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How much of the night sky one could see from various locations

  1. Mar 14, 2014 #1
    I was wondering about how much of the night sky one could see from various locations on the Earth.

    At any one time, near sea level, how much is visible? My guess is that much less than half is visible, with the rest being blocked from view by the earth. Is that correct? How small a portion are we viewing when we look up?

    And over the course of a year, it seems that different parts of the Earth would see a larger or smaller proportion of the whole. For example, if one were at the north pole, one would see nearly the same portion of the sky every night for half a year, and see nothing the other half due to the sun. If one were on the equator instead, one would see huge new vistas as the Earth traveled around the Sun, depending on the time of year.

    So how small is this "at any one time" portion, and do people at the equator see pretty much everything over the course of a year, or is a significant portion still invisible, even to them? Do people at the equator ever see the North Star?
  2. jcsd
  3. Mar 14, 2014 #2


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    At any one time you see half of the sky. Why do you think it is less? You need to remember that the "celestial sphere" is effectively infinitely far away. Actually, since the Earth is round, if you are a non-zero distance off of the ground, you actually see slightly more than half the sky, as the attached diagram shows.

    If you ignore the sun for the moment, then over the course of one day how much of the sky you see depends on where on the Earth you are, as you said. At the poles, you only see half the sky, while at the equator, you see it all. If you are at the equator, the north star is sitting on the horizon.

    Attached Files:

    • sky.pdf
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  4. Mar 16, 2014 #3
    At sea, you can see slightly more than one half of the celestial sphere... refraction of the atmosphere lets you see slightly below the horizon, neglecting crud in the air between you and whatever you are looking at.
  5. Mar 16, 2014 #4
    Which is not quite negligible.
    suggests 3...4 magnitudes extinction at 4 degrees altitude... which would make Polaris hard to see.
  6. Mar 18, 2014 #5
    Thanks guys. This sort of info is exactly what i was wondering about.
  7. Mar 21, 2014 #6
    Now I'm maybe even more confused. I'm under the impression that the Earth blocks out much of the night sky.

    So if you always see 1/2 of the night sky, then please tell me if this follows:

    If you are standing at the North Pole, and compare what you see with your friend at the equator, then you see all the stars that are directly above his head, and all along the line that forms the equator, and you see all the stars north of that line. Similarly, Polaris is directly above your head, and Polaris is visible to your friend at the Equator.

    I still don't get it. I would think that you could only see stars that are "north" of a plane tangent to the Earth, and that you would not be able to see stars that are directly over the equator. The diagram you attached shows the "Theoretical Horizon" as just such a tangent plane. For values of h equal to a couple of meters, it seems like much of the night sky would be blocked out, much more than half, and that Polaris would NOT be visible at the equator when it is nearly straight up over the north pole.

    I'm sorry if I am being dense. Am I missing something obvious? Is Polaris always visible at the Equator, low on the North horizon? Why isn't it well below the Theoretical Horizon?
    Last edited: Mar 21, 2014
  8. Mar 21, 2014 #7


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    This is true. However, you would also be able to see stars that are on the other side of the Earth that your friend cannot see. Ignoring things like hills, mountains, and trees, you would be able to see Polaris if you stood at the equator. It would be very, very close to the horizon and would stay there as the Earth rotates. You would also be able to see all the way to the southern celestial pole.

    A person standing at the north pole would be able to see everything above the celestial equator all at once, but would never be able to see anything in the southern celestial hemisphere. The reverse is true for someone standing at the south pole.

    You're not factoring in the distance between us and the stars. A plane tangent to the surface of the Earth at the north pole is practically identical to a plane that passes through the equator. For objects close by, such as the Moon, other planets, and asteroids, the difference in the few thousand miles between the two planes is easily noticeable. This is why eclipses only cover part of the Earth instead of the whole thing, and also why the transit of an asteroid across Regulus a few days ago was only visible in parts of the Eastern U.S.

    At the multi-lightyear distances of the stars, this parallax is not easily noticeable. At least not without high precision instruments out in space like the Hipparcos and Gaia space observatories. And even these rely on the much larger change in position that results from our orbit around the Sun.
  9. Mar 21, 2014 #8
    Thanks. Yes indeed, that is what I was failing to realize.
  10. Mar 22, 2014 #9
    I opened my world clock apps clicked "+" and it gave me a globe. Rough estimate of my coordinate is 27N49E. After sundown I could see the Orion overhead, and Canopus low in the south about 10 degrees above the horizon, facing north I could see Polaris about 20 degrees above the horizon and it is in between two constellations.. Cassiopeia at the left and the Big Dipper tail down at the right of Polaris. My android compass from playstore said I'm facing the right direction. Before dawn when the Orion had set I could see the Scorpion in the south eastern sky, and Cassiopeia and Big Dipper switched position, Big Dipper tail up at the left of Polaris and Cassiopeia at the right. I think I have a fair share of night sky though light pollution from oil refineries mess it up :D
  11. Mar 22, 2014 #10
    The other thing that you should consider is that the portion of the entire sky which the sun has influence over (visibly) is quite small. Eg, an hour after the sun goes down, the entire sky is dark enough to see stars in all directions, and vice versa, a hour before the sun comes up you are able to see stars in all directions. So on the equator at around this time (the equinox), you are literally able to view 90% of the entire night time sky in one night! And at my latitude (35deg south) I can see the 90% minus the 35deg either side of the north celestial pole.

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