Scintillation or twinkling are generic terms for rapid variations in apparent brightness or color of a distant luminous object viewed through a medium, most commonly the atmosphere

Earth's atmosphere

The atmosphere of Earth is a layer of gases surrounding the planet Earth that is retained by Earth's gravity. The atmosphere protects life on Earth by absorbing ultraviolet solar radiation, warming the surface through heat retention , and reducing temperature extremes between day and night...

 (atmospheric scintillation).

If the object lies outside the Earth's atmosphere, as in the case of stars and planets, the phenomenon is termed astronomical scintillation; if the luminous source lies within the atmosphere, the phenomenon is termed terrestrial scintillation.

As one of the three principal factors governing astronomical seeing

Astronomical seeing

Astronomical seeing refers to the blurring and twinkling of astronomical objects such as stars caused by turbulent mixing in the Earth's atmosphere varying the optical refractive index...

, atmospheric scintillation is defined as variations in illuminance

Illuminance

In photometry, illuminance is the total luminous flux incident on a surface, per unit area. It is a measure of the intensity of the incident light, wavelength-weighted by the luminosity function to correlate with human brightness perception. Similarly, luminous emittance is the luminous flux per...

 only, and so twinkling does not cause blurring of astronomical images. It is clearly established that almost all scintillation effects are caused by anomalous refraction

Refraction

Refraction is the change in direction of a wave due to a change in its speed. It is essentially a surface phenomenon . The phenomenon is mainly in governance to the law of conservation of energy. The proper explanation would be that due to change of medium, the phase velocity of the wave is changed...

 caused by small-scale fluctuations in air density usually related to temperature gradients. Normal wind motion transporting such fluctuations across the observer's line of sight produces the irregular changes in intensity characteristic of scintillation.

Scintillation effects are always much more pronounced near the horizon than near the zenith

Zenith

The zenith is an imaginary point directly "above" a particular location, on the imaginary celestial sphere. "Above" means in the vertical direction opposite to the apparent gravitational force at that location. The opposite direction, i.e...

 (straight up). Parcels of air with sizes of the order of only centimeters to decimeters are believed to produce most of the scintillatory irregularities in the atmosphere. Atmospheric scintillation is measured quantitatively using a scintillometer

Scintillometer

A scintillometer is a scientific device used to measure small fluctuations of the refractive index of air caused by variations in temperature, humidity, and pressure. It consists of an optical or radio wave transmitter and a receiver at both ends of an atmospheric propagation path...

.

Scintillation effects are reduced by using a larger receiver aperture. This effect is known as aperture averaging.

Planetary scintillation

Light from both planets and stars can suffer from scintillation. . Dave Kornreich explains why: . Both stars and planets are circular discs. The light from a star enters the pupil of our eye and comes to a focus as a point at the back of the eye. If that light is followed backwards to the star, it forms a cylindrical bundle the size of the pupil but gradually widening as it nears the star. But so gradual is this widening that even at the edge of the atmosphere, 100 km up, the bundle is still about the same as the pupil, say half a cm across. This is so narrow that wind currents of different temperature and refractive index can disturb the light and cause the star to twinkle. A planet is so much nearer that at the edge of the atmosphere the bundle is a disc metres across. The light from the planet that forms our image of it comes from hundreds of different pathways in this bundle and it is unlikely that many will be disrupted at the same time. The amount of disturbance is therefore much less and the planet appears as a steady point.

See also

• Scintillation (disambiguation)
• Astronomical seeing
  Astronomical seeing
  Astronomical seeing refers to the blurring and twinkling of astronomical objects such as stars caused by turbulent mixing in the Earth's atmosphere varying the optical refractive index...
  
  
• Earth's atmosphere
  Earth's atmosphere
  The atmosphere of Earth is a layer of gases surrounding the planet Earth that is retained by Earth's gravity. The atmosphere protects life on Earth by absorbing ultraviolet solar radiation, warming the surface through heat retention , and reducing temperature extremes between day and night...
  
  
• Interplanetary scintillation
  Interplanetary scintillation
  In astronomy, interplanetary scintillation refers to random fluctuations in the intensity of radio waves of celestial origin, on the timescale of a few seconds. It is analogous to the twinkling one sees looking at stars in the sky at night, but in the radio part of the electromagnetic spectrum...
  
  
• Observational astronomy
  Observational astronomy
  Observational astronomy is a division of the astronomical science that is concerned with getting data, in contrast with theoretical astrophysics which is mainly concerned with finding out the measurable implications of physical models...
  
  
• Twinkle Twinkle Little Star
  Twinkle Twinkle Little Star
  "Twinkle, Twinkle, Little Star" is a popular English nursery rhyme. The lyrics are from an early nineteenth-century English poem, "The Star" by Jane Taylor. The poem, which is in couplet form, was first published in 1806 in Rhymes for the Nursery, a collection of poems by Taylor and her sister Ann...