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Celestial sphere and visible stars

  1. Mar 11, 2012 #1
    OK, maybe this is a silly question but, if you look at the sky in a program like Stellarium say, and then look north, why can you see stars at points on the celestial sphere corresponding to points on the other side of earth from where you are?

    I hope this question is clear, what I mean is the celestial sphere is a projection of each point on the earth outwards to an imaginary sphere at infinity; lets say you're 40 degrees north and on the prime meridian, then you look northward into the sky, why then projections from 180 degrees longitude and 20 degrees lat visible to you (so 20 degrees ascension...but projected from 180deg around long wise)

    I would have thought the pole star would have appeared just above my horizon as I looked north, not high up, and I wouldn't have imagined I would have been able to see any stars corresponding to points on earth that are 180 degrees around (longitude) from me...

    hope that makes sense...maybe it has something to do with the projection being infinite distance away..
    Last edited: Mar 11, 2012
  2. jcsd
  3. Mar 12, 2012 #2
    In theory you can see 180 degrees of the star latitude and longitude, which means someone on the equator could see 100% of the night sky over a year and someone at the polls could see the same 50% of the night sky every day.

    The reality though is that seeing and other obscuring factors get worse as you approach the horizon, so you can never truly see an entire hemisphere of the celestial sphere, though you can view pretty close to that in some locations.

    If you are in the Northern Hemisphere, the poll star (polaris) will appear at the same degrees above the ground in the northward direction as your latitude. That means that on the poll it will be overhead, at 40 degrees it will be 40 degrees above the horizon, and at the equator it will be just on the horizon.
  4. Mar 12, 2012 #3


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    In the program the stars should all corrospond to how they look from where you are on Earth as long as you set the location correctly. The North star appears about 40 degrees high in the sky from a latitude of 40 degrees north. Over the course of the night, you can see over 75% of it as the Earth rotates depending on your latitude. The positions of the stars at any specific time will vary over the year as the Earth moves around the Sun. So a star that appears at zenith at midnight tonight will not appear at zenith at midnight a month from now.
  5. Mar 12, 2012 #4
    Maybe attached diagram will make my question clearer.

    If you look at north star in say stellarium and put on the celestial grid, then you can tilt your head downwards and see 80 deg, 70 deg...20 deg ....why? (I'm not talking about seeing them if you look at the north star and tilt your head upwards, that is obvious, as light can easily reach your location from these points...) Hopefully the diagram will make what I mean clear...

    Also I've attached a screenshot from the program: e.g why can I see Lacerta? from my location on earth at around 40deg lat, 0 long, when lacerta's position in celestial sphere is projected from a point on earth that is something like 30-40 deg lat, and 180 deg long...naively one would not expect to see this (projections from 20-40 deg lat and 0 deg long, I am of course happy with...as they are points on celestial sphere projected from near me on earth so obv I will be able to see these)

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  6. Mar 12, 2012 #5
    I guess the reason is the same reason that someone on the equator can actually see polaris; namely the sphere is really not a finite radius but infinite radius (so the two lines become "parallel" in some sense). It seems obvious when I think of it like this, and there is northing special about a point on the equator after all)
  7. Mar 12, 2012 #6


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    I think I understand. Imagine you are on the North Pole. You can see everything from 0 degrees and up all the time. Now start moving south and you will see your view change. At 45 degrees approximately half of the sky that you used to be able to see is blocked by the Earth, but another half has become visible. Lacerta simply falls into that area of the sky that is still visible. If you were to move another 10-15 degrees south then Lacerta would no longer be visible as it would be under the horizon.

    In your diagram you would need to make the circle MUCH bigger, as objects in space are so far away that for all intents and purposes of the conversation we can consider them an infinite distance away.
  8. Mar 14, 2012 #7
    I see it gives FOV as 196 degrees (Stellarium.)
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