Thread: Planets in the solar system!

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/top...rrestrial.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!

Originally Posted by kingwinner

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!

In other words you can see the passage of time in the collection of craters and so there hasn't been any changes big enough to erase the craters that were formed long time ago.

Originally Posted by kingwinner

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

Is the above statement true?
http://www.planetary.org/explore/top...rrestrial.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.............

The website is correct the a venus day is just over half a venus year. If anyone claimed that a venus day was longer then they used incorrect terminology or they are mistakenly thinking that a venus rotation is the same as a venus day.

By the way the situation is reversed in the case of mercury because it rotates in the positive direction. It rotates once in exactly two thirds the time it takes to go around the sun, but a mercury day actually exactly twice as long as a mercury year. The exact relationship means that mercury's rotation is tidally linked to its orbit.

Originally Posted by kingwinner

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?

Correct. Although another way to think of Venus is that it has a tilt which is nearly 180 degrees and the only planet which is close to this is the planet uranus which has a tilt near 90 degrees.

Originally Posted by kingwinner

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......

In other words all the planets except uranus has their axis of rotation which is nearly perpendicular to the plane of their orbit. None of them are exactly perpendicular and yes earth is 23.5 degrees off perpendicular.

Originally Posted by kingwinner

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!

Apparently the tilt of uranus is slightly more than 90 degrees which is why its rotation is given as negative in your list.

Pluto is stranger than what you imply. It is extremely close in both size and distance to its moon charon and is tidally locked to its moon just as all moons in the solar system (including charon and our own moon) are tidally locked to the planet they orbit. This means that pluto rotates exactly in the same time as its moon charon orbits pluto and that pluto always shows the same face to its moon. If you stand on charon, pluto is always in the same place in the sky and looks exactly the same (as if not rotating). Likewise if you stand on pluto, charon is always in the same place in the sky and never changes (as if it did not rotate). So the axis of rotation of both pluto and charon are perpendicular to their mutual orbit. The above sign must mean that this is in a negative direction compared to other planet like mercury and the earth.

If you stand on our moon then the earth is always in the same place in the sky but because the earth is not tidally locked to the moon then you would see it rotating in the sky of the moon. On the earth the moon is not always in the same place in the sky but we always see the same face of the moon.

Originally Posted by kingwinner

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!

Since Mercury has no molten inner core, and no plate tectonics, it is not changing out it's surface. Therefore, what you see on the surface is what has happened since it solidified. And there is no other evidence that it has been formed any later than the other planets.

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

Is the above statement true?
http://www.planetary.org/explore/top...rrestrial.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 it takes 243 "earth days" for Venus to make one complete rotation, and it takes 225 "earth days" for venus to rotate the sun, then it's day is indeed longer than it's year.

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?

That is correct. I have forgotten the accepted wisdom as to why right now.

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......

Earth's axial tilt changes with time, from about 11% to 27%, approximately. Uranus apparantly was clobbered by something that caused it's axis to tilt further than others.

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!

I'm not certain about this last one. Perhaps someone else can cover that one for us, ok? :wink:

Originally Posted by John L

If it takes 243 "earth days" for Venus to make one complete rotation, and it takes 225 "earth days" for venus to rotate the sun, then it's day is indeed longer than it's year.

No, incorrect. One rotation is the same as a sidereal day which is how long it takes the stars to pass over the horizon, rise again and return to the same position in the sky. The sidereal day is very close to the solar day on earth because the earth day is so much shorter than the earth year. But the revolution of the the planet around the sun makes the number of sidereal days per year differ from the number of solar days by one day depending on which way the planet is rotating. There is one less solar day per year if the planet rotates like mercury and the earth but there is one more solar day per year if the planet rotates the other direction like venus.

For example in the case of Mercury since it rotates once every two thirds of a Mercury year then in two Mercury years it will have rotated three times. So that is one and a half sidereal days per Mercury year. Subtracting one day gives only half a solar day per Mercury year.

In the case of venus you have 225/243 rotations (sidereal days) per venus year so adding one we get 225/243+1 = 1.9259 solar days per venus year.

Alright, I'll take your word for it.

Now, what about my thread on Small Comets? I'm still hoping that someone can give me some more info on what is happening there lately.

Got any ideas?

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...+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 in the solar system...

2) 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..........

And yet if uranus had been tilted by only 89 degrees rather than 97.86 degrees we would have said that uranus had normal rather than retrograde rotation. The significance with regard to the relationship between sidreal and solar days is a bit obscured on uranus where the sun spends a small part of the year moving in spiral around the north pole and a small part of the year moving in a spiral around the south pole. One more turn or one less in these spirals is a pretty small technical matter don't you think?

Originally Posted by kingwinner

2) 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..........

Yes we ususually assume that the word day refers to a solar day. You demonstrate that John L need not feel so bad for getting it wrong, since your teacher made the same mistake. Even with a masters in physics I might have made the same mistake if I had not been studying astronomy with such great detail in the past three years to make a realistic simulation of the planets, stars and galaxies. After all, the length of time which we call a venus day does not have much significance to us living on the earth and with the immense greenhouse effect on venus it would not have so very great a significance even on venus. It really is only a minor curiosity.