I Orientation of the Earth, Sun and Solar System in the Milky Way

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I keep pointing out that the sun's motion around the Milky Way is not an "orbit" in the sense that we usually think of it. It is not even a closed curve. Orbits in a central potential like the solar system are closed curves, but orbits in a potential well like the Milky Way's are typically not. Even if the Milky Way's potential were static in time the orbit would not be a closed curve, and it is definitely not static in time. So it is best to think of the sun's motion as an approximate orbit, where each path around the Milky Way is a different curve.
No argument there - I agree that the sun won’t return to the same place in another 230 million years or so for the reasons you have noted. “Approximate orbit” is a more accurate term than just “orbit.” In a sense though, the earth doesn’t return to the same spot either after one trip around the sun since it is moving through both space and time.

I have amended my diagram so that it doesn't show the sun’s orbit as a closed curve.

Sun's Approx. Orbit around Milky Way (08Nov2018).jpg


But the original intent in my previous post was to point out that the sun moves in unison with our rotating galaxy - not against it, and that its up and down motion is relatively very small.
 

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sophiecentaur

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While the galaxy does have an overall angular momentum, which the solar system goes with,
This is not like a planet orbiting a star, whose motion is highly predictable.
Predictability is something that people tend to assume and we have had a very short fraction of a period in which to observe any motions outside the Solar System. We have been making 'fairly' accurate observations for less than a hundred years yet making predictions about many thousands of years in the future. They must be a bit speculative, surely.
Even motion within the Solar System is subject to Chaos so I have to wonder about the accuracy of predictions with the galactic many-body problem.
(Not that it really matters to us, of course.)
 
I've been tinkering with a few diagrams in an attempt to illustrate the motion of the solar system in its journey around the Milky Way. I also wanted portray how the celestial, ecliptic and galactic coordinate systems are related to each other in a single picture. Note: in the Celestial, or Equatorial system, the Celestial North Pole (an extension of the Earth's axis of rotation), uses the default setting of North as "up." The Ecliptic and Galactic also use North as "up" with reference to the Celestial North Pole. Some people say that in space there is no such thing as "up" or "down," but in determining the position of a celestial object (e.g., declination and right ascension of a star or deep-sky object) is DOES matter.

Please have a look at these diagrams and feel free to comment on any errors, or make suggestions as to how I could make them better. I drew these images, but anyone is free to re-use them without restriction.

Figure 1 shows the motion of the Earth and Sun around the Milky Way. The solar system is actually well within the galactic disk, which is about 1,000 light years thick. The sun and the planets that circle it is roughly 50 light years above the galactic plane, and passed northward through it about 3 million years ago in its undulating path around the galactic center. Note: this diagram is not to scale. The northernmost excursion of the solar system takes it about 250 light years above the galactic plane. This means it would only subtend an angle of about 0.55° relative to the galactic center.

Figure 1. Motion of Earth and Sun around the Milky Way

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Figures 2. and 3. show the orientation of the Earth, Sun & Solar System in the Milky Way - similar diagrams, just presented in different ways.

Figure 2. Orientation of Celestial, Ecliptic and Galactic Poles and Planes

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Figure 3. Orientation of astronomical coordinates projected on the Celestial Sphere.

View attachment 107280


The angle between Celestial Equator (an imaginary plane passing through the earth's equator) and the Ecliptic Plane (an imaginary plane extended through the Sun's equator) is 23.4°. The angle between the North Celestial Pole (an imaginary line extending through Earth's axis of rotation) and the North Ecliptic Pole (an imaginary line extending through the Sun's axis of rotation) is the same - 23.4°. This is the familiar value for the "tilt" of the Earth in its path around the Sun.

The angle between the Ecliptic Plane and the Galactic Equator (an imaginary plane passing through, and parallel to, the disk of the Milky Way) is 60.2°. The angle between the North Ecliptic Pole and the North Galactic Pole (an imaginary line extending through the Milky Way's axis of rotation) is also 60.2°.

The angle between the Celestial Equator and the Galactic Equator is 62.9°, as is the angle between the North Celestial Pole and the North Galactic Pole.

