Within Bright Stars
Why Does Sirius Flash Like a Hovering Aircraft?
Sirius can flash red, blue and other colors near the horizon, making a distant star resemble a hovering craft with changing lights.
On this page
- Why Sirius produces such striking color flashes
- Why low altitude makes the effect stronger
- Clues that distinguish Sirius from an aerial object
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Introduction
Sirius can look remarkably like a distant hovering aircraft when it is low in the sky. Instead of shining as a steady white point, it may pulse in brightness and flash red, blue, green, orange or white in rapid succession. The effect is real, but the colours are normally produced in Earth’s atmosphere rather than by changing lights on an object. Astronomers call the rapid fluctuations scintillation.[International Cloud Atlas]cloudatlas.wmo.intInternational Cloud Atlas Scintillation | International Cloud AtlasInternational Cloud Atlas Scintillation | International Cloud Atlas
Sirius is especially conspicuous because it is the brightest star in the night sky. NASA identifies it as the brightest star as seen from Earth, while Royal Museums Greenwich specifically notes that Sirius can appear to flash blue, red and other colours. That combination — exceptional brightness, dramatic colour changes and little obvious short-term movement — can make it resemble an aircraft, drone or other hovering aerial light.[NASA Science]science.nasa.govScience Skywatching FAQNASA ScienceSkywatching FAQ - NASA ScienceSeptember 3, 2024…
Why Sirius produces such striking colour flashes
Sirius itself is not rapidly switching between red, blue and green. Its visible light contains a range of wavelengths, and that light has already travelled roughly 8.6 light-years before it encounters the comparatively tiny layer responsible for the spectacle: Earth’s atmosphere. Sirius A is so much brighter than its white-dwarf companion Sirius B that the companion does not account for the naked-eye flashing effect.[NASA Science]science.nasa.govScience The Dog Star, Sirius, and its Tiny CompanionNASA ScienceThe Dog Star, Sirius, and its Tiny Companion - NASA ScienceDecember 13, 2005…
The important process begins when the incoming starlight crosses turbulent air. Temperature and density variations create corresponding fluctuations in the atmosphere’s refractive index. The World Meteorological Organization describes scintillation as rapid variations in light in which the apparent brilliance, colour and position of stars can all change because of these refractive-index fluctuations.[International Cloud Atlas]cloudatlas.wmo.intInternational Cloud Atlas Scintillation | International Cloud AtlasInternational Cloud Atlas Scintillation | International Cloud Atlas
Different wavelengths of visible light are also refracted by different amounts. The American Meteorological Society notes that fluctuations affecting different frequency components can create chromatic scintillation, including the random red-and-blue twinkling of bright stars near the horizon. Optical research likewise distinguishes ordinary scintillation caused by random atmospheric density fluctuations from chromatic refraction and the colour-dependent effects produced when the two interact.[Glossary of Meteorology]glossary.ametsoc.orgGlossary of MeteorologyscintillationGlossary of Meteorology…
For an observer, these processes happen too rapidly and at too small an angular scale to resolve as separate images of Sirius. Instead, the eye receives a bright point whose colour and intensity continually change. At one instant red light may be favoured; moments later blue, green or another mixture can dominate. NASA’s Chandra astronomy material describes Sirius as sometimes appearing to change red, green and white several times per second, particularly when it is close to the horizon or the atmosphere is turbulent.[Chandra X-ray Observatory]chandra.harvard.eduChandra X-ray ObservatoryChandra:: Resources:: Q&A: General Astronomy and Space ScienceSeptember 15, 2008…
That rapid alternation is what makes Sirius more deceptive than an ordinary gently twinkling star. The sequence can resemble separate coloured lamps switching on and off. Royal Museums Greenwich consequently gives colour-changing twinkling as a useful clue that a mysterious bright object is probably a star, with Sirius offered as the specific example.[Royal Museums Greenwich]rmg.co.ukOpen source on rmg.co.uk.
