Discerning planets with the naked eye

DelRioPilot

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Is it at all possible to discern a planet's disk with the naked eye? I'm not talking simply seeing the glowing star-like dot in the sky, but rather actually being able to see the circular shape of the planet itself.

I assume only Venus would be possible?

Also, if you were on Venus (and assuming there were no atmosphere), would it be possible to see the Earth and the moon as two separate points of light? Or would they both blur together?

Kind of random questions. But just something I was thinking about recently.
 

Codz

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Surface details? Extremely unlikely. You might be able to discern the bare disk of something like Jupiter or Venus on an extremely clear night, with no light pollution, and perfect visual acuity. Even then, I'd be highly skeptical.
 

PhantomCruiser

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I can't. But I know Jupiter, Venus and Mars when I see them. Same with Vega, Arcturus and Spica. I just "know" where they are in the sky.

I can see the Galillean moons of Jupiter with my cheap pair of binoculars. But when I see Saturn with them, I can discern the rings, but can tell that it's an oval(ish) blob instead of a round disk like the other planets.
 

Unstung

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Venus can reach an angular diameter of 66 arcseconds in the sky. The human eye with no vision problems can resolve something as small as 1 arcminute, or 60 arcseconds. That may be just enough to discern the disc of Venus at its closest distance to Earth, if an individual could even notice it. Much smaller and Venus would appear as a point light source, not to mention the interfering effects of atmospheric distortion and how glaringly bright Venus is compared to other objects in the night sky. However, without atmospheric distortion, technically Venus' disc is discernible.

Earth is slightly wider than Venus, so its disc could technically be seen too. The moon is considerably smaller than Earth, therefore no disc can be resolved by the eye. The distance between the Earth and the moon is much longer than Earth's diameter, so it is logical that the moon and Earth can be resolved separately from Venus, unlike Pluto and Charon which are really far away and orbit closely to each other. Being able to see the moon at all with the human eye, which I don't know the answer to, depends on its apparent magnitude from Venus, a result of its albedo (lower than Earth) and size.

From Wikipedia:
The maximum angular resolution of the human eye at a distance of 1 km is typically 30 to 60 cm. This gives an angular resolution of between 0.02 to 0.03 degrees, which is roughly 0.6 arcminute per line pair, which implies a pixel spacing of 0.3 arcminute. 20/20 vision is defined as the ability to resolve two points of light separated by a visual angle of one minute of arc.

 
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Quick_Nick

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Venus is so glaringly bright through my 4" telescope that it is hard to see the disc even then. And my eyes have certainly never discerned any details of the ISS!
 

Artlav

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Hm.
That brings another question - is it possible to see the Galillean moons of Jupiter with naked eye?
I remember reading about such a claim in some 19th century novel, and it was treated as something close to extraordinary.
 

PhantomCruiser

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It seems to me that if I squint my eyes "just so" that I think I can make them out. But I could just be imagining something that I know is there to see.

I remember how elated my daughter was when she first saw them. It was a pretty cool Daddy-Daughter moment.
 

garyw

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Hm.
That brings another question - is it possible to see the Galillean moons of Jupiter with naked eye?
I remember reading about such a claim in some 19th century novel, and it was treated as something close to extraordinary.

It might have been possible in the 19th century. No street lights or much in the way of pollution and if you were at sea with no moon the night sky would be spectacular.

This left image shows what we should see, the right shows what we can see thanks to light and other pollutants:

 

Unstung

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It might have been possible in the 19th century. No street lights or much in the way of pollution and if you were at sea with no moon the night sky would be spectacular.
The Galilean moons are around magnitude 5, and the human eye can see down to magnitude 8 under "perfect" sky conditions. However, the references mention that naked eye viewing isn't possible because the moons "are close to one of the brightest objects in the sky, so [they] tend to be washed out by Jupiter's brilliance," therefore "f it were not for their proximity to Jupiter, all the Galilean satellites would be visible to the naked eye".

These are simple Google searches. Come on, people.
 

MattBaker

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I have to say I disagree with that chart.
On a clear pass of the ISS nearly directly above me (magnitudes around -3 to -3.5) I can, I don't want to say clearly but noticeable, make out a double-T shape.
And since that was done with my old glasses which are 0.5 dipoters too weak I can say I could make it out with no 20/20 vision but slight short-sightedness.
 
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Urwumpe

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I can, I don't want to say clearly but noticeable, make out a double-T shape.

You really can - or do you just believe that you can see something, that isn't there, but added to the visual by your mind?
 

MattBaker

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You really can - or do you just believe that you can see something, that isn't there, but added to the visual by your mind?

The first time I noticed it I was like "Hey, that isn't a dot anymore, is it?". So I guess it should be possible.

Although I have to note that this was mostly done before I moved to a big city so a few months ago. And heavens-above.com puts the ISS 10 kilometers lower at that point.
I don't know how much 10 kilometers would matter, especially since I can't remember if they were apogee/perigee or something in between passes.

---------- Post added at 02:20 PM ---------- Previous post was at 02:18 PM ----------

Tomorrow would be such a pass but it's mostly cloudy here...
 

Urwumpe

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The first time I noticed it I was like "Hey, that isn't a dot anymore, is it?". So I guess it should be possible.

Although I have to note that this was mostly done before I moved to a big city so a few months ago. And heavens-above.com puts the ISS 10 kilometers lower at that point.

Well, the problem is: You already knew that it was the ISS.

Also, you think that the solar arrays had been visible to you in a clear double-T shape, but: The solar arrays are perpendicular to the sun. Can you really be sure that the ISS will appear like that what you expected?

And then remember: The best seeing disk diameter is 0.4 arcseconds and only possible on few high places in the world - anything smaller than 0.4 arcseconds will arrive a blob of light that is 0.4 arcseconds wide - regardless of the optics that then further add errors.

As example:


And that has a VERY favorable condition for seeing the solar arrays, since a HTV is about to dock to the ISS. (With the solar arrays turned horizontal and thus perpendicular to an observer on Earth)

The ISS is about 150 times bigger than that ideal, but if you are watching the ISS for example from turbulent european lower altitude, the ISS will only be about 10 times bigger than the seeing disk and the solar arrays already even with a telescope impossible to tell apart.

(I am already happy to notice that orange color of the ISS)
 
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T.Neo

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IIRC it is speculated that it may be possible for those with good eyesight to make out the crescent of Venus, there are various early descriptions of Venus, or deities associated with Venus, being 'horned'.
 
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