is at the same point in the sky (the 24 hour day we measure). But the 360 degree rotation is four minutes shorter.

How this would work with Venus's retrograde rotation is, I'm afraid, totally beyond me! The rotation being a substantial fraction of the length of the year means the difference in sidereal and solar days is, as we have seen, quite significant.

Thanks for the correction, Silas. I should have taken the time to visualise it properly!

Imagine three scenarios -
A non-rotating planet... One solar day = 1 year
A planet rotating once per solar year... The planet is tidally locked, and has no solar day at all.
A planet rotating backward once per solar year... If you're standing on the equator, how many time is the Sun overhead during a year? (This is almost the situation with Venus, and explains why the solar year is the length it is).

...A planet rotating backward once per solar year... If you're standing on the equator, how many time is the Sun overhead during a year? ...twice

I'm trying to come up with a formula to calculate the length of a solar day given any of this information:
- The length of a sidereal day
- The length of a year
- The orbital radius
- The planetary radius
- The speed of the planet as it revolves around the sun
- The rate of rotation of the planet

Unfortunately, every formula I try works fine for Earth but gives 19 days as the length of a solar day on Venus. Does anyone else know a formula that works?

I also come up with a solar day length of 505.8 days fairly often... Still no closer, though.

The formula I would use is

day_{solar}= \frac{1}{\frac{1}{day_{sidereal}}-\frac{1}{year_{sidereal}}

The trick here is that since Venus' rotation is retrograde, you have to enter the period of the sidreal day as a negative number.

The formula I would use is

day_{solar}= \frac{1}{\frac{1}{day_{sidereal}}-\frac{1}{year_{sidereal}}

The trick here is that since Venus' rotation is retrograde, you have to enter the period of the sidreal day as a negative number.

Awsome! Thankyouthankyouthankyou, I've spent the last two days trying to figure something out using angles, times, distances, ... and gotten nowhere with Venus (even trying all of the signing permutations), but this works perfectly! :D

Do you know what that equation is called? I was trying to look it up to learn more about its uses, but I don't know how to search for it.

It's the equation for synodic period.

It generally is used to find the period between conjunctions of planets. In that case instead of using the sidereal day and year for one planet, you use the sidereal year for each planet.

First, which are you really trying to figure out rotation or revolution.

Rotation is the time duration the planet takes to turn 360 degrees on its axis.

Revolution is the time it takes the planet to travel a complete 360 degrees circle around the sun.

DwayneD.L.Rabon

Oh, I already figured out what I needed (I'm using it in a program I'm working on), I was just interested in looking up the equation to learn more about it, since at this point it seems to give me the correct answer magically :)