Why low altitude makes the effect stronger
The most dramatic displays occur when Sirius is relatively close to the horizon because its light then crosses much more atmosphere before reaching the observer. The World Meteorological Organization explicitly states that scintillation becomes more pronounced as the atmospheric path grows longer, making stellar twinkling stronger near the horizon than near the zenith.[International Cloud Atlas]cloudatlas.wmo.intInternational Cloud Atlas Scintillation | International Cloud AtlasInternational Cloud Atlas Scintillation | International Cloud Atlas
The increase in atmospheric path length is substantial. Astronomers express it using airmass, with one airmass representing approximately the amount of atmosphere encountered when looking straight upwards. A star 30° above the horizon is viewed through roughly two airmasses, while at an altitude of only 10° the figure is about 5.6. Conditions become still more extreme very close to the horizon.[Sky & Telescope]skyandtelescope.orgSky & Telescope Transparency and Atmospheric ExtinctionSky & TelescopeTransparency and Atmospheric Extinction - Sky & TelescopeJune 10, 2008…
More atmosphere means more opportunities for moving layers and pockets of differently heated air to refract the light. It also makes wavelength-dependent atmospheric effects more important. Research into stellar photometry confirms that atmospheric dispersion can amplify colour scintillation away from the zenith, while modern astronomical instruments have to account explicitly for wavelength-dependent atmospheric refraction.[arXiv]arxiv.orgarXiv How achromatic is the stellar scintillation on large telescopes?arXiv How achromatic is the stellar scintillation on large telescopes?
The result can be much more vivid than the word twinkling suggests. A 2024 observation highlighted by BBC Sky at Night recorded Sirius producing conspicuous green flashes as it approached the horizon. Such observations demonstrate that apparently exotic colours are not merely theoretical consequences of atmospheric optics; they can be conspicuous enough to record directly.[Sky at Night Magazine]skyatnightmagazine.comOpen source on skyatnightmagazine.com.
Conditions also vary from night to night. The amount and distribution of atmospheric turbulence change, so Sirius may look fairly steady on one occasion and extraordinarily restless on another. Astronomers take scintillation seriously enough to measure atmospheric turbulence from the intensity variations of bright stars, and scintillation remains a quantifiable noise source in precision ground-based photometry.[CTIO]ctio.noirlab.eduCTIORINGSS Turbulence ProfilerCTIORINGSS Turbulence Profiler
This variability helps explain why someone familiar with the night sky can still be surprised. Seeing Sirius behaving normally on previous evenings does not mean it will display the same degree of flashing under different atmospheric conditions.
Why the flashes can resemble aircraft lights
Aircraft provide an intuitive interpretation for a bright point showing red, white or other changing lights. There is an additional complication: an aircraft approaching almost directly towards an observer can itself appear nearly stationary for a time. Royal Museums Greenwich therefore treats aircraft and scintillating stars as genuinely confusable possibilities, noting both Sirius’s coloured flashes and the temporary stationary appearance of an approaching aeroplane.[Royal Museums Greenwich]rmg.co.ukOpen source on rmg.co.uk.
Sirius can create the reverse illusion. It really is effectively stationary on the timescale of a brief observation, aside from the slow apparent movement of the sky caused by Earth’s rotation, but atmospheric fluctuations supply something that looks like aviation-style flashing. In darkness, without distance or size cues, the observer cannot determine merely from apparent brightness whether the point represents a brilliant star or a much dimmer lamp at terrestrial distance.
There is evidence that this confusion occurs outside astronomy textbooks. Sky & Telescope describes a case in which an astronomy club was shown video of an alleged UFO characterised by brilliance and rapid pulsations. The object’s identity could be established from the reported time and direction: it was Sirius. The same account identifies Sirius as a recurrent source of UFO reports because of its brightness and strong twinkling.[Sky & Telescope]skyandtelescope.orgSky & Telescope Why Are There So Many UFOs?Sky & Telescope Why Are There So Many UFOs?
The broader identification problem is longstanding. Project Blue Book, the US Air Force’s historical UFO investigation programme, explicitly placed bright stars within its category of astronomical identifications. The CIA’s summary of Blue Book investigative practice likewise notes that investigators recorded time, location, direction, weather and the object’s manner of appearance because those details allow apparently unusual aerial lights to be tested against astronomical and other conventional possibilities.[CIA]cia.govHow To Investigate a Flying SaucerHow To Investigate a Flying Saucer
Sirius is a particularly good example of why an identification should depend on geometry rather than appearance alone. Saying that something “looks like Sirius” is weaker evidence than demonstrating that Sirius occupied the reported position at the reported time. Conversely, coloured flashing by itself is not evidence that an object is an aircraft: scintillation can generate a remarkably similar visual signal.
Clues that distinguish Sirius from an aerial object
The strongest clues come from combining the light’s behaviour with its position and movement. No single rule is infallible, but several observations together can make a Sirius identification quite strong.
