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

  1. Jun 18, 2018 #76

    Bandersnatch

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    Ah. You're right. Well spotted. It does look like it's pointing in the wrong direction, and should indeed be deflected towards the reader rather than in the plane of the picture.
    Maybe @fizixfan will stop by and take a shot at correcting it. Although I imagine it might be difficult to render it clearly in two dimensions.
     
  2. Jun 19, 2018 #77
    I've been following this thread since Loki sullivan started posting, but I've been unable to fully understand what's being said. This may not answer your question, but the Earth's Axis of Rotation is tilted "away" from the Sun in the Winter, which is why it gets colder in the Northern Hemisphere during winter (you probably already know that), and tilted "toward" the Sun in the Northern Hemisphere in the Summer. The Earth is also situated between the Sun and the Galactic Center in the summer (Sun - Earth - Galactic Center). That's why we have such great viewing of the Milky Way in the summer months - because the Sun isn't in the way at night and we're looking toward the Galactic Center. In winter the Sun is between the Earth and Galactic Center (Earth-Sun-Galactic Center), so no viewing of the Milky Way in Winter from the NH.

    The Earth, in a physical sense, is about 50 light years north of the Galactic Equator, so change in position of 186 million miles from over the course of a year isn't going to change our position with respect to the Galactic Plane. What does change is our point of view, since we are on a tilted, spinning sphere orbiting the sun.

    The axial tilt of the earth does not vary relative to the Galactic North on a seasonal basis or even during a human lifetime - although it does precess (Google that term if necessary). But for the purposes of this discussion let's just say the North Celestial Pole (Earth's Axis of Rotation) and the North Galactic Pole (Milky Way's Axis of Rotation) do not vary. The angle between the NCP and NGP is 62.9°, although this can only be determined using spherical trigonometry, since the Celestial Equator, Ecliptic Plane and Galactic Plane do not intersect at a single point. See Figures 2 and 3 in my original post. I'm including another crude drawing I did that may help. Words are really kind of hopeless in explaining three different celestial coordinate systems - which is why I prefer pictures.

    drawings-orientation-of-earth-sun-solar-system-in-milky-way-crop-annotated copy.jpg

    It might also help if you read all the posts in this thread, but especially, I would encourage you to go to a place where there are dark skies during the summer months and look up. I recently purchased a telescope for my camera, and have found some amateur astrophotographers to hang out with. I would also recommend uploading Stellarium - a free planetarium program for your PC, and Skywatch for you mobile device.
     
    Last edited: Jun 19, 2018
  3. Jun 19, 2018 #78
    Let me first say, thank you for sharing your work. I have learned much from your illustrations. As far as my inquiry, I hope you understand I am not criticizing your work but seeking clarification for my own understanding. I DO understand the axial tilt of the earth and its affect on seasonality. My question was referring to the direction of axial tilt of the earth. In the illustration, the angle of the earth axis in relation to galactic plane is apparent, but the axial tilt direction appears to be towards the sun when near the winter solstice. If I am understanding that the northern hemisphere is shown to the left, then wouldn’t the direction of axial tilt be away from the sun at winter solstice? It may just be a misinterpretation due to the nature of 2D renderings, so I ask for clarification for my own understanding.
     
  4. Jun 19, 2018 #79
    It's the nature of the 2D rendering, I'm afraid.
     
  5. Jul 19, 2018 at 12:23 AM #80
    You'll be interested to know that, since the Earth's equator is inclined so steeply (60.2 degrees) to the plane of the Milky Way, that there are features of the Milky Way, such as the Galactic Center and Galactic Bulge, that can only be seen from the Northern Hemisphere, and features such as the Coalsack Nebula and the small and large Magellanic Clouds, that can only be seen from the Southern Hemisphere!
     
  6. Jul 19, 2018 at 1:42 AM #81

    davenn

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    you sure about that ? :wink:

    the galactic centre is in Sagittarius which comes well above the horizon in the southern hemisphere.
    In fact at this time of the year, it passes right overhead
    Maybe you take a trip "down under" one day and I and some other Australian astronomers will treat you to the delights of the southern sky


    Dave
     
    Last edited: Jul 19, 2018 at 2:38 AM
  7. Jul 19, 2018 at 3:16 AM #82

    davenn

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    there ya go one of my own pix looking straight into the core complete with bulge :smile:

    upload_2018-7-19_18-16-0.png


    upload_2018-7-19_18-16-35.png




    Dave
     
  8. Jul 19, 2018 at 8:19 AM #83
    Obviously I'm mistaken - Thanks for letting me know! I guess I'm not as familiar with the southern hemisphere as I'd like to be!
     
  9. Jul 19, 2018 at 1:36 PM #84

    Janus

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    Here's the relationship between the Celestial Equator, Galactic Equator, and Ecliptic.
    coords.jpg
    The white grid are celestial coordinates and the light blue grid galactic coordinates.
    Note that the Galactic Center is ~29 degrees South of the Celestial equator. This means that it is visible in the entire Southern Hemisphere and not visible for points above ~61 degrees North Latitude. The ecliptic crosses the Galactic equator ~6.5 degrees from the Galactic center and the Celestial equator crosses the galactic equator 33 degrees from the galactic center along the galactic equator.
     
  10. Jul 19, 2018 at 9:55 PM #85
    Thanks, Janus - Are there any parts of the Milky Way that aren't observable from the Southern Hemisphere?
     
  11. Jul 19, 2018 at 10:06 PM #86

    davenn

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    Not really, as you can see from my pic. you can see it from one thin end on the left, through the centre and out to the thin end on the right side

    What we don't see from the southern hemisphere are the constellations that are well away from the galactic plane ... ie. close to the north celestial pole

    Ursa Major and minor, Cassiopeia, Draco, Cepheus, Hercules ... to name a few


    Dave
     
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