Editing Twinkling

{{Short description|Variation in brightness and the position of stars due to atmospheric refraction}}
{{Other uses|Twinkle (disambiguation)|Scintillation (disambiguation){{!}}Scintillation}}

[[File:Szintillation.Sirius.480.webm|thumb|The twinkling of the [[brightest star]] in the [[night sky]] [[Sirius]] ([[apparent magnitude]] of -1.1), shortly before upper [[culmination]] at the [[meridian (astronomy)|meridian]], at 20° above the southern horizon. In 29 seconds, Sirius [[diurnal motion|appears to move]] 7.5 [[minute and second of arc|arcminutes]] from left to right.]]
[[File:twinkling_principle.svg|thumb|Comparison of twinkling of a star (top) and a planet (bottom). The turbulent atmosphere (shaded blue) distorts their wavefronts (cyan lines) differently with time, like caustics on a swimming pool floor. When a dark part hits the observer (white circle), the object appears dark, and vice versa. An object with larger angular size smears the pattern, yielding less change in intensity.]]
'''Twinkling''', also called '''scintillation''', is a generic term for variations in [[apparent brightness]], [[colour]], or [[apparent place|position]] of a distant luminous object viewed through a medium.<ref name="intech">Wang, Ting-I; Williams, Donn; [http://www.isa.org/InTechTemplate.cfm?Section=Article_Index1&template=/ContentManagement/ContentDisplay.cfm&ContentID=44354 "Scintillation technology bests NIST"]. {{Webarchive|url=https://web.archive.org/web/20131004215902/http://www.isa.org/InTechTemplate.cfm?Section=Article_Index1&template=%2FContentManagement%2FContentDisplay.cfm&ContentID=44354 |date=2013-10-04 }}, ''InTech'', May 1, 2005.</ref> If the object lies outside the [[Earth's atmosphere]], as in the case of stars and [[planet]]s, the phenomenon is termed ''astronomical scintillation''; for objects within the atmosphere, the phenomenon is termed ''terrestrial scintillation''.<ref>[https://web.archive.org/web/20050201052818/http://www.hq.nasa.gov/office/hqlibrary/aerospacedictionary/508/508index.htm "NASA Aerospace Science and Technology Dictionary"], ''NASA.gov''.</ref> As one of the three principal factors governing [[astronomical seeing]] (the others being [[light pollution]] and [[cloud cover]]), atmospheric scintillation is defined as variations in [[illuminance]] only.

In simple terms, twinkling of stars is caused by the passing of light through [[air mass (astronomy)|different layers]] of a [[Atmospheric turbulence|turbulent atmosphere]]. Most scintillation effects are caused by anomalous [[atmospheric refraction]] caused by small-scale fluctuations in [[air density]] usually related to temperature gradients.<ref name="Sofieva Dalaudier Vernin 2013 p. 20120174">{{cite journal|last1=Sofieva|first1=V. F.|last2=Dalaudier|first2=F.|last3=Vernin|first3=J.|title=Using stellar scintillation for studies of turbulence in the Earth's atmosphere|journal=Philosophical Transactions of the Royal Society|publisher=The Royal Society|volume=371|issue=1982|date=2013-01-13|issn=1364-503X|doi=10.1098/rsta.2012.0174|page=20120174|doi-access=free|pmid=23185055 }}</ref><ref>VanCleave, Janice; [http://scienceprojectideasforkids.com/2010/stellar-scintillation-twinkling-stars/ "Stellar Scintillation: Twinkling Stars"]. ''JVC's Science Fair Projects'', May 2, 2010.</ref> Scintillation effects are always much more pronounced near the [[horizon]] than near the [[zenith]] (directly overhead),<ref>[http://www.nws.noaa.gov/om/educ/activit/scint.htm "Scintillation or Atmospheric Boil"], ''noaa.gov''.</ref> since light rays near the horizon must have longer paths through the atmosphere before reaching the observer. Atmospheric twinkling is measured quantitatively using a [[scintillometer]].<ref>Chun, M.; Avila, R; [http://adsabs.harvard.edu/full/2002ASPC..266...72C "Turbulence profiling using a scanning scintillometer"], ''Astronomical Site Evaluation in the Visible and Radio Range'', Astronomical Society of the Pacific '''266''':72–78.</ref>

