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Orientation of the moon

  1. May 19, 2015 #1
    Edit: since creating this post i found a video showing all of the rotations and phases of the moon for a year



    So for my purposes that should be sufficient. Thanks.

    ------------------------------original message-----------------------------------------------

    Hi, Can somebody explain why the view of the moon as seen in pictures from the same location on Earth is rotated compared to other pictures while the view of the earth seen in the photos is correct?

    I am wanting to show a northern hemisphere view of the moon from Gibraltar at 36N and then the upside view of the moon from Auckland New Zealand at 37S but i have ran into problems with that rotation.

    images?q=tbn:ANd9GcQj2Huw7BcNcSEgo4vb4qkj6HvahFEin21Rb2XSfKSfurp-dhD6.jpg

    NightPierreCilliers.jpg



    The Gibraltar moon

    IMG_1592.JPG

    Thanks

    Andrew
     
    Last edited: May 19, 2015
  2. jcsd
  3. May 19, 2015 #2

    mfb

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    You can rotate the the southern hemisphere photos by 180 degrees to get the same orientation of the moon. Then the different orientation of earth is simply from the curvature of earth, as the pictures are taken from opposite sides.
     
  4. May 19, 2015 #3

    russ_watters

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    Note that they are all different orientations, so that answer is incomplete.

    The larger issue here is the revolution, not the curvature. Even in a flat system, an object's left side touches the left horizon and the right side touches the right horizon. If the revolution axis is perpendicular to the ground, the object flips 180 degrees as it passes the zenith. Any other orientation and the object rotates as it traverses the sky, and above 90 degrees inclination it rotates one way while below 90 degrees it rotates the other. But what changed wasn't actually the direction of rotation, but rather that you turned around (to keep the inclination below 90 degrees).

    The dividing line between the two is thus not necessarily at the equator. By convention, you tend to orient yourself so you are facing most of the path it takes through the sky, but that path only bisects the equator twice a month.
     
  5. May 19, 2015 #4
    As explained in my edited original post the surface of the moon we see changes when viewed from the same point on the earth.

    From wiki

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

    Lunar libration
    The Moon generally has one hemisphere facing the Earth, due to tidal locking. Therefore, humans' first view of the far side of the Moon resulted from lunar exploration in the 1960s; however, this simple picture is only approximately true: over time, slightly more than half (about 59%) of the Moon's surface is seen from Earth due to libration.[1]

    Libration is manifested as a slow rocking back and forth of the Moon as viewed from Earth, permitting an observer to see slightly different halves of the surface at different times.

    There are three types of lunar libration:

    • Libration in longitude results from the eccentricity of the Moon's orbit around Earth; the Moon's rotation sometimes leads and sometimes lags its orbital position.
    • Libration in latitude results from a slight inclination between the Moon's axis of rotation and the normal to the plane of its orbit around Earth. Its origin is analogous to how the seasons arise from Earth's revolution about the Sun.
    • Diurnal libration is a small daily oscillation due to the Earth's rotation, which carries an observer first to one side and then to the other side of the straight line joining Earth's and the Moon's centers, allowing the observer to look first around one side of the Moon and then around the other—because the observer is on the surface of the Earth, not at its center.
     
  6. May 19, 2015 #5

    russ_watters

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    The apparent rotation of the view has nothing to do with libration. The axes of the two effects are perpendicular to each other and either can exist without the other.

    The effect may be easier to visualize by just considering circumpolar constellations, which rotate as they revolve around the axis of the pole.
     
  7. May 19, 2015 #6
    I am a bit confused.

    I am not saying you are wrong or I know better, but I do want to just check you have thought about this before and do know what the answer is.

    1. I assume you are talking about the constant rotation of the stars around the pole star? Whereas the observed rotation here is a wobble back and forth

    2. Nasa say "our changing view of the Moon makes it look like it's wobbling. This wobble is called libration."

    http://svs.gsfc.nasa.gov/cgi-bin/details.cgi?aid=4000

    3. Can you expand please on this:

    The axes of the two effects are perpendicular to each other and either can exist without the other.

    Thanks
     
    Last edited: May 19, 2015
  8. May 19, 2015 #7

    russ_watters

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    Let me try to be clear: in the OP, you said "rotation", not "wobble", and provided pictures showing the dark area forming a C, that faced in many different directions (up, down, left, right). Is that what you want to know about?

    For the rotation, the axis is poking you in the eye and the moon rotates 360 degrees around it. For libration, the axis is perpendicular to that (so it causes no rotation in your view plane whatsoever) and only results in you seeing a bit more than half of the moon instead of exactly half.
     
