38 replies to this topic

[the L]

Thanks MID. I will check the site. I hope everthing goes well with hurricane.
btw in my country we have joke about hurricanes in America.

"If you survive Irene we will send you Jadranka" (our prime minister)

Thanks on answers. But somehow I know I will be back with more questions.
Cheers.

[MID]

the L, on 16 September 2011 - 03:50 AM, said:

Thanks MID. I will check the site. I hope everthing goes well with hurricane.
btw in my country we have joke about hurricanes in America.

"If you survive Irene we will send you Jadranka" (our prime minister)

Thanks on answers. But somehow I know I will be back with more questions.
Cheers.

My pleasure!
Thanks for the Joke!
And questions are always welcome!

[the L]

Why earths rotate itself and mercury dont?

[Mentalcase]

the N, on 13 May 2012 - 02:52 PM, said:

Why earths rotate itself and mercury dont?

It does spin The N (the L, Melo) it's rotation period is once every 58.65 Earth days.

[the L]

Mentalcase, on 13 May 2012 - 03:27 PM, said:

It does spin The N (the L, Melo) it's rotation period is once every 58.65 Earth days.

This is getting funny. I must changed my name. Evryone call me L yet display name is N.
I heard that ones Mecury side is always dark as moon...

Why moon doesnt rotate itself?

[jules99]

the N, on 13 May 2012 - 06:39 PM, said:

This is getting funny. I must changed my name. Evryone call me L yet display name is N.
I heard that ones Mecury side is always dark as moon...

Why moon doesnt rotate itself?

Hi The N

Tidal locking;

http://en.wikipedia....i/Tidal_locking

[Mentalcase]

the N, on 13 May 2012 - 06:39 PM, said:

This is getting funny. I must changed my name. Evryone call me L yet display name is N.
I heard that ones Mecury side is always dark as moon...

Why moon doesnt rotate itself?

Actually the Moon rotates as well.

Quote

"How it works: If you go out on several different nights and look at the Moon, you will always see the same features, at about the same position. It looks as if the Moon doesn't rotate! Ah, but it does.

This can be seen using a model. Grab two oranges (or apples, or baseballs, or whatever roughly spherical objects you have handy). Mark one with an "X"; this represents a feature on the Moon. Now put the other one down on a table; this is the Earth. Place the Moon model on the table about 30 centimeters (one foot) away with the X facing the Earth model. Now move the Moon model as if it were orbiting the Earth, taking care to make sure that the X faces the Earth model at all times.

Surprise! You'll see that to keep the X facing the Earth model, you have to rotate the Moon model as it goes around the Earth model. Furthermore, you can see you have to spin it exactly once every orbit to keep the X facing the Earth model. If you don't rotate it, the Moon model will show all of its "sides" to the Earth model as it goes around.

Now, I have been a bit tricky here. We are talking about two different frames of reference; one on the surface of the Earth looking out at the Moon, and one outside the Earth-Moon system looking in. You performed the experiment from the latter frame, and saw the Moon rotating. From the former, however, you can see for yourself the Moon does not rotate. What is being argued here is that in one frame the Moon rotates, in another it does not.

We've actually learned three things:
# 1) the Moon rotates as it orbits the Earth (as seen by an outside observer);
# 2) it rotates one time for every orbit (to that observer); and
# 3) if it didn't rotate, we would eventually see all of the Moon as it orbited the Earth. "

