<p>It's true that planets usually don't flicker, or twinkle (or scincillate, as astronomers like to call it). The reason is that they are close enough that they are actually seen as a disk with a larger diameter than the atmosphere can "wash out" (the <em>seeing disk</em>). Stars, on the other hand, are point sources, and this point jumps back and forth on the sky, as the light takes slightly different paths through the atmosphere. This is what we see as twinkling.</p>

<p>I can think of three reasons that may make a planet twinkle nevertheless:</p>

<ol>
<li><p>If it stands very close to the horizon, so that the total column of air that the light passees (the "airmass") is very high. If this is the case, it should also look a bit redder than usual.</p></li>
<li><p>If observed close to some building that emits heat, like an exhaust, so that the light passes through parcels of hot and cold air on small scales.</p></li>
<li><p>If an airplane passed by, emitting hot air.</p></li>
</ol>

CC BY-SA 3.0
2015-04-13T18:56:09.553Z
The planet at an elevation of about 20-25 degrees; would that be enough? It wasn't redder than usual, though. Items 2 or 3 are for sure not true in this case.
TomCho
CC BY-SA 3.0
2015-04-13T20:46:55.743Z
Hmm… at that elevation, the air mass is 2-3. Pretty high for astronomical standards, but the angular size of Venus should still be larger than the seeing disk. The only other reason I can think of is small patches of clouds. This could make its brightness vary, which could perhaps be perceived as flickering.
pela
CC BY-SA 3.0
2015-04-13T18:19:25.827Z
2015-04-13T18:19:25.827Z
pela
<h3>Venus (and Mercury) are highly <em>predisposed</em> to twinkling seen from Earth!</h3>
<p>The other answers try to explain based on atmospherics alone, without stopping to consider the following:</p>
<p>While the outer planets (Mars and Jupiter et al.) are always almost fully illuminated, the <em>inner planets</em> Mercury and Venus are often crescents and so at least in one direction -- parallel to the ecliptic -- they can have nearly stellar sizes of only a few arcsecs.</p>
<p>This means that at these times <strong>we should expect them to twinkle</strong> unless observing under incredibly good conditions. And of course they are thinnest when closest to the Sun and so will be low to the horizon after sunset when their light travels through an enormous amount of air and temperatures are changing rapidly, so sort-of maximum convection/turbulence.</p>
<p>For more on how Venus's dance changes distance from Earth and illuminated fraction in such a way that it's almost always between -3.8 and -4.8 magnitude, see answers to</p>
<ul>
<li><a href="https://astronomy.stackexchange.com/q/33797/7982">Venus&#39; magnitude during inferior conjunction</a></li>
</ul>
<hr />
<p><a href="https://i.sstatic.net/23E6B.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/23E6B.jpg" alt="Phases of Venus, from https://commons.wikimedia.org/wiki/File:Phases_Venus.jpg" /></a></p>
<p><strong>above:</strong> Phases of Venus, originally from <a href="https://commons.wikimedia.org/wiki/File:Phases_Venus.jpg" rel="nofollow noreferrer">Wikimedia</a> but annotated.</p>
<p>For three times in 2004 (Feb 27, May 7, and June 8) I've taken the angular diameters of Venus from <a href="https://ssd.jpl.nasa.gov/horizons/app.html#/" rel="nofollow noreferrer">JPL's Horizons</a> and using the largest (June 8) as a calibration for the image shown the full diameter of Venus.</p>
<p>Then I've drawn a thin rectangle to eyeball the width of the bright part of the limb that would contribute most of the light. The annotated numbers (15/5, 40/3, 58/1) are the apparent diameters of the full disk and the width of the bright limb <em>in arcseconds</em>.</p>

