I have always found September a perfect time for looking beyond my beloved atmosphere to the
heavens above. The evenings are often still warm enough to linger comfortably outdoors and sit
and gaze upward. Yet, sunset is early enough and darkness quickly complete enough to enjoy the
celestial show without staying up half the night.
The weather, of course, plays an important role in star and planet watching since an overcast
night kills any chance of seeing beyond the cloud deck. Air pollution, combined with light
pollution, also affects what we see or how much we see. They are related as dust and other
pollution particles in the air not only diminish the incoming starlight but can scatter urban light
and render all but the brightest bodies invisible. But let's look at this topic from a location away
from strong human influences. We'll also eliminate the clouds, although the right cloud-moon
combination can bring many fascinating features to nighttime sky watching.
What is the first thing you usually notice about stars on these clear nights? I'll give you a hint
from our youth: "Twinkle, twinkle little star...." Yes, stars twinkle. The "little" part may refer to the fact that the planets, which have much of the
same appearance as stars, do not twinkle.
The technical term for twinkling is scintillation, the rapid variation in apparent position, colour
or brightness of a luminous object when viewed through a turbulent media, in this case, the
atmosphere. Stars, as we know, are large masses of glowing gas similar to our sun, but they are located so far
away that they appear to us as bright pin-points. Their light travels relatively straight and true
across the light-years of interstellar space, reaching the top of Earth's atmosphere as a steady
point of light (how they would appear to viewers on the International Space Station). When
starlight enters the relatively dense atmosphere (compared to the vacuum of space), its rays are
diverted from their direct path by changes in air density on their way toward the surface. This is
called refraction.
If the atmosphere were just a dense immovable coating around the earth, stars would appear
slightly off their true location due to the refraction of the atmosphere. The atmosphere, however, is in fairly constant motion
and becomes increasingly dense as one moves closer to the surface, though not uniformly so.
Light rays bend differently when they pass through cold air and hot air regions, and always bend
toward the colder air. This is because cold air is more dense than warm air (assuming all other
factors are equal). The continual horizontal and vertical motions of hot and cold air pockets
cause light rays moving through the naturally turbulent atmosphere to change direction
continuously.
The lower atmosphere, where we stand to view the stars, is mottled with pockets of varying
density caused by rising and falling air parcels and strong horizontal winds. When the local air
density changes rapidly with time, a condition termed turbulent, the light ray's path also alters
rapidly. This slight but perceptible refraction bends the path one way, one moment, slightly
different the next. This constant, but random shifting results in the star's image jiving and
jiggling, fading in and out, and even changing colours before our eyes. This stellar dancing is
what we call twinkling.
In addition to this constant jitter in the apparent position of a star, the turbulent air pockets of
also focus and de-focus the starlight, making the stars appear to randomly change brightness.
And since the amount of refraction also depends on the wavelength of the light, various colours
in the ray may dominate at times, giving the twinkling star hints of colour change. (This effect is
not as apparent since our eyes are less light-sensitive in the dark.)
Often on a clear night with calm surface winds, wild twinkling of the stars indicates strong
winds, such as the jet stream, high in the atmosphere above the viewer. The greater the
atmospheric turbulence, the greater the twinkle effect. Lots of twinkling stars indicate a very
unstable, turbulent atmosphere above the viewer.
Twinkling only affects those distant objects whose visual size is smaller than the refractive shifts
caused by the atmospheric turbulence. That is why planets do not twinkle. Even though they
appear as stars, they present a visually large disk compared to the level of turbulent refraction.
So although their light scintillates, the refractions of different light rays coming from across a
planet's disk tend to cancel each other out, and the planet's light shines rather steady. In fact, one
way, therefore, to determine whether a sky object is a star or planet is to see if it twinkles.
On rare occasions, the turbulence may be strong enough to show some planetary twinkle, but
usually only when the planet is near the horizon. A similar effect to twinkling can often be seen
in distance surface light sources, such as individual city lights, when there is strong surface layer
turbulence. Shimmer over hot deserts or other hot surfaces is another form of scintillation. One of the reasons that astronomical observatories are located on high mountains is to reduce
the effects of the lower atmosphere on the light coming from distant stars.
Learn More From These Relevant Books Chosen by The Weather Doctor
Written by Keith C. Heidorn, PhD, THE WEATHER DOCTOR,
November 1, 2005 A version of this material was previously published by Keith Heidorn on Suite 101: Science of the Sky, 2002
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I have recently added many of my lifetime collection of photographs and art works to an on-line shop where you can purchase notecards, posters, and greeting cards, etc. of my best images.
If the atmosphere were just a dense immovable coating around the earth, stars would appear
slightly off their true location due to the refraction of the atmosphere. The atmosphere, however, is in fairly constant motion
and becomes increasingly dense as one moves closer to the surface, though not uniformly so.
In addition to this constant jitter in the apparent position of a star, the turbulent air pockets of
also focus and de-focus the starlight, making the stars appear to randomly change brightness.
And since the amount of refraction also depends on the wavelength of the light, various colours
in the ray may dominate at times, giving the twinkling star hints of colour change. (This effect is
not as apparent since our eyes are less light-sensitive in the dark.)