- Check the exact direction and time. Sirius has a predictable astronomical position. If a sky chart places it exactly where the flashing object was observed, that is much stronger evidence than resemblance alone. NASA notes that Orion provides a convenient landmark: following Orion’s Belt towards the left and downwards leads to Sirius in the winter sky.[NASA Science]science.nasa.govwhats up december 2024 skywatching tips from nasawhats up december 2024 skywatching tips from nasa
- Watch for sustained positional stability. An approaching aircraft can temporarily appear stationary, but it will eventually develop obvious angular motion as its geometry changes. Sirius instead follows the slow, smooth apparent motion of the celestial sphere. Royal Museums Greenwich specifically notes this eventual sideways or upward movement as a way an aircraft can reveal itself.[Royal Museums Greenwich]rmg.co.ukOpen source on rmg.co.uk.
- Look at the rhythm of the colours. Sirius’s flashes are irregular because they are driven by changing atmospheric turbulence. They need not repeat like engineered navigation or anti-collision lights. The World Meteorological Organization describes scintillation as fluctuations in brilliance, colour and apparent position rather than a fixed blinking sequence.[International Cloud Atlas]cloudatlas.wmo.intInternational Cloud Atlas Scintillation | International Cloud AtlasInternational Cloud Atlas Scintillation | International Cloud Atlas
- Notice whether the effect strengthens near the horizon. Strong multicoloured scintillation from a brilliant point low in the sky fits Sirius particularly well because the long atmospheric path magnifies the effect.[International Cloud Atlas]cloudatlas.wmo.intInternational Cloud Atlas Scintillation | International Cloud AtlasInternational Cloud Atlas Scintillation | International Cloud Atlas
- Recheck the location later or on another night. Sirius will return to a predictable position according to the ordinary progression of the night sky. Repeated observation can therefore turn an initially ambiguous hovering light into a straightforward astronomical identification.
- Treat extreme camera zoom cautiously. A poorly focused point source can become a large disc or irregular blob in video, while atmospheric fluctuations remain visible as changing colours and brightness. A magnified coloured “orb” is therefore not, by itself, evidence that the source has a visible physical surface. Sky & Telescope’s Sirius case describes both hand-induced motion and autofocus changes contributing to the extraordinary appearance of the recorded light.[Sky & Telescope]skyandtelescope.orgSky & Telescope Why Are There So Many UFOs?Sky & Telescope Why Are There So Many UFOs?
The decisive distinction is therefore not whether Sirius looks like an aircraft. Under favourable atmospheric conditions, it genuinely can. The useful question is whether the object’s position, irregular scintillation and subsequent motion match a star whose location can be independently predicted.
Why Sirius is such a useful identification case
Sirius illustrates an important feature of unusual aerial-light reports: a conventional source does not necessarily have to look conventional. A witness may correctly report an exceptionally bright light that remains in roughly the same place and repeatedly flashes different colours. None of those observations has to be imagined or fabricated for the interpretation to be mistaken.
What changes the assessment is the mechanism. Sirius is the brightest star in Earth’s night sky, its point-like image is particularly conspicuous, and a low viewing angle sends its light through a long, turbulent atmospheric path. Refraction, atmospheric dispersion and scintillation can then turn its normally white appearance into an irregular sequence of brilliant colours.[nasa.gov]science.nasa.govScience Skywatching FAQNASA ScienceSkywatching FAQ - NASA ScienceSeptember 3, 2024…
That makes Sirius a useful test case within the wider category of bright stars mistaken for hovering aerial objects. Red and blue flashes do not automatically imply navigation lights; apparent hovering does not establish terrestrial distance; and spectacular changes seen through a camera do not establish that the source itself is changing shape or colour. When the reported time, observing location and direction place Sirius at the same point in the sky, atmospheric scintillation provides a well-understood physical explanation for precisely the behaviour that initially made the light seem unusual.
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93.
Source: skyandtelescope.org
Title: Zodiacal Light in the Evening
Link:https://skyandtelescope.org/astronomy-news/observing-news/zodiacal-light-in-the-evening/
94.
Source: ctio.noirlab.edu
Link:https://www.ctio.noirlab.edu/~atokovin/profiler/index.html
95.
Source: space.com
Title: Doorstep Astronomy: Sirius Gets Serious | Space
Link:https://www.space.com/3479-doorstep-astronomy-sirius.html
96.
Source: skyandtelescope.org
Title: Everything You Ever Wanted to Know About Light Pollution
Link:https://skyandtelescope.org/astronomy-resources/everything-you-ever-wanted-to-know-about-light-pollution/
97.
Source: tuc-tuc.noirlab.edu
Link:https://tuc-www.tuc.noirlab.edu/survey-archives/bcs/ObservingLogs/msc051208.html
98.
Source: space.com
Title: Canine Constellations: Night Sky Goes to the Dogs | Space
Link:https://www.space.com/1018-canine-constellations-night-sky-dogs.html
99.
Source: space.com
Title: Great Star of the South | Space
Link:https://www.space.com/803-great-star-south.html
100.