    Last edited: May 19, 2015
  9. May 19, 2015 #8
    In my question i was talking about the difficulty making sense of pictures from a single 'down under´ Earth location namely Auckland where the pictures are rotated in an arc about 45 degree either side of vertical. Later i learnt that from a single earthly location the view of the moon wobbles back and forth rotationally so i began using that word to describe what i was seeing. The view wobbles back and forth rotationally but does not reverse or invert. I included the northern hemisphere picture I wanted to compare to which had the view 'the right way up'.

    You are now talking about the view of the moon rotating 360 degrees so i am totally confused by what you are wanting to tell me:-(

    I think we might be talking at crossed purposes somehow??

    i can see though you are saying libration does not cause the back and forth rotation that we see when comparing images of the moon from the same location on earth

    When you said this earlier:

    ---------------
    The larger issue here is the revolution, not the curvature. Even in a flat system, an object's left side touches the left horizon and the right side touches the right horizon. If the revolution axis is perpendicular to the ground, the object flips 180 degrees as it passes the zenith. Any other orientation and the object rotates as it traverses the sky, and above 90 degrees inclination it rotates one way while below 90 degrees it rotates the other. But what changed wasn't actually the direction of rotation, but rather that you turned around (to keep the inclination below 90 degrees).

    The dividing line between the two is thus not necessarily at the equator. By convention, you tend to orient yourself so you are facing most of the path it takes through the sky, but that path only bisects the equator twice a month.
    -------------------

    I am sorry but you have totally lost me as to what you are saying there.
     
    Last edited: May 19, 2015
  10. May 19, 2015 #9

    Bandersnatch

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    Let me restate what russ_waters said. Maybe a different angle will help.

    Go to this site:
    http://neave.com/planetarium/
    and find a celestial pole. Make sure constellation outlines are on. Advance time and observe the constellations near the pole - they'll rotate. They'll complete one revolution (360deg) every 24h. They do flip upside down every 12h even though you're staying on the same hemisphere!

    You can see this easily near the poles, since the constellations there stay in a narrow field of vision, but it's also what every other constellation does. It's just that for those near the horizon you can't see them all the time (when they're below the horizon), and you need to turn yourself around to track them over the passing hours. But even so, as you look at any constellation rising over the horizon and then at the same one setting, you'll see that it's not oriented the same way.

    It's the same for the planets and the Moon. The planetarium linked above doesn't feature lunar surface details, but it can show changing lunar phases. The horns of the crescent of the Moon will point towards different direction when it rises than when it sets (you'll find the moon near the horizon from your location). This is what causes the clockwise/counter-clockwise rotation seen on the pictures, so that e.g. Mare Crisium (the nearly circular feature near the limb) appears to be at different 'hour' on the Moon's face. Libration would cause the same Mare Crisium to move more/less towards the centre of the Moon (change latitude and longitude).

    It's called 'field rotation', and a bit of pain in the neck for users of certain kinds of telescopes (with alt-azimuth mounts).

    That Phil Plait's video you linked to, however, factors out such daily observational effects. The rotation you see there is, I think, an additional, but similar effect due to how the Moon climbs and descends in its inclined orbit over the month - but I have to admit I'm not on a very sure footing here.
     
  11. May 19, 2015 #10

    russ_watters

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    If you stand next to a car and watch the wheels as the car moves away, you see the axis pointed at you, with the wheel rolling 360 degrees (and more). That's the rotation in the pictures in the OP.

    If you stand behind a car and it rocks back and forth a half a meter, you see slightly more than half of the tread, facing you (about 59% or +-15 degress). That's libration.

    Notice how the "C" shaped dark area is always pointed down and to the right, whereas in the pictures you posted in the OP, it is sometimes pointed up.
     
    Last edited by a moderator: Apr 16, 2017
  12. May 19, 2015 #11

    Drakkith

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    There's a few issues here. First, the picture of the Gibraltar Moon gives us no indication as to where in the sky the Moon was at the time, so there's no way to know what orientation to use in the first place, as the orientation of the Moon changes as it moves across the sky.

    Second, the orientation of the Moon when viewed from different hemispheres will not necessarily be 180 degrees apart. See the following:

    When the Moon is rising, people at every latitude will be facing approximately the same direction (East) to view the Moon, so the difference in the apparent orientation will be minimal. As the Moon approaches it's highest position in the sky, the difference in the orientation between observers in the northern and southern hemispheres will become more and more pronounced, and when the Moon is highest in the sky observers in the south that are facing north will see the Moon as being upside down compared to an observer in the northern hemisphere looking south. As the Moon begins to set, the difference starts to lessen, and when the Moon is very close to the western horizon all observers will be seeing approximately the same orientation again.

    Note that the side of the Moon that first rises above the eastern horizon is the side that first falls below the western horizon. In other words, when rising, the orientation is around 180 degrees different than it is when the Moon is setting. I say 'around' because things like the observers latitude, the tilt of the Earth's rotational axis, and the time of year will make it so that the orientation changes by less than 180 degrees over the course of the night. You can avert this orientation change by standing on your head/hands while watching the Moon set. Please remember to avoid any alcohol consumption before attempting this highly technical astronomical observing pose, as it can lead to disastrous consequences and copious amounts of laughing. At you.

