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Raleigh
2007-Aug-17, 03:04 PM
Friends-
This is a question for any knowledgeable person.
Imagine a round dinner plate on a flat level table. The plate can be moved sideways over the table. Or it can be lifted upwards. Now imagine the plate becoming the Solar System, with the Sun at its center and dwarf planet Pluto at its outer edge. The other planets are somewhere between the two, all rotating around the Sun on approximately the same plane counterclockwise--some faster, some slower. (http://en.wikipedia.org/wiki/Solar_System, Layout and Structure section)
Now as the Sun orbits around the center of the Milky Way Galaxy at approximately 220km/sec (Ibid.) would the plate be moving sideways over the table, or upwards?
In the former case, the Earth and other planets would be in front of the Sun for a while, then would rotate slowly as the Sun passes them, then would be behind the Sun, then they would have to travel at a much higher speed to catch up to and again be in front of the Sun.
In the later case, if the plate were lifted upwards, the planets would have a more even speed in their elliptic and would essentially be spiraling counterclockwise, not orbiting, around the Sun to keep up with it as it circles the center of the galaxy.
The plate could also be moved at any angle between up and flat.
My question is, then, at what angle does this Solar System move through space, from up to flat? And what is the range of speed for the Earth as it keeps up with the Sun?
One knowledgeable Internet correspondent says that the Earth's speed ranges from 340 to 400 km/sec. But these figures cannot be correct if the 220 km/sec speed of the Sun is correct, in which case if direction is up Earth speed would range from perhaps +10% to +20%; or if flat + or- 50%.
This question is important because of the question of gravity.
Perplexed. Hope someone can help.
Peace, Raleigh

loglo
2007-Aug-17, 04:52 PM
I would say that the Earth's velocity you quote is with respect to the Sun and not to the galaxy and the Sun's velocity you quote is with respect to the Galactic centre. You are trying to compare apples and pears. If you are trying to find a function for velocity and direction of the Earth as it orbits the Sun as the Sun orbits the galaxy I doubt one exists.

As for the galactic plane, from Cosmos:-
"The galactic plane is tilted at an angle of 63 degrees to the celestial equator. Since the ecliptic (the path of the Sun on the sky) is inclined at an angle of 23.5 degrees to the celestial equator, the galactic plane and the ecliptic are nearly at right angles (63 + 23.5 = 86.5 degrees), although this is purely coincidental."

01101001
2007-Aug-17, 05:14 PM
And the net motion of the sun and its system, relative to galactic center, is not a nice nearly circular ellipse, but a 3D merry-go-round-horse path, bobbing up and down through the galactic plane as it goes around.

John Mendenhall
2007-Aug-17, 05:20 PM
There is no absolute reference frame against which to measure the speed of the solar system and its planets (or anything else, a fundamental concept of relativity.) You can compare the motions of the planets with other celestial objects, but one is as good as another. Locally, the solar system sort of bumbles along through the interstellar medium of very thin gas and a little dust. You might check Wiki for the "Heliopause", the shell where the interstellar magnetic field wins out over the magnetic field of the solar system. As I recall, the shock wave defining the shell is supersonic.

Hornblower
2007-Aug-17, 10:38 PM
As for the galactic plane, from Cosmos:-
"The galactic plane is tilted at an angle of 63 degrees to the celestial equator. Since the ecliptic (the path of the Sun on the sky) is inclined at an angle of 23.5 degrees to the celestial equator, the galactic plane and the ecliptic are nearly at right angles (63 + 23.5 = 86.5 degrees), although this is purely coincidental."
Cosmos is mistaken. The galactic equator intersects the ecliptic almost exactly at the solstice points, not the equinoxes. Thus they intersect at an angle of 63 degrees. I just checked it in my Sky Atlas 2000.0.

Let's imagine our table as being in the plane of the galaxy, while looking toward Sagittarius with the north galactic pole up. Our plate would be tipped 63 degrees counterclockwise from the horizontal position and moving left, toward Cygnus.

01101001
2007-Aug-17, 10:48 PM
Let's imagine our table as being in the plane of the galaxy, while looking toward Sagittarius with the north galactic pole up. Our plate would be tipped 63 degrees counterclockwise from the horizontal position and moving left, toward Cygnus.

If we were spinning, I'd be nauseated. Wait, we are spinning...

