Why do planets not twinkle like stars?

[Uplooker]

As title suggests "Why do planets not twinkle like stars?"

Sorry if this is a daft question or has been asked numerous times already

[barkis]

Not a daft question at all.

Stars appear to scintillate, or twinkle, because they are at such vast distances, they can only be seen as a point source, and as such, are susceptible to earths atmosphere interrupting their light, and therefore they appear to switch on and off rapidly.

Planets on the other hand, are larger objects, and less prone to that effect, although it is not impossible for a planet to twinkle. When I say larger, it is because they are relatively close to earth, and are extended objects.

I'm sure someone quite recently on the forum, said he had seen Saturn twinkling.

Ron.

[Peter Drew]

Stars twinkle because as they are so distant you can only see the light from them and not their actual diameter, the light is then distorted by atmospheric disturbance. Planets do have discernable diameters and as a result do not to the same degree suffer from this effect.

[Gina]

I think Saturn will twinkle if the seeing is poor.

[ollypenrice]

There are various ways of putting this. One would be to say that from a point source there is a single thin beam which can be distracted this way or that. From a non point source there are beams from top, bottom, left, right, middle... and that distortions of one will be neutralized by distortions of another, and so on.

Olly

[rabbithutch]

These responses confuse me, but I understand the atmosphere's effect on the seeing.

I'm confused by the statement that planets are bigger than stars. I should think that generally the reverse is true (unless, of course, you meant that closer proximity made them appear larger).

I'm also confused about the statement that planets don't have discernible diameters. Could you please explain? Perhaps I don't understand 'discernible'' in the context of 'diameters'.

[barkis]

I modified my post to clear up the confusion about planet size.

Of course planets are miniscule compared to stars, it's due to their closer proximity to earth that their size makes it more difficult for earths atmosphere to interfere with their reflected light, compared to the point source of stars.

I think if you read Peters post again, you'll find he said "planets do have discernible diameters."

Ron.

[Mikea]

It helps if you think about the angular diameter of the object. See this:

Angular diameter - Wikipedia, the free encyclopedia

[rabbithutch]

AHA! My old eyes failed me again. I put a word in that was not there. It makes more sense, too.

Thanks for clarifying your point about size.

[FraserClarke]

There was a most beautiful APOD showing this effect the other day. Took me a little while to intepret the image, but once I did I thought this is one of the most fantastic astronomical images I've ever seen. Packed full of physics ;

APOD: 2011 April 28 - Scintillating

[twotter]

Cheers TeaDwarf - I've now got a new wallpaper on my laptop

[Uplooker]

Thank you all. TeaDwarf - I particularly like the picture that you provided a link to.

I am familiar with how the atmosphere affects the "seeing", but could not work out why it affected stars more than planets.

Cheers,

Ian

[acey]

I was at a planetarium presentation recently and the guide asked the audience: "What's the difference between a star and a planet?" I assumed he was going to say something about their composition but instead he announced, "Stars twinkle, planets don't." This factoid has become generally received wisdom, the reasoning being, as has been stated, that planets have finite apparent angular size; but as has also been stated, planets will twinkle if there is sufficient turbulence in the air: a planet seen low over the horizon will be apt to twinkle. And stars will twinkle very little if they are high in the sky and there is little turbulence.

[chemtom24]

this may be a silly question as well since im not sure if planets far away are visible, but lets imagine they are - then surely they too would twinkle and would be indistinguishable from a star to the naked eye? or does light from distant planets simply not reach us bright enough for us to see?

[acey]

Light never runs out completely, it just gets fainter. With a big enough telescope (or other equipment) you can see that light. In the case of planets out to Saturn, reflected light from the surface is bright enough to see easily with the naked eye.

Twinkling, as has been said, is due to atmospheric turbulence, and is more pronounced the smaller the source, since a "point source" (a star) will be completely displaced by the effect. But you can see an analogous glimmering of terrestrial lights (e.g. streetlights) if they are far away and the air is turbulent. Similarly, the planets can appear to glimmer or twinkle, just like stars. And bright stars high up on steady nights can shine with a very steady light. So twinkling or lack of twinkling is not, I think, a good diagnostic test of whether something is a planet or a star. A surer way is to watch the same object over a course of many nights and see if it moves with respect to the other stars - that's what the ancients did, and it's why they're called planets (which means "wanderer").