CC BY-SA 4.0
2022-11-07T07:09:05.263Z
And what was the phase on 12 April 2015 when the twinkling occurred?
ProfRob
CC BY-SA 4.0
2022-11-07T09:37:01.183Z
@ProfRob I don't care. I simply added a supplemental answer to *Why does Venus flicker?* which is essentially *Because it's an inner planet*. All the other stuff applies to outer planets as well, but I have a strong hunch that if Jupiter were sitting right next to Venus on 12 April 2015 it would not have twinkled to the same extent.
uhoh
CC BY-SA 4.0
2022-11-07T09:41:51.657Z
I think Venus was about 75% illuminated at the time, with an angular size of 15 arcseconds.
ProfRob
CC BY-SA 4.0
2022-11-07T09:46:01.723Z
@ProfRob oic then if the OP saw twinkling then at that moment it for all the other reasons and it wasn't specifically related to it being an inner planet. Still what's discussed here is relevant to future readers, if not to the folks in 2015.
uhoh
CC BY-SA 4.0
2022-11-07T10:56:58.643Z
@ProfRob so I've just asked [Has "crescent-twinkling" even been demonstrated or at least calculated/predicted? Any "twinkleometer" data for Venus out there?](https://astronomy.stackexchange.com/q/50983/7982)
uhoh
CC BY-SA 4.0
2022-11-07T15:49:53.083Z
If you have posted that question, then how reliable is this answer?  Is it in fact a hypothesis? What is the justification/explanation for why a limited angular size in one dimension would lead to more scintillation?
ProfRob
CC BY-SA 4.0
2022-11-07T21:53:55.687Z
@ProfRob Optics is optics. The same localized random phenomenon in the upper atmosphere that makes an object of ~arcsec extent in 2 dimensions twinkle must also make an object of ~arcsec extent in 1 dimension twinkle, though by a somewhat lesser amount. This is inescapable physics and is not open to debate. What I've asked is if anyone has measured it and/or *formally predicted it* in a rigorous and peer-reviewed way. I've added the [resource-request] tag to the question to make that clearer.
uhoh
CC BY-SA 4.0
2022-11-07T03:50:45.253Z
2022-11-07T03:50:45.253Z
uhoh
<p>Because planets actually do twinkle. Most people were told that the major difference between stars and planets is that only the former twinkle - but that's an oversimplification. Given the right conditions, planets will twinkle too, it just happens more rarely.</p>

<p>Several factors that contribute to it:</p>

<ul>
<li><p>lots of air turbulence; or, as astronomers call it, "bad seeing"</p></li>
<li><p>closeness to horizon; if the planets are high in the sky, the air column is shorter so there's less chance they will twinkle; but when they are low, their light goes through more air and so it is perturbed to a larger degree</p></li>
</ul>

<p>The observation you've made, Venus twinkling, is not very unusual. Many stargazers are used to seeing that once in a while. I've seen Venus scintillate several times in the past, always at sunset when it was about to drop below horizon; I would presume you could see the same behavior very early in the morning as Venus has just risen.</p>

CC BY-SA 3.0
2015-09-17T17:18:38.033Z
2015-09-17T17:18:38.033Z
Florin Andrei
<p>I was watching Venus with the naked eye yesterday at about 7 pm and I noticed that it was flickering, almost like a star.</p>

<p>I have always learned that planets don't flicker to the naked eye, only stars (indeed, every other time I've seen a planet it wasn't flickering), so I was rather confused. I even checked Stellarium to make sure that what I was seeing was, in fact, Venus.</p>

<p>Does anybody have an explanation for that behaviour?</p>

<p>Thank you all.</p>

CC BY-SA 3.0
2016-02-03T00:55:33.350Z
Personally I've seen Mars scintillate almost every time I've watched it, so I can tell you that planets are capable of flickering. In Mars' case it's more of a change in color than a change in brightness; I'm not sure why. I've never seen Venus scintillate, but then I don't go looking for Venus very much at all. I've never seen Jupiter or Saturn scintillate though, probably because I only notice them when they're high in the sky at night.
DrZ214
CC BY-SA 3.0
2015-04-13T19:58:39.763Z
Very poor seeing would do it. http://en.wikipedia.org/wiki/Astronomical_seeing
asawyer
<observational-astronomy><amateur-observing><venus>
Why does Venus flicker?
TomCho
# Why does Venus flicker?

(Asked by: username_0 on 2015-04-13)

I was watching Venus with the naked eye yesterday at about 7 pm and I noticed that it was flickering, almost like a star.

I have always learned that planets don't flicker to the naked eye, only stars (indeed, every other time I've seen a planet it wasn't flickering), so I was rather confused. I even checked Stellarium to make sure that what I was seeing was, in fact, Venus.

Does anybody have an explanation for that behaviour?

Thank you all.

**Tags:** observational-astronomy, amateur-observing, venus

> **Comments:**
> **username_2**: Very poor seeing would do it. http://en.wikipedia.org/wiki/Astronomical_seeing  
> **username_1**: Personally I've seen Mars scintillate almost every time I've watched it, so I can tell you that planets are capable of flickering. In Mars' case it's more of a change in color than a change in brightness; I'm not sure why. I've never seen Venus scintillate, but then I don't go looking for Venus very much at all. I've never seen Jupiter or Saturn scintillate though, probably because I only notice them when they're high in the sky at night.  

---

## 3 answers

### Answer by username_6 on 2015-09-17. Score: 10 | Accepted answer

Because planets actually do twinkle. Most people were told that the major difference between stars and planets is that only the former twinkle - but that's an oversimplification. Given the right conditions, planets will twinkle too, it just happens more rarely.

Several factors that contribute to it:

* lots of air turbulence; or, as astronomers call it, "bad seeing"
* closeness to horizon; if the planets are high in the sky, the air column is shorter so there's less chance they will twinkle; but when they are low, their light goes through more air and so it is perturbed to a larger degree

The observation you've made, Venus twinkling, is not very unusual. Many stargazers are used to seeing that once in a while. I've seen Venus scintillate several times in the past, always at sunset when it was about to drop below horizon; I would presume you could see the same behavior very early in the morning as Venus has just risen.