The effects of twinkling are reduced by using a larger receiver [[aperture]]; this effect is known as ''aperture averaging''.<ref>Perlot, N.; Fritzsche, D. [http://elib.dlr.de/7341/1/LASE2004-5338-29_Perlot_ApAv_measur_HANDOUT.pdf "Aperture-Averaging – Theory and Measurements"], ''elib – Electronic Library''.</ref><ref>{{cite journal |last1=Andrews |first1=C. |last2=Phillips |first2=R. L. |last3=Hopen |first3=C. |year=2000 |title=Aperture averaging of optical scintillations |journal=Waves in Random Media |volume=10 |issue=1 |pages=53–70 |publisher=[[Taylor & Francis]]|doi=10.1088/0959-7174/10/1/305 |s2cid=120797868 }}</ref>  Many modern large telescopes also use [[Adaptive optics|adaptive optical systems]] which precisely deform the figure of a mirror in order to compensate for scintillation.<ref>{{cite journal |last=Beckers |first=J.M. |title=Adaptive Optics for Astronomy: Principles, Performance, and Applications |journal=Annual Review of Astronomy and Astrophysics |date=1993 |volume=31 |issue=1 |pages=13–62 |doi=10.1146/annurev.aa.31.090193.000305 |bibcode = 1993ARA&A..31...13B}}</ref>

While light from stars and other [[astronomical object]]s is likely to twinkle,<ref>{{cite book|last=Wheelon|first=Albert D.|title=Electromagnetic Scintillation: Volume 2, Weak Scattering|url=https://books.google.com/books?id=CNPtmZJZWCAC|year=2003|publisher=Cambridge University Press|isbn=978-1-139-43960-2}}</ref> twinkling usually does not cause images of planets to flicker appreciably.<ref>Kenyon, S.&nbsp;L.; Lawrence, M. et al; [http://www.phys.unsw.edu.au/~mcba/pubs/kenyon06.pdf "Atmospheric Scintillation at Dome C, Antarctica"], ''Astronomical Society of the Pacific'' '''118''', 924–932.</ref><ref>{{cite journal |last1=Ellison |first1=M. W. |year=1952 |title=Why do Stars Twinkle? |journal=Irish Astronomical Journal |volume=2 |issue=1 |pages=5–8 |bibcode=1952IrAJ....2....5E }}</ref>
Stars twinkle because they are so far from Earth that they appear as point sources of light easily disturbed by Earth's atmospheric turbulence, which acts like lenses and prisms diverting the light's path. Large astronomical objects closer to Earth, like the [[Moon]] and other planets, can be resolved as objects with observable diameters. With multiple observed points of light traversing the atmosphere, their light's deviations average out and the viewer perceives less variation in light coming from them.<ref>Graham, John A. [http://www.scientificamerican.com/article.cfm?id=why-do-stars-twinkle "Why do stars twinkle?"] ''Scientific American'', October 2005.</ref><ref>Byrd, Deborah; [http://earthsky.org/space/why-dont-planets-twinkle-as-stars-do "Why don’t planets twinkle as stars do?"], ''Earthsky'', October 24, 2005.</ref>

==See also==
{{wiktionary|scintillate|twinkle}}
* [[Adaptive optics]]
* [[Interplanetary scintillation]]
* [[Observational astronomy]]
* ''[[Twinkle, Twinkle, Little Star]]''
*[[Twelve Variations on "Ah vous dirai-je, Maman"]]

== References ==
{{Reflist}}

{{DEFAULTSORT:Scintillation (Astronomy)}}
[[Category:Observational astronomy]]
[[Category:Atmospheric optical phenomena]]
[[Category:Articles containing video clips]]