dipper@utastro.UUCP (06/11/85)
When you're above Earth's atmosphere, the stars don't appear to twinkle. More -- in a minute. June 11 Where the Stars Don't Twinkle When we stand on the Earth, we see the universe of outer space while looking through the blanket of Earth's atmosphere. Looking through it with powerful telescopes is a little like opening your eyes under water -- things are a bit blurred. The atmosphere also slightly dims the light of the stars. Stars look about 20% brighter to those working in space -- say, in the orbit of the shuttle -- compared to what we see down on the surface of our world, several hundred miles below. So, once you get beyond the atmosphere, the stars look sharper and a bit brighter -- and they don't appear to twinkle as they do from Earth's surface. Instead, stars seen from space all appear as unwavering points of light. The twinkling we notice from the ground comes from the effect of the atmosphere on incoming waves of starlight. The atmosphere makes the light waves ripple -- incoming waves from a single star interfere with each other as they descend through Earth's blanket of air -- to our eyes. This rippling produces the brightness and color changes which we see as the twinkling of the stars. From an orbit around our Earth, the atmosphere also provides a spectacular view of the brightest star -- and the closest one -- our sun. From the shuttle, while that craft moves in orbit around our world, the astronauts watch as the sun moves out from behind the body of the Earth every ninety minutes -- a brilliant diamond centered on the backlighted crescent of the Earth's atmosphere. Script by Diana Hadley, Harlan Smith and Deborah Byrd. (c) Copyright 1984, 1985 McDonald Observatory, University of Texas at Austin
neal@denelvx.UUCP (Neal Weidenhofer) (06/22/85)
****************************************************************************** > The twinkling we notice from the ground > comes from the effect of the atmosphere on incoming waves of > starlight. The atmosphere makes the light waves ripple -- incoming > waves from a single star interfere with each other as they descend > through Earth's blanket of air -- to our eyes. This rippling produces > the brightness and color changes which we see as the twinkling of the > stars. > > (c) Copyright 1984, 1985 McDonald Observatory, University of Texas at Austin This brings to mind a question that's been on my mind for a number of years: Given that the twinkling effect is caused by tthe atmosphere as stated above, why is it that the stars twinkle but the planets don't. The explanation I've always heard is that it's because the planets are closer--but I don't see what that has to do with it; after all, they aren't closer than the atmosphere. (Are they? :-) In other words, why does the atmosphere have that effect on starlight but not on the light reflected from the planets? And while the future's there for anyone To change, still you know it Regards, seems Neal Weidenhofer It would be easier sometimes Denelcor, Inc. to change the past. <hao|csu-cs|brl-bmd>!denelcor!neal
ables@mcc-db.UUCP (King Ables) (06/24/85)
> ... why is it that the stars twinkle but the planets don't.
The reason the planets don't twinkle is about the same reason the
Sun doesn't (although not as exagerated). The stars are so far
away (thus, so small) that most would appear as just a point of
light if you were in space looking at them. As that one point
source of the light is seen through the atmosphere, it is affected
as described in the Stardate posting. A planet, while looking about
the same from the ground, actually is larger to our sight (say from
orbit) and is therefore larger than a single point source of light.
Thus, the process that causes the twinkle isn't as pronounced with
a planet (although, I believe some slight flicker might be perceived
when looking at [for example] Pluto). The effect is undetectable
with sunlight since the sun appears as such a large object.
O'm French, wha do you think I've got this out-rr-rageous ac-cent?
-King
ARPA: ables@mcc
UUCP: {ihnp4,seismo,ctvax}!ut-sally!mcc-db!ablesjp@lanl.ARPA (06/24/85)
> ****************************************************************************** > > The twinkling we notice from the ground > > comes from the effect of the atmosphere on incoming waves of > > Given that the twinkling effect is caused by tthe atmosphere as > stated above, why is it that the stars twinkle but the planets don't. > > The explanation I've always heard is that it's because the planets are > closer--but I don't see what that has to do with it; after all, they aren't > closer than the atmosphere. (Are they? :-) > > In other words, why does the atmosphere have that effect on > starlight but not on the light reflected from the planets? > Your explanation is partly correct. The stars are so far away that they may be considered point sources of light. The planets, on the other hand, are close enough that they appear to be disks of light. The effect of the atmosphere is the same on all sources of light originating outside it, but the light reflected from a planet can orginate at many points. The effect of the atmosphere causes each point to "twinkle" just as for the stars, but the average of many twinkling points (not synchronized because each ray takes a slightly different path) tends to be a steadier, non-twinkling, image. Jim Potter jp@lanl.arpa
GMS@psuvm.BITNET (06/24/85)
Actually, planets will be seen to twinkle if they are near the horizon, or
if the air is extremely unsteady. (Check out Jupiter or Venus just after
they have risen) The fact that they normally do not appear to twinkle is
a spinoff of the fact thet they are much nearer to us. Since they are
relatively close, they present tiny disks which, although not discernable
by naked eye, are significantly larger than the very tiny apparant disks
of the stars. Thus while the light waves are still refracted about, the
effect is somewhat mediated by the larger apparant disks of the planets.
Gerry Santoro
Penn State University
. . . !psuvax1!santoro
. . . !psuvax1!psuvm.bitnet!gms
jwl@ucbvax.ARPA (James Wilbur Lewis) (06/26/85)
Stars twinkle because they are, for all practical purposes, point sources. Planets, since they are much closer to Earth, have visible disks. Since the light you see from a planet comes from a larger area, it is less affected by the atmospheric turbulence that makes the stars twinkle. -- Jim Lewis U.C. Berkeley ...!ucbvax!jwl
cramer@kontron.UUCP (Clayton Cramer) (07/04/85)
> Stars twinkle because they are, for all practical purposes, point sources. > Planets, since they are much closer to Earth, have visible disks. Since > the light you see from a planet comes from a larger area, it is less affected > by the atmospheric turbulence that makes the stars twinkle. > > -- Jim Lewis > U.C. Berkeley > ...!ucbvax!jwl A real trivial minor point: Mercury, I've noticed, tends to twinkle, partly because the only times you see is at dusk, when heat disturbances are quite pronounced, and also because it has the smallest visible disk of the visible planets.
acy@pucc-i (Linda Sears) (07/11/85)
Newsgroups: net.astro Subject: Re: StarDate: June 11 Where the Stars Don't Twinkle References: <224@utastro.UUCP> <71@denelvx.UUCP>