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jimmy
2003-Oct-31, 03:28 AM
Please don't laugh.....O.K. if it's funny to you, laugh.
Somewhere a long time ago I heard that stars twinkle and planets do not.
This evening, 10/30/03 at about 6 PM I was out looking at Venus just about 10 minutes before it dissappeared below the horizon. It was definitely twinkling. I called my brother and he saw it too.
Can someone tell me what's up?

Josh
2003-Oct-31, 03:43 AM
You heard right, stars twinkle and planets don't. Both of them will shimmer and shake a little because of atmospheric effects though. The reason star shimmer however (as i understand it) is because they are only tiny beams of light reaching the earth. If ou look through a telescope the planets can be seen to be flat disk shapes while the stars still look like pin points of light regardless of if you look with the naked eye or through a telescope.

The atmosphere has a much greater effect on these thin beams of light and the stars appear to twinkle. Because the planets are much closer (and disk like through a telescope) means that this effect is cancelled out somehow across the area of the disk. Perhaps the reason Venus appeared to twinkle (assuming it wasn't just minor shimmering) is that when it's about to set you are looking through a lot more of the atmosphere than when looking straight over head. therefore, atmospheric effects would be greater. Also if there is pollution in the sky above your city then this will greatly effect the view of anything through it.

jimmy
2003-Oct-31, 06:13 AM
Originally posted by Josh@Oct 31 2003, 03:43 AM
You heard right, stars twinkle and planets don't. Both of them will shimmer and shake a little because of atmospheric effects though. The reason star shimmer however (as i understand it) is because they are only tiny beams of light reaching the earth. If ou look through a telescope the planets can be seen to be flat disk shapes while the stars still look like pin points of light regardless of if you look with the naked eye or through a telescope.

The atmosphere has a much greater effect on these thin beams of light and the stars appear to twinkle. Because the planets are much closer (and disk like through a telescope) means that this effect is cancelled out somehow across the area of the disk. therefore, atmospheric effects would be greater. Also if there is pollution in the sky above your city then this will greatly effect the view of anything through it.
Perhaps the reason Venus appeared to twinkle (assuming it wasn't just minor shimmering) is that when it's about to set you are looking through a lot more of the atmosphere than when looking straight over head.

Yeah, that's the only thing I could figure. I'm about 60 miles SSE of New Orleans and in the country so there is not much visible pollution.
Thanks for the info. Josh.

Matthew
2003-Oct-31, 11:36 PM
Stars and planets 'twinkle' because of the Earths atmosphere. Out in space they do not twinkle. The twinkling occurs because the atmosphere diffracts the light of the stars.

kashi
2003-Nov-05, 06:40 AM
My understanding is that Mathew is correct. Of course they both twinkle. The only reason that they do this is because of atmospheric disturbance. Planets may "twinkle" less because they are closer and therefore appear larger.

jimmy
2003-Nov-06, 06:32 AM
Thank you all...

professor
2003-Nov-06, 02:29 PM
Originally posted by Josh@Oct 31 2003, 03:43 AM
Both of them will shimmer and shake a little because of atmospheric effects though. The reason star shimmer however (as i understand it) is because they are only tiny beams of light reaching the earth. If ou look through a telescope the planets can be seen to be flat disk shapes while the stars still look like pin points of light regardless of if you look with the naked eye or through a telescope.


[QUOTE]
This is absolutely correct. It is called Astronomical Refraction (the symbol is greek "Ro" or somewhere just "R"), and it is one of five basic desturbancies included in calculus of the ephemerides (other four are Abberation, Paralax, Nutation and Precession). The influence of refraction is the biggest near horizont, and thatīs why planets have a little shimmer there. And more, the refraction is the reason why the Moon is bigger near horizon then upper in the sky, and why the Sun looks more eliptical just before it sets under horizon. And that is why it is stil visible after the sunset; you can read without artificial light until the Sun reaches 15th degree below horizont (calculate a little: at subpolar places at winter, Sun doesnīt reach that 15th degree for a long time; those are so called "white nights"). The local climate is very important too, in the matter of calculating the nearest value of refraction, but finally for every serious observations of celestial objects, the less is the distance from zenith, the results are more considerable and precise.
For the conclusion some mathematics:
Ro = Z-Z', where Z, Zī are real and visible distance from zenith, respectively;
value of normal refraction is
Ro = 60'.25tan[Z']; this value is not considerable when Z > 70š;
instead,
Ro = g(Atan[Z'] + B(tan[Z']exp3) + C(tan[Z']exp5) +...), where A,B,C are atmospherical coeficients

Thanks for your patience :ph34r:

Dave Mitsky
2003-Nov-06, 03:03 PM
Even though it is not a point source like a star a planet that is close to the horizon will twinkle to the naked-eye when the "seeing" is very bad (unsteady atmospheric conditions). Prismatic dispersion due to the thick atmosphere "near" the horizon will cause false colors to be seen as well.

Dave Mitsky

Dave Mitsky
2003-Nov-06, 03:14 PM
"And more, the refraction is the reason why the Moon is bigger near horizon then upper in the sky..."

This is not correct. The real reason for the lunar illusion is still under dispute but refraction is definitely not it.

http://archive.abcnews.go.com/sections/tec...geek990930.html (http://archive.abcnews.go.com/sections/tech/Geek/geek990930.html)

http://facstaff.uww.edu/mccreadd/

http://griffithobs.org/IPSMoonIllus.html

http://www.space.com/scienceastronomy/sola...oon_000105.html (http://www.space.com/scienceastronomy/solarsystem/bigmoon_000105.html)

Dave Mitsky

DippyHippy
2003-Nov-07, 02:57 AM
Dave's absolutely right... it's one of the most persistent astronomical misconceptions... I didn't know it was a myth either until I hit upon Phil Plait's famous Bad Astronomy (http://www.badastronomy.com) website.

If you're looking for the specific page about the Moon, that's here. (http://www.badastronomy.com/bad/misc/moonbig.html) :)

professor
2003-Nov-11, 11:56 AM
Calculate the refraction in every point of the circle made by the moonīs edge and then compare that new figure with original one.

OverTheStars
2003-Nov-13, 01:59 AM
any star is pretty far away, far enough for objects to pass by it every second. think of a flashlight, and a bunch of objects moving by in front of it. it would block the light for a second or two. the planets we can see in our solar system wouldnt twinkle, because there arent enough objects and space to pass by it every second(or less). but im sure it could twinkle once or twice, there are objects flying around in our solar system, but i dontt hink i've ever seen a planet twinkle.

OverTheStars
2003-Nov-13, 02:00 AM
ehh, i think i may be wrong then. ignore my reply. sorry.

jimmy
2003-Nov-13, 04:55 AM
Hi Over the Stars,
I'm in Southeast Louisiana and the planet Venus is the first thing you can see in the western sky just after sunset. As it gets closer to the horizon, you can see it twinkling.
It's been pointed out to me that this is because I'm looking through more of the atmosphere at that angle, and therefore the light from Venus is refracted moreso than if it were straight up. Then it wouldn't twinkle.