These three angles = 23.4°, 60.2° and 62.9° cannot be shown or calculated in two dimensions, because they represent separate planes which do not intersect at a common point. If you look at Figure 3, you can see that this is so.

References:
https://en.wikipedia.org/wiki/Celestial_coordinate_system#Galactic_system
https://www.eso.org/public/news/eso0932/
http://www.engineeringanddesign.com/1/054.htm
I believe this is why we have shorter days in the winter but the moon is out longer at night and the opposite in the summer. Thoughts?
 

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sophiecentaur

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The relative angles of the Earth’s tilt and the plane of the Moon’s orbit are only affected very slightly by our Galactic situation. The height of the Moon in the sky over the year doesn’t change a lot. In winter, the Moon is seen for more hours against a dark sky than in summer. Is that what you mean by “out”? A ‘pale’ moon can be seen in a light sky in summer for many hours.
 
The relative angles of the Earth’s tilt and the plane of the Moon’s orbit are only affected very slightly by our Galactic situation. The height of the Moon in the sky over the year doesn’t change a lot. In winter, the Moon is seen for more hours against a dark sky than in summer. Is that what you mean by “out”? A ‘pale’ moon can be seen in a light sky in summer for many hours.
Thanks for your response. Yes the big picture of this thread is in regards to the position of the solar system relative to the mid plane of the Galaxy. I hope this discussion is still relevant and not deviating too far.

I'm focusing in on the celestial plain and lunar plain relative to earth. It seems the celestial plane and lunar plain are tilted in opposite directions leading to the trend of the Moon passing higher through the sky during winter nights and the sun passing lower in the sky during winter days. Yes, like you said longer hours of moonlight in the winter. I'm speaking from a northern hemisphere reference and also I'm curious if this is naturally also applicable for the southern hemisphere? Hope this isn't coming across as a homework question but actually as in sincere curiosity the heavenly bodies.
Kindest Regards
 
Thanks for your response. Yes the big picture of this thread is in regards to the position of the solar system relative to the mid plane of the Galaxy. I hope this discussion is still relevant and not deviating too far.

I'm focusing in on the celestial plain and lunar plain relative to earth. It seems the celestial plane and lunar plain are tilted in opposite directions leading to the trend of the Moon passing higher through the sky during winter nights and the sun passing lower in the sky during winter days. Yes, like you said longer hours of moonlight in the winter. I'm speaking from a northern hemisphere reference and also I'm curious if this is naturally also applicable for the southern hemisphere? Hope this isn't coming across as a homework question but actually as in sincere curiosity the heavenly bodies.
Kindest Regards
If I may correct my terminology stating the lunar plane and Celestial plane are tilted in opposite directions is not very well worded. Basically I'm just saying the lunar plain is tilted higher than the celestial plain ( speaking from a northern hemisphere Viewpoint near winter solstice). I sure could use a better way to word that.
 

sophiecentaur

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I'm curious if this is naturally also applicable for the southern hemisphere?
The Moon's orbit has to around the CM of Earth / Moon so the plane must go through the middle. I think that implies that the effects are the same, top and bottom.
Edit: I think that comment needs to be modified to include the idea of 'average over time'.
 
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Janus

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If I may correct my terminology stating the lunar plane and Celestial plane are tilted in opposite directions is not very well worded. Basically I'm just saying the lunar plain is tilted higher than the celestial plain ( speaking from a northern hemisphere Viewpoint near winter solstice). I sure could use a better way to word that.
"Tilted higher" is a bit of an arbitrary statement. The Moon's orbit is tilted at 5 degrees to the ecliptic (the Earth solar orbit plane). The Moon's orbit also has a nodal precession; It "wobbles". The period of this precession is 18.6 years. This, in turn means that the Lunar orbit varies from being 18.5 to 28.5 degrees in tilt with respect to the celestial equator over that 18.6 year period.

While in one year, at the winter solstice, the Lunar orbit will align so that the full moon can appear higher in the Southern sky (as seen from the Northern hemisphere) than the Sun does on the Summer solstice(The difference between maximum and minimum declination will be the greatest), 9.3 years later, the full moon will never get as high in the sky as the Sun does on the Summer solstice (the difference between declinations will be the least).
 

