1. Abishua

    Abishua Member

    I heard a statement and not sure how to verify it, so asking here. So the statement is that from lets say 40 degrees north lat you can see all the northern constellations from any longitude, but the same is not true if you are at -40 degrees lat.. or southern hemisphere. So at south at that latitude you cannot see all the southern constellations like you can see all the northern ones from the north lat.

    Is this true or bunk?
  2. Henk001

    Henk001 Active Member

    Its bunk.
    The northern and southern hemisphere are completely symmetrical. As soon as you cross the equator you will be able to see the whole southern hemisphere (with the south celestial pole initially on the horizon and all the constellations circling around it).
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  3. Henk001

    Henk001 Active Member

    (Amateur) astronomers all over the world use Stellarium. Try it. Choose any location you like and watch what is visable.
    Don't worry about the fact that it is a computer program. Those amateur astronomers would immediately notice if things were wrong
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  4. Abishua

    Abishua Member

    Yes.. I was just looking at stelarium for the past hour or so.. to put it more clearly, the claim is that from all meridians at the equator simultaneously you can see ursa minor, major and polaris when looking north, but when looking south you cannot see simultaneously from all of them the south pole star and southern cross constellation.
  5. Trailspotter

    Trailspotter Senior Member

    No, it is not possible to see Ursa Major from all meridians at the Equator simultaneously. However, it is possible to see both Ursa Major and Crux (Southern Cross) from the Equator simultaneously, just not all time. I actually have seen them both simultaneously from Amboseli NP, Kenya, 2.5° south of the Equator.
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  6. Mick West

    Mick West Administrator Staff Member

    This type of claim dates back to Samuel Rowbotham, and is repeated by modern Flat Earth promoters. It was very hard to check back in the 1800s, but is very easy to check now.

    Here's an example from Dubay's book, and his supporting quotes:

    Totally wrong. the latitudes a star is visible from depends only on its declination, equivalent to latitude in the celestial sphere (and maybe a couple of degrees due to refraction at the horizon). So Polaris, at 89° can only be viewed down to 2 or 3 degrees below the equator (under ideal viewing conditions).

    Dubay seems to be repeating a misconception that the tilt of the Earth allows the stars to be seen. The tilt of the earth is only relevant to the Solar System, the axis of the the Earth points in a relatively fixed position year to year, and the pole stars are just the stars closest to the point where the axis happens to point.

    Again, the visibility of any star is directly related to the its declination. The star in Ursa Major closest to the celestial North Pole is the amusingly named Dubhe, which has a declination of about 61.5° So from 30° South, at it's highest, Dubhe still fails to rise above the horizon (unless refraction bumps it up a degree or so). Here's the view showing the horizon. This intermittent visibility of (most of ) Ursa Major is entirely expected and very straightforward.
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  7. Henk001

    Henk001 Active Member

    Last edited: May 18, 2017
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  8. Amber Robot

    Amber Robot Member

    Further, it is very easy to show with simple trigonometry that in Dubay's model the elevation angle of Polaris as a function of latitude doesn't agree with reality, disproving his flat earth model. He may mention observability, but he doesn't mention the actual angles they're observed at.
  9. Mick West

    Mick West Administrator Staff Member

    This all related to the "Ground Truth" idea I wrote about here:
    Stellarium matches observed reality. Stellarium shows a round Earth. Case closed.
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  10. Abishua

    Abishua Member

    Thanks for all your responses.. lovely picture Henk! if some people from equator do not provide photographic evidence of this I agree.. case closed.
  11. Rory

    Rory Active Member

    There is another factor in this, which I discovered when challenged by a flat earther that he had evidence that Polaris could be seen from below 3°S - that of elevation.

    It turns out he was right: but the image he was citing was taken from Kilimanjaro, at over 19,000 feet above sea level - an elevation which would give a view, by rough calculations, something similar to what would be seen from about 182 miles, or 2.7 degrees, further north.

    Not sure if there are any mountains further south than that from which Polaris could be seen. All the big ones in Ecuador are a little closer to the equator. And, of course, that still leaves another 86.9 degrees worth of globe from which the north star can't be seen.
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  12. Enricks

    Enricks New Member

    Does refraction affect the visibility? On the wikipedia article about circumpolar stars it reads:

  13. Mick West

    Mick West Administrator Staff Member

    It does, but not a lot. The easiest way to remember and visualize the effect of refraction near the horizon is to note that when the sun first touches the horizon, it has (geometrically) just gone below the horizon. i.e. the refraction at the horizon is (on average) about the angular size of the sun, or half a degree.