http://www.badastron.../moon_spin.html

Quote

ranging from 46 to 70 million kilometers. It takes 87.969 earth days to complete an orbit. The diagram on the right illustrates the effects of the eccentricity, showing Mercury's orbit overlaid with a circular orbit having the same semi-major axis. The higher velocity of the planet when it is near perihelion is clear from the greater distance it covers in each 5-day interval. The size of the spheres, inversely proportional to their distance from the Sun, is used to illustrate the varying heliocentric distance. This varying distance to the Sun, combined with a 3:2 spin-orbit resonance of the planet's rotation around its axis, result in complex variations of the surface temperature.^[13] This resonance makes a single day on Mercury last exactly two Mercury years, or about 176 Earth days.^[69]
Mercury's orbit is inclined by 7 degrees to the plane of Earth's orbit (the ecliptic), as shown in the diagram on the right. As a result, transits of Mercury across the face of the Sun can only occur when the planet is crossing the plane of the ecliptic at the time it lies between the Earth and the Sun. This occurs about every seven years on average.^[70]
Mercury's axial tilt is almost zero,^[71] with the best measured value as low as 0.027 degrees.^[7] This is significantly smaller than that of Jupiter, which has the second smallest axial tilt of all planets at 3.1 degrees. This means that to an observer at Mercury's poles, the center of the Sun never rises more than 2.1 arcminutes above the horizon.^[7]
At certain points on Mercury's surface, an observer would be able to see the Sun rise about halfway, then reverse and set before rising again, all within the same Mercurian day. This is because approximately four Earth days before perihelion, Mercury's angular orbital velocity exactly equals its angular rotational velocity so that the Sun's apparent motion ceases; at perihelion, Mercury's angular orbital velocity then exceeds the angular rotational velocity. Thus, to a hypothetical observer on Mercury, the Sun appears to move in a retrograde direction. Four days after perihelion, the Sun’s normal apparent motion resumes at these points.^[13]
Mercury attains inferior conjunction (near approach to the Earth) every 116 Earth days on average,^[3] but this interval can range from 105 days to 129 days due to the planet’s eccentric orbit. Mercury can come as close as 77.3 million km to the Earth,^[3] but it will not be closer to Earth than 80 Gm until AD 28,622. The next approach to within 82.1 Gm is in 2679, and to within 82 Gm in 4487.^[72] Its period of retrograde motion as seen from Earth can vary from 8 to 15 days on either side of inferior conjunction. This large range arises from the planet's high orbital eccentricity.^[13]
Spin–orbit resonance




After one orbit, Mercury has rotated 1.5 times, so after two complete orbits the same hemisphere is again illuminated.
For many years it was thought that Mercury was synchronously tidally locked with the Sun, rotating once for each orbit and always keeping the same face directed towards the Sun, in the same way that the same side of the Moon always faces the Earth. Radar observations in 1965 proved that the planet has a 3:2 spin–orbit resonance, rotating three times for every two revolutions around the Sun; the eccentricity of Mercury’s orbit makes this resonance stable—at perihelion, when the solar tide is strongest, the Sun is nearly still in Mercury’s sky.^[73]
The original reason astronomers thought it was synchronously locked was that, whenever Mercury was best placed for observation, it was always nearly at the same point in its 3:2 resonance, hence showing the same face. This is because, coincidentally, Mercury's rotation period is almost exactly half of its synodic period with respect to Earth. Due to Mercury's 3:2 spin–orbit resonance, a solar day (the length between two meridian transits of the Sun) lasts about 176 Earth days.^[13] A sidereal day (the period of rotation) lasts about 58.7 Earth days.^[13]
Simulations indicate that the orbital eccentricity of Mercury varies chaotically from nearly zero (circular) to more than 0.45 over millions of years due to perturbations from the other planets.^[13][74] This is thought to explain Mercury's 3:2 spin-orbit resonance (rather than the more usual 1:1), since this state is more likely to arise during a period of high eccentricity.^[75] Numerical simulations show that a future secular orbital resonant perihelion interaction with Jupiter may cause the eccentricity of Mercury's orbit to increase to the point where there is a 1% chance that the planet may collide with Venus within the next five billion years.

http://en.wikipedia....ury_(astronomy)



http://www.solarview...moon/vmoon1.mpg

[the L]

Thanks guys.

[MID]

the L, on 15 May 2012 - 05:44 PM, said:

Thanks guys.