---

### Answer by username_3 on 2015-04-13. Score: 5

It's true that planets usually don't flicker, or twinkle (or scincillate, as astronomers like to call it). The reason is that they are close enough that they are actually seen as a disk with a larger diameter than the atmosphere can "wash out" (the *seeing disk*). Stars, on the other hand, are point sources, and this point jumps back and forth on the sky, as the light takes slightly different paths through the atmosphere. This is what we see as twinkling.

I can think of three reasons that may make a planet twinkle nevertheless:

1. If it stands very close to the horizon, so that the total column of air that the light passees (the "airmass") is very high. If this is the case, it should also look a bit redder than usual.
2. If observed close to some building that emits heat, like an exhaust, so that the light passes through parcels of hot and cold air on small scales.
3. If an airplane passed by, emitting hot air.

> **Comments:**
> **username_0**: The planet at an elevation of about 20-25 degrees; would that be enough? It wasn't redder than usual, though. Items 2 or 3 are for sure not true in this case.  
> **username_3**: Hmm… at that elevation, the air mass is 2-3. Pretty high for astronomical standards, but the angular size of Venus should still be larger than the seeing disk. The only other reason I can think of is small patches of clouds. This could make its brightness vary, which could perhaps be perceived as flickering.  

---

### Answer by username_4 on 2022-11-07. Score: 2

### Venus (and Mercury) are highly *predisposed* to twinkling seen from Earth!

The other answers try to explain based on atmospherics alone, without stopping to consider the following:

While the outer planets (Mars and Jupiter et al.) are always almost fully illuminated, the *inner planets* Mercury and Venus are often crescents and so at least in one direction -- parallel to the ecliptic -- they can have nearly stellar sizes of only a few arcsecs.

This means that at these times **we should expect them to twinkle** unless observing under incredibly good conditions. And of course they are thinnest when closest to the Sun and so will be low to the horizon after sunset when their light travels through an enormous amount of air and temperatures are changing rapidly, so sort-of maximum convection/turbulence.

For more on how Venus's dance changes distance from Earth and illuminated fraction in such a way that it's almost always between -3.8 and -4.8 magnitude, see answers to

* [Venus' magnitude during inferior conjunction](https://astronomy.stackexchange.com/q/33797/7982)

---

[![Phases of Venus, from https://commons.wikimedia.org/wiki/File:Phases_Venus.jpg](https://i.sstatic.net/23E6B.jpg)](https://i.sstatic.net/23E6B.jpg)

**above:** Phases of Venus, originally from [Wikimedia](https://commons.wikimedia.org/wiki/File:Phases_Venus.jpg) but annotated.

For three times in 2004 (Feb 27, May 7, and June 8) I've taken the angular diameters of Venus from [JPL's Horizons](https://ssd.jpl.nasa.gov/horizons/app.html#/) and using the largest (June 8) as a calibration for the image shown the full diameter of Venus.

Then I've drawn a thin rectangle to eyeball the width of the bright part of the limb that would contribute most of the light. The annotated numbers (15/5, 40/3, 58/1) are the apparent diameters of the full disk and the width of the bright limb *in arcseconds*.

> **Comments:**
> **username_5**: And what was the phase on 12 April 2015 when the twinkling occurred?  
> **username_4**: @username_5 I don't care. I simply added a supplemental answer to *Why does Venus flicker?* which is essentially *Because it's an inner planet*. All the other stuff applies to outer planets as well, but I have a strong hunch that if Jupiter were sitting right next to Venus on 12 April 2015 it would not have twinkled to the same extent.  
> **username_5**: I think Venus was about 75% illuminated at the time, with an angular size of 15 arcseconds.  
> **username_4**: @username_5 oic then if the OP saw twinkling then at that moment it for all the other reasons and it wasn't specifically related to it being an inner planet. Still what's discussed here is relevant to future readers, if not to the folks in 2015.  
> **username_4**: @username_5 so I've just asked [Has "crescent-twinkling" even been demonstrated or at least calculated/predicted? Any "twinkleometer" data for Venus out there?](https://astronomy.stackexchange.com/q/50983/7982)  
> **username_5**: If you have posted that question, then how reliable is this answer?  Is it in fact a hypothesis? What is the justification/explanation for why a limited angular size in one dimension would lead to more scintillation?  
> **username_4**: @username_5 Optics is optics. The same localized random phenomenon in the upper atmosphere that makes an object of ~arcsec extent in 2 dimensions twinkle must also make an object of ~arcsec extent in 1 dimension twinkle, though by a somewhat lesser amount. This is inescapable physics and is not open to debate. What I've asked is if anyone has measured it and/or *formally predicted it* in a rigorous and peer-reviewed way. I've added the [resource-request] tag to the question to make that clearer.  

---

