Expert: Tom Whiting - 9/24/2007

Question
I remember when I was in school that they said if you saw a planet in the sky it would fliker red ,blue,green, and white is this true?(Looks like a star that changes color)We saw it tonight and I was making sure I was right!! Thank you so much for any help!!!

Answer
Hi Amanda,
Actually, it's the reverse!!....it's the brighter stars that our atmosphere causes to twinkle, change color violently, and
flicker the light beam.  Planets, on the other hand, are a more
steady light with an unsteady atmosphere.  The reason for this is that light from a star is coming in as a single pin-point of light, a singular stream of photons, so the wavering atmosphere can cause violent scintillation and violent color changes to that single-file line of photons.  Bright planets, on the other hand, are actually small disks in our sky, so their light is composed of multi, overlapping, parallel lines of incoming photons....so with the naked eye they, bright planets,  appear to be more steady, even though they really aren't.
 For instance, even in a small telescope, when the observer sees the disks of planets under poor seeing condtions, the edges of the planets appear to be "swimming or boiling" under poor seeing conditions. Also their entire disks are
void of any sharp detail due to bad seeing.   And twinkling stars are a real tip-off that the seeing is not at its best conditions....astronomers like to see the stars very steady,
for high resolution....sharp details, on the planets being viewed.   

Probably the bright object you were viewing tonight was the bright star Capella, just rising in the northeast as darkness falls....with bad seeing, it can twinkle and change colors violently.  Whereas bright Jupiter in the southwestern sky as darkness falls, is fairly steady to the naked-eye.  (Jupiter is our only bright planet in the evening sky now, until Mars rises around 1 am.) So in the early evening sky, right now,  all other bright luminaries are...stars.
Hope this helps,
Clear Skies,
Tom Whiting
Erie PA


FOLLOW UP:
Oh, Amanda,
For future reference, anytime you see a star twinkling, a star
fluctuating in color, a change in color or shape of an object like the flattening orange sun or moon down near the horizon, that's our atmosphere doing it.  IF we didn't have an atmosphere (like on the moon) then the stars would always be steady pin-points of light, no brightness or color change; the planets, sun, and moon would never change shape or color; everything would be the same, all the way down to the horizon where, at the moment of setting, the star or planet would simply "wink out" almost instantaneously when the horizon presented a blockage of light.   Just thought you'd like to know.
Clear Skies,
Tom W.
Erie, PA