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Why has no one seen Neptune with the naked eye?

  1. Dec 11, 2009 #1
    According to http://en.wikipedia.org/wiki/Apparent_magnitude" [Broken] Neptune could be visible to the naked eye.

    Neptune's maximum brightness is magnitude 7.78 while the faintest star known to be observed with the naked eye is magnitude 7.72. But that star was only observed as a guide to observe the galaxy M81 by the naked eye by Brian skiff. So, it is not a hard limit at all.

    http://www.skyandtelescope.com/resources/darksky/3304011.html" [Broken]
    So, it seems to me that Neptune is just within reach of the naked eye. However, it seems that no one has actually seen it with the naked eye. Why not? :confused:
    Last edited by a moderator: May 4, 2017
  2. jcsd
  3. Dec 12, 2009 #2
    Interesting question and good logic. However, Neptune is big but it is really far away. As far as the values for apparent magnitude go, they are approximates, as the Wikipedia article states "These are only approximate values at visible wavelengths". Seeing that the limiting magnitude for the visible eye is around 7.6 ({approximate} and with effort) and Neptune has a 7.78 apparent magnitude ({approximate}), that's a .18 difference, with approximations - not much. My guess is its just barely out of our unaided eye viewing. Small binoculars may work though. Additionally, light pollution could play into our inability to see Neptune with the unaided eye.
  4. Dec 12, 2009 #3
    7.6 limit is an extreme, one-in-a-million limit that can be achieved by a person with excellent visual acuity in perfect conditions (perhaps at sea 200 km from the nearest coast, on a moonless night in a cloudless sky). An average person is not likely to see beyond magnitude 4 in the cities and beyond magnitude 6 in the country.

    Notice that the table gives maximum apparent magnitudes of 5.1 and 5.5 for Uranus and Vesta, considerably brighter than Neptune's 7.78, and even those weren't discovered before the age of telescope.

    Here's another reason. Do you know how many objects of magnitude less than 7.6 are there in the sky? The answer is, approximately 30 thousand! It is relatively easy to notice "the big 5" and to identify them as planets, because there are only two stars in the whole sky brighter than Saturn at opposition. But, unless you have photographic memory or you make detailed star charts for a living, you won't be able to recognize Uranus, Vesta, and Neptune as planets, at best you'll notice them and consider them regular stars.
    Last edited: Dec 12, 2009
  5. Dec 12, 2009 #4


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    Interesting question! I had a hunt around, and found this... it seems that Brian Skiff has attempted to view Neptune, and not succeeded. Here's an email: [AZ-Observing] First attempt at Neptune naked-eye, Brian Skiff, 6 Jul 2005:
    I had the opportunity tonight to spend some time looking at the field including Neptune in Capricornus. I first plotted the planet on the Tirion atlas then looked quickly with 6x30mm binoculars to see that this was correct. The planet is well placed in the sense of being uncrowded and within an asterism of convenient stars so that further reference to the atlas was unnecessary.

    In brief, I couldn't see it. A star presently not far southeast of the planet, HD 202890, was visible on numerous occasions. This is a K0 giant with V=6.9. I also got more intermittent glimpses of nearby 31 Cap, at V=7.1. Neptune, at V=7.7, I think won't be visible from this latitude until it gets north of the Equator. Gonna have to wait awhile! It ought to be straightforward with patience from Chile or elsewhere in the south.

    Cheers -- sylas
  6. Dec 12, 2009 #5
    Sylas, thanks for digging this up!

    Hamster, I agree that people would certainly not have noticed Neptune without actively searching for it. However, there are many amateur astronomers who try to spot faint objects with the naked eye.
  7. Dec 28, 2009 #6

    Last edited by a moderator: Apr 24, 2017
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