    Even that's an oversimplification though, stars are a lot smaller, so when a star stars sets over the ocean (theoretically instantly) it's at the absolute maximum position for refraction, and so it's theoretically possible for it to be visible for a bit more. In practice though if refraction boosts it another degree then it's likely to be lost in the haze anyway.

    Here's a video showing stars seeming to slow down a bit as they get closer to the horizon.

    Source: https://vimeo.com/188149183

    And an analysis:

    Source: https://www.youtube.com/watch?v=m-xXhrTG3Sk
  14. Enricks

    Enricks New Member

    Thank you. So, because of refraction near the horizon, it is possible to see stars which have a lower declination and you cannot theoretically see from your latitude, like stated in the article I quoted?
  15. Mick West

    Mick West Administrator Staff Member

    And a more quantative answer can be found here:

    A range of extreme observations though history has been collected by Andrew Young

    So 1 to 2 degrees happens occasionally, over 2° is very unlikely and there are no really reliable measurements of this. Over 4° can probably be consider to be a hard limit.
  16. Mick West

    Mick West Administrator Staff Member

    Yes it is, but as noted above, only by a few degrees.
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  17. Enricks

    Enricks New Member

    So, applying this to a circumpolar star. You would see Vega as circumpolar between 51° 13' and Cornwall (roughly 50°) even if you shouldn't be able to, because of refraction (like stated in the article).
    As it starts to go up in the sky, its position relatively to the other stars should change, as refraction is no longer there far from the horizon. Conversely, as it starts to go down, its position should change again, appearing on the horizon instead of going down. (to put is simply, its circular path in the sky is not regular).

    Am I right?
    Last edited: Jul 2, 2017
  18. Mick West

    Mick West Administrator Staff Member

    Yes, but if you look at star trail timelapse you'd see this is a pretty small effect over the ocean horizon. You really won't see much with the naked eye - other than stars occasionally being visible a bit past where they should not be.

    In the image below there's a slight flattening of the trails behind the lighthouse as they are raised up very slightly.
    Image source: http://www.photosbykev.com/wordpress/2014/01/18/star-trail-photography/
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  19. Enricks

    Enricks New Member

    The slight flattening on the horizon also happens for non-circumpolar star-trails? If I'm not mistaken, the video you showed me a few post before (the one with the green lines) showed a slight changing in the star trails' path near the horizon. It could be that.
  20. Mick West

    Mick West Administrator Staff Member

    It's a compression of the entire (unrefracted) image near the horizon. It's non-linear, so the closer the horizon you get, the more distorted it is.

    So take this image, imagine these are the perfect geometrical paths of the stars, like you'se see if there was no atmosphere, and the earth was invisible.
    add a horizon

    And approximate the refraction - exaggerated a bit so you can see what is happening.

    The entire portion of the image near the horizon is raised up and compressed a bit. The circumpolar stars (the stars that describe a full circle around the pole without being hidden) have a flattened bottom to their paths, but little to no distortion higher up. The other stars (that set) has a slight deviation to their path - but the steeper the setting angle the less noticeable this is

    Slider comparison:
    20170702-080640-rg373. 20170702-081037-3ermj.
  21. JFDee

    JFDee Senior Member

    Isn't the horizon itself 'elevated' by refraction? I think it should be higher compared to the one in the unrefracted version.
    Last edited: Jul 4, 2017
  22. Mick West

    Mick West Administrator Staff Member

    It is, yes, but less than the stars as it's a lot closer (i.e goes through less atmosphere)
  23. Henk001

    Henk001 Active Member

    I think you could be right considering this schematic drawing:
    The grey line corresponds with the direction of the "non-refracted"horizon; the purple with with the refracted horizon; there is an angle between the two: the refracted horizon appears to be higher. I don't know in what amount though
  24. Enricks

    Enricks New Member

    Does the sun have this deviation too? (ie. the rise/set azimuths are different from the expected ones)
  25. Henk001

    Henk001 Active Member

    Yes. And so does the moon. This is the general idea:
  26. Enricks

    Enricks New Member

    Thanks. So, are the azimuths reported in rise/set charts already adjusted for this?
  27. Mick West

    Mick West Administrator Staff Member

    Generally yes. Remember these charts originally came from ground-based observations, rather than some mathematical model of the Earth and sun viewed from space.