<p>This depends on eye biology, so there's no purely astronomical answer.  I'll note that Venus is just about capable of casting a shadow (at greatest elongation, with ideal conditions) so let's say that a magnitude -5 object is at about the biological limit of casting a shadow.</p>
<p>The sun has a magnitude -27, so that is 22 magnitudes brighter than Venus.  5 magnitudes is a factor of 100 in brightness, so 20 magnitudes is a factor of 100000000 (100 million)  and 22 magnitudes is about 600 million.</p>
<p>But light follows an inverse square law, The square root of 600 million is about 25000.  So at about 25000 times the distance of Earth, or 0.4 light-years, the sun would be as dim as Venus at its brightest and perhaps marginally able to cast a shadow for someone with acute vision, on a good matt white surface. This puts our astronaut in the Oort cloud.</p>
<p>There's a good deal of uncertainty in this estimate, based on eye acuity. So let's say &quot;something between 0.1 and 1 light-years&quot;</p>

CC BY-SA 4.0
2020-07-16T14:03:35.463Z
I am talking of someone (a human) who sees well, or someone who doesn't but is wearing glasses or contact lenses. Thus, the difference can't be 0.1-1 ly. I think 0.4-0.5 ly would be a good guess.
Ioannes
CC BY-SA 4.0
2020-07-16T14:09:42.350Z
The difference is a shadow which everyone agrees is visible and one which you can kind of sort of see, if you look at it sideways, The outer parts of the eye have more rod cells that are sensitive to lower levels of light. Psycologically it is harder for people to see something that is only visible when they don't look at it.  My initial estimate of "-5" could easily be -4 or -7" and this has a big effect on the final result.  I'm willing to guess "between 0.1 and 1 light-year, depending on biological and psychological factors."
James K
CC BY-SA 4.0
2020-07-16T14:50:59.447Z
Or in other words, within 0.1 ly everyone would see their shadow and beyond 1 ly noone would recognize any.
Ioannes
CC BY-SA 4.0
2020-07-16T13:08:56.193Z
2020-07-16T13:08:56.193Z
James K
<p>How far from the Sun would a (dwarf) planet have to be so that the Sun is dim enough that it doesn't cast a shadow when you're standing on the planet? What magnitude is the border in brightness beyond which a shadow isn't visible anymore?</p>

CC BY-SA 4.0
2020-07-16T08:14:50.833Z
A shadow is _always_ cast unless there's something that's blocking the light. Another question is that the shadows from a dim light source are dim, too, but that's just a matter of your perception abilities. Even the farthest visible light sources like distant stars and galaxies do cast shadows.
Yellow Sky
CC BY-SA 4.0
2020-07-16T09:08:50.113Z
@YellowSky You know that's not what I'm talking about.
Ioannes
CC BY-SA 4.0
2020-07-16T09:41:41.860Z
Your question lacks focus and needs more detail. I can well read what you wrote: _the Sun is dim enough that it **doesn't cast a shadow**_, and _a shadow isn't cast anymore_, so I tell you that shadow is _always_ cast, the distance from and the brightness of the light source doesn't influence whether the shadow is cast or not. How can anyone know what degree of contrast is enough for _you_ to consider it a shadow? It's very subjective and opinion-based. Can you specify what is "a shadow" for you? Can you give any _objective_ criteria?
Yellow Sky
CC BY-SA 4.0
2020-07-16T10:34:04.340Z
@YellowSky Next time Tau Ceti is in the sky, take an image of your shadow cast by Tau Ceti. If no shadow is visible, that's what I mean.
Ioannes
CC BY-SA 4.0
2020-07-16T10:52:06.007Z
Unfortunately, I'm on Earth now, the atmosphere of the planet will block and disperse a substantial part of the light of Tau Ceti. Besides, there are many light sources in the sky here that are much brighter than Tau Ceti, so their light will compensate the shadows from Tau Ceti and make them indiscernible in the array of superimposed shadows and lit areas produced by the other light sources.
Yellow Sky
CC BY-SA 4.0
2020-07-16T11:28:35.977Z
@YellowSky Then travel to a rogue atmosphereless planet midway between the Sun and Sirius and there try again, please. If you don't see your shadow cast by Tau Ceti that's the sense of the question. :-)
Ioannes
CC BY-SA 4.0
2020-07-16T12:55:18.463Z
@Greenhorn also visible to what? The human eye? A camera? Besides the already mentioned light scattering, the recording device is also of importance. You can still very much see a terminator and shadows being cast on the New Horizons images of Pluto and Charon.
Adwaenyth
CC BY-SA 4.0
2020-07-16T14:01:16.817Z
@Adwaenyth The human eye or a usual hand camera, I think they're similar in recognizing any shadows.
Ioannes
CC BY-SA 4.0
2020-07-16T14:54:48.623Z
@Greenhorn   please don't be obstinate.  You have the germ of an interesting question here,but we're trying to point out that several parameters have to be well-defined before we can calculate an answer.
Carl Witthoft
CC BY-SA 4.0
2020-07-16T15:00:20.777Z
@CarlWitthoft Well, my talk with YellowSky is outdated since I edited my question in the meantime. The question is how far from the Sun a shadow is recognizeable for someone on a planet. One is free to provide answers including different factors (like atmospheric scattering, looking through an infrared camera or different levels of sharpness).
Ioannes
CC BY-SA 4.0
2020-07-18T20:19:11.330Z
Greenhorn, on: possibly some QM could say a number for you, for example if we have not enough photon to have a shadow, then we have no shadow. This number will be big and probably unusable for you. Greenhorn, off: fusion is not permanent also in the "normal" stars. But I voted to accept :-)
peterh
CC BY-SA 4.0
2020-07-19T05:04:32.907Z
@peterh-ReinstateMonica Yeah but with "permanent" I mean "until the star goes supernova or ejects a planetary nebula". Brown dwarfs don't do this, they cool down only. But they _do_ start nuclear fusion though not of hydrogen but deuterium and more massive ones lithium.
Ioannes
<the-sun><light><distances>
At what distance wouldn't the Sun cast a visible shadow anymore?
Ioannes
# At what distance wouldn't the Sun cast a visible shadow anymore?