Source: space.com
Title: Doorstep Astronomy: See Scorching Sirius | Space
Link:https://www.space.com/765-doorstep-astronomy-scorching-sirius.html
101.
Source: ctio.noirlab.edu
Title: photometric zero points color terms
Link:https://www.ctio.noirlab.edu/noao/content/photometric-zero-points-color-terms
102.
Source: astro.pas.rochester.edu
Link:https://astro.pas.rochester.edu/idldoc/airmass.html
103.
Source: cloudatlas.wmo.int
Link:https://cloudatlas.wmo.int/en/scintillation.html
104.
Source: wikizero.net
Title: Project Blue Book
Link:https://www.wikizero.net/wiki/en/Project_Blue_Book
105.
Source: internationalflyingsaucerbureau.com
Title: Project Blue Book
Link:https://www.internationalflyingsaucerbureau.com/project-blue-book/
106.
Source: rmg.co.uk
Link:https://www.rmg.co.uk/stories/space-astronomy/solar-system-through-your-own-telescope
107.
Source: rmg.co.uk
Link:https://www.rmg.co.uk/stories/space-astronomy/astronomy-naked-eye
108.
Source: rmg.co.uk
Link:https://www.rmg.co.uk/stories/space-astronomy/[noctilucent-clouds
109.
Source: rmg.co.uk
Title: delta aquariid meteor shower 2026 when where see it uk
Link:https://www.rmg.co.uk/stories/space-astronomy/delta-aquariid-meteor-shower-2026-when-where-see-it-uk
110.
Source: rmg.co.uk
Title: space astronomy highlights 2026
Link:https://www.rmg.co.uk/stories/space-astronomy/space-astronomy-highlights-2026
111.
Source: rmg.co.uk
Link:https://www.rmg.co.uk/stories/space-astronomy/why-sky-blue?wvideo=w19jaayxo3
112.
Source: rmg.co.uk
Link:https://www.rmg.co.uk/collections/objects/rmgc-object-12898
113.
Source: ctio.noirlab.edu
Link:https://www.ctio.noirlab.edu/~atokovin/profiler/archive.html
114.
Source: ctio.noirlab.edu
Title: cerro tololo trails
Link:https://www.ctio.noirlab.edu/noao/content/cerro-tololo-trails
115.
Source: ctio.noirlab.edu
Link:https://www.ctio.noirlab.edu/~atokovin/tutorial/part1/turb.html
116.
Source: ctio.noirlab.edu
Link:https://www.ctio.noirlab.edu/~atokovin/tutorial/part3/wfs.html
117.
Source: ctio.noirlab.edu
Link:https://www.ctio.noirlab.edu/~atokovin/profiler/instrument.html
118.
Source: mysteriousworld.fandom.com
Title: Project Blue Book
Link:https://mysteriousworld.fandom.com/wiki/Project_Blue_Book
119.
Source: military-history.fandom.com
Title: Project Blue Book
Link:https://military-history.fandom.com/wiki/Project_Blue_Book
Additional References
120.
Source: pubmed.ncbi.nlm.nih.gov
Link:https://pubmed.ncbi.nlm.nih.gov/18357067/
Source snippet
Chromatic refraction with global ozone monitoring by occultation of stars. I. Description and scintillation correction - PubMed...
121.
Source: youtube.com
Title: Sirius at 500 fps with a 10” LX200 defocused in steps
Link:https://www.youtube.com/watch?v=dsH9b738J7w
Source snippet
Twinkle, twinkle little Sirius - Scintillation action This video demonstrates atmospheric scintillation causing the star Sirius to rapidl...
122.
Source: archives.gov
Link:https://www.archives.gov/research/catalog/catalog-bulk-downloads/uap-bulk-download
123.
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124.
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125.
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126.
Source: archives.gov
Link:https://www.archives.gov/research/topics/uaps/textual-and-microfilm
127.
Source: youtube.com
Title: FACETS OF THE STARS
Link:https://www.youtube.com/watch?v=ZDwuQgETvOo
Source snippet
Sirius at 500 fps with a 10" LX200 defocused in steps...
128.
Source: visit.archives.gov
Title: gov50 Years Ago: Government Stops Investigating UFOs | National Archives Museum
Link:https://visit.archives.gov/whats-on/explore-exhibits/50-years-ago-government-stops-investigating-ufos
129.
Source: youtube.com
Title: Twinkle, twinkle little Sirius. Scintillation action
Link:https://www.youtube.com/watch?v=y1yZSIs7Nwc
Source snippet
Knipperende UFO Blinking UFO - the Star Sirius...