Why has no one seen Neptune with the naked eye?

[Count Iblis]

According to http://en.wikipedia.org/wiki/Apparent_magnitude" 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"

Class 1: Excellent dark-sky site. The zodiacal light, gegenschein, and zodiacal band (S&T: October 2000, page 116) are all visible — the zodiacal light to a striking degree, and the zodiacal band spanning the entire sky. Even with direct vision, the galaxy M33 is an obvious naked-eye object. The Scorpius and Sagittarius region of the Milky Way casts obvious diffuse shadows on the ground. To the unaided eye the limiting magnitude is 7.6 to 8.0 (with effort); the presence of Jupiter or Venus in the sky seems to degrade dark adaptation. Airglow (a very faint, naturally occurring glow most evident within about 15° of the horizon) is readily apparent. With a 32-centimeter (12½-inch) scope, stars to magnitude 17.5 can be detected with effort, while a 50-cm (20-inch) instrument used with moderate magnification will reach 19th magnitude. If you are observing on a grass-covered field bordered by trees, your telescope, companions, and vehicle are almost totally invisible. This is an observer's Nirvana!

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?

 

[nickdk]

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.

 

[hamster143]

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.

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.

 

[sylas]

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.

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

 

[Count Iblis]

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.

 

[Count Iblis]

http://www.tass-survey.org/richmond/answers/mag_limit.html"

For some years Brent Archinal and I have wondered about the naked-eye limiting magnitudes determined by Heber Cutis at Lick Observatory at the turn of the Century. He claimed to see down below mag. 8, but we wondered what the modern standard V magnitudes of those stars were. I've dug out the relevant publication (1901 Lick Obs. Bulletin, 2, 67), and have looked up the stars (fortunately a short and well-identified list). The names are given below along with V and B-V from the Hipparcos/Tycho catalogues, and the magnitude Curtis gave for the same stars. I use the H/T data for convenience; they ought to be reliable to within a couple percent, and comparison with data collected in SIMBAD from ordinary sources suggest these numbers are fine for the present purpose. Curtis's comments are also shown for most of the stars.

As can be seen, the faintest star he saw reliably is V = 8.44, and two others below V=8.0 and one at 7.98 were seen as well. As might be expected, he did rather better overall on the near-overhead field around T UMa than on the T Vir field, several degrees south of the Equator (Lick is at about latitude +37.4). Just that alone tells you a lot about observing other than close to the meridian.

These in essence reproduce the results of Dave Nash at the Nebraska Star Party several years ago, when he did a double-blind test using stars in the head of Draco, and saw down to about V=8.2. On winter and spring nights at our Anderson Mesa site, I use a star in Coma at V=7.8 as a transparency test, and usually see it.

group near T Vir

Star V B-V Curtis remarks
HD 106384 6.56 0.28 6.52 [FG Vir, sl var]
HD 107830 7.19 0.43 7.20 seen easily
HD 105654 7.23 0.40 7.31 seen quite easily
HD 106515 7.34 0.82 7.42 seen easily
HD 106622 7.47 0.93 8.1 seen without difficulty on last two nights
HD 106579 8.44 0.44 8.3 seen with considerable difficulty; perhaps
one-fifth of trials failed

group near T UMa

HD 110275 7.98 0.24 8.1 seen; one or two failures
HD 110408 8.08 0.53 8.2 seen
HD 110104 8.21 1.12 8.3 seen with difficulty
BD+60 1415 8.98 1.35 8.5 glimpsed at intervals; very doubtful