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Big Bad Voodoo

I admit ignorance about Astronomy

39 posts in this topic

Sun's distance to the milky way's galactic center (supermassive blackhole) is roughly 25,000 light years. The milkyway is aproximately 100,000 LY in diameter.

I forget how long it takes a spiral arm to make a complete rotation. Depending on reference the milky way rotates at about 600 km per second. One facinating aspect of a spiral is the speed at which stars rotate. In our solar system, the closer you are to the sun, the faster you move (i.e. Mercury verses pluto). However, with a spiral galaxy this isn't the case. Some stars on the outer spiral arms rotate faster around the glactic center then stars much closer. This is presented evidence for the existense of dark matter.

Demensions are a completely different subject (and very speculative and near impossible to ever detect, if possible at all).. As we know of only 3 spacial demnsions and time as the fourth, there are another ten or so theorized demensions. A subect that is really, extremely hard to comprehend. I think L, you would appreciate a course in theoretical physics.

TimeA temporal dimension is a dimension of time. Time is often referred to as the "fourth dimension" for this reason, but that is not to imply that it is a spatial dimension. A temporal dimension is one way to measure physical change. It is perceived differently from the three spatial dimensions in that there is only one of it, and that we cannot move freely in time but subjectively move in one direction.

The equations used in physics to model reality do not treat time in the same way that humans commonly perceive it. The equations of classical mechanics are symmetric with respect to time, and equations of quantum mechanics are typically symmetric if both time and other quantities (such as charge and parity) are reversed. In these models, the perception of time flowing in one direction is an artifact of the laws of thermodynamics (we perceive time as flowing in the direction of increasing entropy).

The best-known treatment of time as a dimension is Poincaré and Einstein's special relativity (and extended to general relativity), which treats perceived space and time as components of a four-dimensional manifold, known as spacetime, and in the special, flat case as Minkowski space.

Additional dimensions Theories such as string theory and M-theory predict that physical space in general has in fact 10 and 11 dimensions, respectively. The extra dimensions are spatial. We perceive only three spatial dimensions, and no physical experiments have confirmed the reality of additional dimensions. A possible explanation that has been suggested is that space acts as if it were "curled up" in the extra dimensions on a subatomic scale, possibly at the quark/string level of scale or below.

http://en.wikipedia.org/wiki/Dimension Edited by Mentalcase

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Thanks on your answers.

I got some new questions.

Whats distance from our solar system and center of our galaxy?

Whats spiral arm and how long periodic movement of that arm lasts? (whatever it was) :innocent:

L,

The distace from here to the galactic center is, as has likely already been said, around 27,000 light years.

That's roughly 160,000,000,000,000,000 miles away, or around a billion and a half times the distance between us and the Sun!

Kind of a big place, eh?

I think you're asking about the period for the galaxy to rotate once?

There are several estimates, but the average value of those is around 240,000,000 years (let's say around 3 million human lifetimes). It takes a while.

Edited by MID

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You need to get out more... What I mean is, find a dark location, away from city and town lights, on a clear night (being in a desert area and/or at a higher altitude helps..). Then spend twenty minutes in absolute darkness to let your eyes fully dark adapt, and then finally.. look up. Well, actually, for northern hemisphere viewers at the moment (September) before midnight, you'll need to look a bit West of South for the thickest part of the Milky Way. Trouble is, the Moon is around at the moment - it washes out the sky, so maybe leave it for a week or two..

Sadly for you northerners, you don't get a very good view of the 'centre' of the Milky Way, which is near Sagittarius. Sagittarius is probably at its highest around July (?) I think.

For us Southerners (eg Australia), it's almost directly overhead at this time of year. So our desert night skies are simply beautiful, as the 'Backbone of the Night' stretches out overhead... OK, that image is cheating, as it's a composite of many selected time exposures, but for a fully dark adapted eye and from a clear desert sky, it can almost look that good..!

It's one of the reasons why I love camping in the middle of nowhere...

Edited by Chrlzs

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Ah but you can, L!

Read very carefully what Chrlzs told you about seeing the "Milky Way"

It's visible.

It does depend on where you are and when you're there.

However, alot of people have a tough time seeing it, when I've pointed it out to them, plain as day, on a dark night here in the mid-Atlantic U.S. Sometimes, the glow of it is massive to me, and yet, I have to teach people how to look without staring (averting the eyes, relaxing them...off center of the object of your desire...an astronomer's technique!).

Maybe it's a little hazy and they're not picking it up...I'll give them a fine binocular and have them look into it. The stars they see in thefield suddenly are amazing!

I suggest consulting star charts for your particular location. They will paint the Milky Way on the chart for you, so you'll know where to look.

Where Chrlzs is now should be pretty good viewing (I think it's coming up on Spring in Australia??). I don't know where you are, so it may not be so

good right now....but generally, everyone has the opportunity to see an arm of our glaxy, if they know where to look!

And if you know what you're looking at...it's a pretty stunning thing!

Might I suggest that you go to www.astronomy.com. Maybe subscribe to that journal (It's great). Look, you're far too interested in this stuff not to check out your sources, and in my opinion, for what it's worth, Astronomy is the best one you can get!

:tu:

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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. :w00t:

Cheers.

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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. :w00t:

Cheers.

My pleasure!

Thanks for the Joke!

And questions are always welcome!

:tu:

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Why earths rotate itself and mercury dont?

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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.

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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. :lol:

I heard that ones Mecury side is always dark as moon...

Why moon doesnt rotate itself?

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This is getting funny. I must changed my name. Evryone call me L yet display name is N. :lol:

I heard that ones Mecury side is always dark as moon...

Why moon doesnt rotate itself?

Hi The N

Tidal locking;

http://en.wikipedia.org/wiki/Tidal_locking

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This is getting funny. I must changed my name. Evryone call me L yet display name is N. :lol:

I heard that ones Mecury side is always dark as moon...

Why moon doesnt rotate itself?

Actually the Moon rotates as well.

"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.badastronomy.com/bad/misc/moon_spin.html

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

220px-Mercury%27s_orbital_resonance.svg.png

magnify-clip.pngAfter 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.org/wiki/Mercury_%28astronomy%29

http://www.solarviews.com/raw/moon/vmoon1.mpg

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Thanks guys.

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Thanks guys.

:tsu:

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