Hey, Ralph!

loglo
2007-Aug-18, 12:56 PM
Cosmos is mistaken. The galactic equator intersects the ecliptic almost exactly at the solstice points, not the equinoxes. Thus they intersect at an angle of 63 degrees. I just checked it in my Sky Atlas 2000.0.

Let's imagine our table as being in the plane of the galaxy, while looking toward Sagittarius with the north galactic pole up. Our plate would be tipped 63 degrees counterclockwise from the horizontal position and moving left, toward Cygnus.

From The American Heritage dictionary:-
"The great circle of the celestial sphere that lies in the plane bisecting the band of the Milky Way, inclined at an angle of approximately 62° to the celestial equator."

From the astronomy glossary at http://www.cascaeducation.ca/files/glossary.html:-
"celestial equator
The imaginary line dividing the northern and southern hemispheres of the celestial sphere. Equivalent to the equator on the Earth."

Your SkyAtlas is wrong or you are mis-interpreting it. The Galactic Equator intersects the Celestial Equator at 62 degrees, not the Ecliptic, which is offset by a further 23.5 degrees.

Here's the
full page. (http://astronomy.swin.edu.au/cms/astro/cosmos/G/Galactic+Equator)

Hornblower
2007-Aug-18, 02:02 PM
From The American Heritage dictionary:-
"The great circle of the celestial sphere that lies in the plane bisecting the band of the Milky Way, inclined at an angle of approximately 62° to the celestial equator."

From the astronomy glossary at http://www.cascaeducation.ca/files/glossary.html:-
"celestial equator
The imaginary line dividing the northern and southern hemispheres of the celestial sphere. Equivalent to the equator on the Earth."

Your SkyAtlas is wrong or you are mis-interpreting it. The Galactic Equator intersects the Celestial Equator at 62 degrees, not the Ecliptic, which is offset by a further 23.5 degrees.

Here's the
full page. (http://astronomy.swin.edu.au/cms/astro/cosmos/G/Galactic+Equator)

My Sky Atlas is as accurate as Wil Tirion knew how to make it, and I am not misinterpreting anything.

The plane of the galaxy crosses the celestial equator at 62 or 63 degrees, depending whose figure we use, and about 23 degrees to the south it crosses the ecliptic at a point where the ecliptic is parallel to the equator. Thus its inclination to the ecliptic at that point is nearly the same as at the equator.

I acknowledge that my initial statement is not completely accurate due to the realities of spherical trig. Since I am not up to speed on the math, I checked up by plotting the circles in question on a globe. With the galaxy inclined 62 degrees at the equator, it crossed the ecliptic at the solstice point at approximately 58 degrees, not at 86 as alleged by Cosmos. That would have required crossing at the September equinox point rather than at the actual crossing almost exactly at the solstice point.

My National Geographic globe made this task easy. It came with a great circle plotting tool and a protractor for measuring angles on its surface.

grant hutchison
2007-Aug-18, 03:04 PM
Some numbers on this:
In ecliptic coordinates, the latitude of the galactic north pole is 30º, implying that the plane of the Earth's orbit is tilted at 60º relative to the galactic equatorial plane.
In equatorial coordinates, the declination of the galactic north pole is 27º, implying that the Earth's equator is tilted at 63º relative to the galactic equatorial plane.
The reason we can't just add the Earth's tilt and the galaxy's tilt is as Hornblower says: the three planes under discussion don't share a line of intersection. The reason the ecliptic and equatorial angles are very similar is again as Hornblower says: because the galactic plane crosses the ecliptic and the equatorial plane close to the solstice points, where tangents to circles drawn around the sun in the two latter planes are almost parallel to each other.

Grant Hutchison

grant hutchison
2007-Aug-18, 03:45 PM
For Rayleigh:
The plane of the solar system is tilted at 60º to the equatorial plane of the galaxy; it so happens that the line of intersection between those two planes points towards the centre of the galaxy.
You can picture the relative positions by taking a plate, standing it on edge, and moving it in a horizontal direction, initially orientating it so that it is flat-side-on to its direction of movement. Now tilt the top edge of the plate backwards relative to the line of movement by 30º. If we imagine that the horizontal plane in your room is the plane of the galaxy, and your ceiling is galactic north, then the plate gives you the orientation of the solar system in its current orbital position in the galaxy.
(In fact, as well as moving "horizontally" in the galactic plane, the solar system is at present climbing a bit towards galactic north, so for added detail you should be making your plate rise at a slight angle while keeping its orientation the same as was achieved previously.)