    Edit: After re-reading your question, I think the key is that the Moon is rising at different points in the year, so the orientation will be slightly different. I'm sure photos taken at different times of the night will also contribute, even if those times are only an hour or so apart. This is especially true during winter months, when the Moon rises at a low latitude. During summer, when the Moon rises essentially right overhead, you don't need to rotate yourself to view the Moon, you just look up further. But during winter the arc the Moon takes through the sky doesn't pass overhead and you have to rotate yourself to keep the Moon in view as it rises. This causes the apparent orientation of the Moon to change. Off the top of my head I'd say the difference in orientation is upwards of 15 degrees per hour, which looks like it could easily explain the differences in your first 2 pictures. (The second picture looks like it was probably taken an hour or two after the first)
     
    Last edited: May 19, 2015
  13. May 19, 2015 #12

    russ_watters

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    Since, the "C" is facing down and to the right, I think we can surmise that the moon is roughly due south....unless the camera was equatorially mounted.

    Otherwise, I agree with everything you said.
     
  14. May 19, 2015 #13
    My reference point for my original comments was the field of tycos seen in this picture swinging from left to right. When i watched the Nasa animation as far as i recall this view of the moon never swung anything like 180 degrees. The maximum swing i saw was about 45 degrees either side of vertical.

    Edit; I can see the comment saying the nasa video has removed the daily rotation which is going to cause the moon to rotate 180 degrees horizon to horizon. I can the comments saying the moon behaves like the stars and appears to rotate around the 'celestial pole'.

    Edit: Presumably just by chance my auckland pictures all showed the field of tycos at the top and the only gibraltar picture i could find showed it at the bottom and then just by chance nasas animation compared with Gibraltar and that has resulted in my confusion..

    These following comments however still stand after my two edits.

    In new zealand the sun and moon rise on the right in the East and move to the left. The view of the moon you see from new zealand is upside down compared to the view of the moon you see from the antipoid point which as far as i knew is around Gibraltar. If a person faces east and watches the moon rise in new zealand necessarily he is standing on his head compared to the person in Gibraltar. I am supposing the person on the opposite side of the earth cannot see the moon rise when it is rising in NZ and instead he sees it setting?

    I am still horribly confused by the comments here. What i need to know is if i can use the moon to demonstrate the world is round by showing at the antipoid point of earth compared to your current location and current time the moons view is upside down when you are down under. It appears you are saying this is possible when the moon is at its highest point.

    Incidently where i was living in NZ it was not possible to see the moon rising or setting against the horizon because of the local hills so the moon was always rotated quite a few degrees by the time i saw it. That might be adding to my confusion.
     
    Last edited: May 20, 2015
  15. May 20, 2015 #14

    Drakkith

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    This is only true if you compare the views when the Moon is at its highest point in the sky at each location. When it is rising, it is far from being upside down for either observer. Offhand I'd guess the difference is about 77 degrees given the difference in latitude between the two locations.

    That's pretty much correct, but the first part doesn't matter since we are talking about the Moon rising at each location. Imagine someone at the same latitude as Gibraltar, but the same longitude as yourself in New Zealand, then imagine both of you looking east to where the Moon is rising. To simplify things let's assume the Moon rises directly over the equator. For the person at Gibraltar's latitude, they have to face a little south of due east to see the Moon, and for yourself in New Zealand you have to look a little north of due east. This means that the orientation of the Moon is different for both of you, but not 180 degrees different.
     
  16. May 20, 2015 #15
    When i place Auckland at the bottom of my globe i find that gibraltar is very near the top of the globe. So one observer must be close to being 'upside down'

    I am talking mainly about creating a proof for round earth, where I became confused by the rotation of the moon where that rotation wobbled some degrees back and forth and it appears this is known as libration. Therefore at the same time the moon rises in NZ it will set i believe in Gibraltar and the views will be the same but inverted?

    By the way my understanding of the moon has evolved tremendously since i started this thread but my need for a proof of round earth via a simple observation of the moon remains so I need to be confidant of my facts.
     
    Last edited: May 20, 2015
  17. May 20, 2015 #16

    Drakkith

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    Just about, yes.

    Yes, it will be close to being completely inverted.
     
  18. May 20, 2015 #17
    Thanks for the replies everybody. Cheers!
     
  19. May 20, 2015 #18

    Bandersnatch

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    One guy made a spreadsheet for calculating position and orientation of the moon:

    (link to the spreadsheet: http://dropcanvas.com/0hn26)
    You can play with the numbers to see how the orientation ('Orientation of illuminated portion relative to horizon' in the document) changes with each passing hour, depending on the time of year and your location.
     
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