Drakkith

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The Moon's orbit also has a nodal precession; It "wobbles". The period of this precession is 18.6 years. This, in turn means that the Lunar orbit varies from being 18.5 to 28.5 degrees in tilt with respect to the celestial equator over that 18.6 year period.
Whoa! That's a lot of wobble! I had no idea the Moon's orbit varied its tilt by 10 degrees.
Any idea where we are in the cycle now?
 

phyzguy

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Whoa! That's a lot of wobble! I had no idea the Moon's orbit varied its tilt by 10 degrees.
Any idea where we are in the cycle now?
The moon's orbit is always inclined by 5 degrees relative to the ecliptic plane. It's just that as it precesses, the inclination to the Earth's equatorial plane ranges from 23.5-5=18.5 to 23.5+5=28.5.
 
Thank you for the feedback I enjoy being an ambassador of this knowledge to my friends and family. Two nights ago we were at the beach early in the morning and I was describing the beginning of the perseid meteor shower when we saw a red fiery meteor shoot overhead. It was about a spaghetti noodle wide with a tail about 20 degrees in arc length. I will study the main thread further.
Cheers.
If your plan is for one year plant rice. If your plan is for ten years plant trees. If your plan is for one hundred years educate children.
Confucius
 
"Tilted higher" is a bit of an arbitrary statement. The Moon's orbit is tilted at 5 degrees to the ecliptic (the Earth solar orbit plane). The Moon's orbit also has a nodal precession; It "wobbles". The period of this precession is 18.6 years. This, in turn means that the Lunar orbit varies from being 18.5 to 28.5 degrees in tilt with respect to the celestial equator over that 18.6 year period.

While in one year, at the winter solstice, the Lunar orbit will align so that the full moon can appear higher in the Southern sky (as seen from the Northern hemisphere) than the Sun does on the Summer solstice(The difference between maximum and minimum declination will be the greatest), 9.3 years later, the full moon will never get as high in the sky as the Sun does on the Summer solstice (the difference between declinations will be the least).
The Elegance of this description will make teaching others more efficient thank you.
 

DaveC426913

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I have always imagined that the Sun revolves around the galaxy in a counterclockwise direction assuming the convention of looking down on it from "North" (i.e. moving to the right in typical pictures).

I never made the connection to the fact that the spiral arms spin out clockwise (as seen from the North/top).

You learn something new every day.
 
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Oh, for a 3D video representation of the entire shebang. . . and (in my case) the 3D means to view it :woot:
 

sophiecentaur

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I never made the connection to the fact that the spiral arms spin out clockwise (as seen from the North/top).
It's worth while pointing out that the arms are not spinning like a Catherine Wheel Firework. They are just a density pattern or wave due to the interaction of each star with its 'close' neighbours. Density waves do not consist of the same stars all the time and do not travel at the same speed as their constituent stars so they can be looked upon as 'virtual'. Wiki gives a fair description of the effect.
 
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That is neato-keen! Almost obvious (particularly the _winding effect_) if you stop to think about it, which I confess I have not until I read the wiki article.

diogenesNY
 
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This is Venus, high in the morning sky, amid the faint pillar of light called the Zodiacal Light. The glow is sunlight reflected off cometary dust in the inner solar system.
Above is the centre of the Galaxy area of Sagittarius. Alan Dyer
https://amazingsky.net/2014/04/06/venus-in-the-zodiacal-light/

Since the Zodiacal Light lies in the ecliptic plane, we can see from this image that the plane of the solar system intersects the galactic centre.

(PF doesn't want to insert the image today.)
 
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This is my "map" of the solar system with the orbits to scale. I have included the orbits of eight planets plus that of Pluto and the parts of Eris' orbit that falls into the frame. I will add some labels and a wire coming out of the map to indicate the passage of the system around the centre of the Milky Way. I'll also post a shot of the inner planets orbits as they are rather small in this picture.
20191018_114821.jpg
 

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