Grant Hutchison

astromark
2007-Aug-19, 12:50 AM
Just in case you by some miracle understand any of this, ie; 23.5deg. 63deg. and then, 30deg and also 60deg., can we be a little more vague? Consider this also. This galaxy 'The Milky Way' is itself ripping across the space it occupies. I have been careful not to sagest which way that movement is because I have no idea... Andromeda is getting closer while most other galactic objects are receding away...

But just to make this as simple as I can.... If you go out doors and look straight up at 6 am. you are looking the way the earth is traveling in its path around the sun. What or how you calculate the direction of the sun and its path around this galactic core is further confused by the movements of every thing you are looking at. If there is one fact I gleam from this subject it is that, 'Nothing is still'. Any reference to velocity is relative. This might be why its called relativaty.... :)

grant hutchison
2007-Aug-19, 12:59 AM
But just to make this as simple as I can.... If you go out doors and look straight up at 4 am. you are looking the way the earth is traveling in its path around the sun.Around 06:00, I think: halfway between midnight and noon on the leading side of the Earth. And then you need to look towards the zodiac band, not necessarily directly overhead.

Grant Hutchison

grant hutchison
2007-Aug-19, 02:07 AM
Just in case you by some miracle understand any of this ...A picture is worth a thousand words. :)
Here (http://www.ghutchison.pwp.blueyonder.co.uk/orbits/galaxy.jpg) is a picture of the solar system generated using Celestia. We're well outside the orbit of Neptune, looking towards the centre of the galaxy. Orbits of the planets are in blue; the plane of the galaxy runs horizontally, with the galactic centre in the middle of the picture. Galactic north is at the top. The solar system is heading towards the left of the picture, and slightly upwards.
The celestial grid is also shown, to demonstrate how the ecliptic angle and the equatorial angle of the galactic plane is about the same.

Grant Hutchison

Ken G
2007-Aug-19, 02:29 AM
And to potentially stave off an errant journey, note that none of this is going to have anything to do with the effects of galactic gravity, which sounds like where the OP is headed. The beauty of the "equivalence principle" is that any orbiting system (like the solar system), that is in "free fall" as part of a larger orbiting system (like the galaxy), can have its internal orbital motions analyzed independently of the gravity of the larger system, except in regard to tidal effects. So the only way the gravity of the galaxy can affect our solar system is via its miniscule tidal influences, so please don't bother taking this down some other path!

As for the 340 to 400 km/s, that would seem to refer to the Earth's speed relative to the average frame of all the galaxies around us, as measured by our motion relative to the cosmic microwave background (not the Milky Way).

grant hutchison
2007-Aug-19, 02:54 AM
And what is the range of speed for the Earth as it keeps up with the Sun?More on this:
The local standard of rest (LSR) is tied to the circular velocity at the sun's distance from the centre of the galaxy: 220 km.s-1.
The sun moves a little faster than the LSR, by about 15 km.s-1, making a total of 235 km.s-1.
The Earth goes around the sun at about 30 km.s-1, but the sixty-degree tilt means that only half that velocity is ever directed parallel to the galactic plane.
So the Earth's velocity around the galaxy will vary by 30 km.s-1, from 220 km.s-1 at the December solstice to 250 km.s-1 at the June solstice.

Grant Hutchison

astromark
2007-Aug-19, 05:41 AM
:)So do not stick ya head out the window...235km per/sec is kinda fast.

loglo
2007-Aug-19, 12:56 PM
Hi Hornblower,
My apologies, I have actually been taught this incorrectly, which is really annoying since I'm doing an astronomy degree! I'll flick the Cosmos people a note and get their slide corrected.

For those who like me have really bad 3d visualisation skills the slide on page 10 of this PDF (www2.jpl.nasa.gov/radioastronomy/Chapter7.pdf) has a good graphic.

grant hutchison
2007-Aug-19, 04:41 PM
My apologies, I have actually been taught this incorrectly, which is really annoying since I'm doing an astronomy degree! I'll flick the Cosmos people a note and get their slide corrected.I thought this seemed vaguely familiar ... We've actually discussed the same webpage before, here at BAUT, about a year and a half ago. The thread is here (http://www.bautforum.com/questions-answers/39408-retrograde.html), with the brief discussion starting at post #8.

Grant Hutchison