(Asked by: username_0 on 2020-07-16)

How far from the Sun would a (dwarf) planet have to be so that the Sun is dim enough that it doesn't cast a shadow when you're standing on the planet? What magnitude is the border in brightness beyond which a shadow isn't visible anymore?

**Tags:** the-sun, light, distances
> **Comments:**
> **username_1**: A shadow is _always_ cast unless there's something that's blocking the light. Another question is that the shadows from a dim light source are dim, too, but that's just a matter of your perception abilities. Even the farthest visible light sources like distant stars and galaxies do cast shadows.  
> **username_0**: @YellowSky You know that's not what I'm talking about.  
> **username_1**: Your question lacks focus and needs more detail. I can well read what you wrote: _the Sun is dim enough that it **doesn't cast a shadow**_, and _a shadow isn't cast anymore_, so I tell you that shadow is _always_ cast, the distance from and the brightness of the light source doesn't influence whether the shadow is cast or not. How can anyone know what degree of contrast is enough for _you_ to consider it a shadow? It's very subjective and opinion-based. Can you specify what is "a shadow" for you? Can you give any _objective_ criteria?  
> **username_0**: @YellowSky Next time Tau Ceti is in the sky, take an image of your shadow cast by Tau Ceti. If no shadow is visible, that's what I mean.  
> **username_1**: Unfortunately, I'm on Earth now, the atmosphere of the planet will block and disperse a substantial part of the light of Tau Ceti. Besides, there are many light sources in the sky here that are much brighter than Tau Ceti, so their light will compensate the shadows from Tau Ceti and make them indiscernible in the array of superimposed shadows and lit areas produced by the other light sources.  
> **username_0**: @YellowSky Then travel to a rogue atmosphereless planet midway between the Sun and Sirius and there try again, please. If you don't see your shadow cast by Tau Ceti that's the sense of the question. :-)  
> **username_2**: @Greenhorn also visible to what? The human eye? A camera? Besides the already mentioned light scattering, the recording device is also of importance. You can still very much see a terminator and shadows being cast on the New Horizons images of Pluto and Charon.  
> **username_0**: @Adwaenyth The human eye or a usual hand camera, I think they're similar in recognizing any shadows.  
> **username_3**: @Greenhorn   please don't be obstinate.  You have the germ of an interesting question here,but we're trying to point out that several parameters have to be well-defined before we can calculate an answer.  
> **username_0**: @CarlWitthoft Well, my talk with YellowSky is outdated since I edited my question in the meantime. The question is how far from the Sun a shadow is recognizeable for someone on a planet. One is free to provide answers including different factors (like atmospheric scattering, looking through an infrared camera or different levels of sharpness).  
> **username_4**: Greenhorn, on: possibly some QM could say a number for you, for example if we have not enough photon to have a shadow, then we have no shadow. This number will be big and probably unusable for you. Greenhorn, off: fusion is not permanent also in the "normal" stars. But I voted to accept :-)  
> **username_0**: @peterh-ReinstateMonica Yeah but with "permanent" I mean "until the star goes supernova or ejects a planetary nebula". Brown dwarfs don't do this, they cool down only. But they _do_ start nuclear fusion though not of hydrogen but deuterium and more massive ones lithium.  

---

## 1 answers

### Answer by username_5 on 2020-07-16. Score: 5 | Accepted answer

This depends on eye biology, so there's no purely astronomical answer. I'll note that Venus is just about capable of casting a shadow (at greatest elongation, with ideal conditions) so let's say that a magnitude -5 object is at about the biological limit of casting a shadow.

The sun has a magnitude -27, so that is 22 magnitudes brighter than Venus. 5 magnitudes is a factor of 100 in brightness, so 20 magnitudes is a factor of 100000000 (100 million) and 22 magnitudes is about 600 million.

But light follows an inverse square law, The square root of 600 million is about 25000. So at about 25000 times the distance of Earth, or 0.4 light-years, the sun would be as dim as Venus at its brightest and perhaps marginally able to cast a shadow for someone with acute vision, on a good matt white surface. This puts our astronaut in the Oort cloud.

There's a good deal of uncertainty in this estimate, based on eye acuity. So let's say "something between 0.1 and 1 light-years"

> **Comments:**
> **username_0**: I am talking of someone (a human) who sees well, or someone who doesn't but is wearing glasses or contact lenses. Thus, the difference can't be 0.1-1 ly. I think 0.4-0.5 ly would be a good guess.  
> **username_5**: The difference is a shadow which everyone agrees is visible and one which you can kind of sort of see, if you look at it sideways, The outer parts of the eye have more rod cells that are sensitive to lower levels of light. Psycologically it is harder for people to see something that is only visible when they don't look at it.  My initial estimate of "-5" could easily be -4 or -7" and this has a big effect on the final result.  I'm willing to guess "between 0.1 and 1 light-year, depending on biological and psychological factors."  
> **username_0**: Or in other words, within 0.1 ly everyone would see their shadow and beyond 1 ly noone would recognize any.  

---

