TWINKLING OF STARS, or SCINTILLATION (scintilla, "a spark of fire"). This term is applied to a phenomenon which has attracted the attention of astronomers and scientific men in all ages. It consists in rapid variations in the brightness of a fixed star when observed with the naked eye, and is often accompanied by changes in the colour, and alterations in the apparent diameter of the star or in the length of the diverging rays which appear to dart from its centre iu different directions. It is commonly stated that the twinkling disappears when the star is viewed through a telescope ; such, however, is not the case, although under such circumstances the phenomena are modified.
This subject occupied but a very small space in scientific works, if indeed it was to be found at all in them, until Arago devoted one of his searching scientific notices to the subject in the Annuaire pour l'An 1852, publi6 par le Bureau des Longitudes ;' nevertheless the importance of the subject has never been forgotten, and Kepler even invited scientific men to a conference on the subject, and appointed Frankfort as the place of rendezvous.
The changes in colour which accompany scintillation, and form one of its most important features, were noticed by early observers. Indeed the name given by the Arabs to Sirius refers to this fact ; they call it barakesch, or "the thousand.coloured star." Tycho, writing in 1572, respecting the new star of that year, compares it to the reflections of a cut diamond moving in the presence of light. Kepler also refers to the Dog Star as presenting by turns all the colours of the rainbow. Hooke, in his Micrographia ' refers to the various colours which accompany the scintillation of stars, appearing red at one moment, yellow at another, and blue at a third. Forster PhiL Mag.,' 1821) remarks that sometimes the intensely red light appears after two dilatations of the star, under other circumstances after three, but often without any apparently regular law. Several observers notice the scintillation of the planets ; but no astronomer refers to their change of colour. Scintillation in their case is a simple change in the intensity of the light.
The scintillation of a star, when viewed through a telescope, was first• described by Simon Marius, who recommends that the eyepiece be removed from the telescope, and the eye be substituted for it, at a time when the sky is very clear and the air tranquil. The scintillation will then appear like a fulmination or ebullition of the substance of the star, and certain determinate and distinct colours will appear in greater or less' abundance according to the stars observed. Thus Sirius presents green, yellow, red, and blue repeatedly following in the same order. Nicholson also in 1813 remarks that the circular disc of • star vacillates in such a manner as to give the idea of a number of discs passing in succession before each other. These discs are of different colours : the light appeared to come from different aides, and the moat frequent colours were blue, steel-blue, pea.green, brilliant copper, red, and white. The same observer viewing Sirius through an achromatic) by Ramsden, magnifying twenty-four times, and the eye-pieco adjusted for distinct vision, the observer struck lightly on the tube • series of rapid blows with his fingers, so as to make. the image of the star describe a luminous line, every part of which displayed the most lively colours, and it was caloulsted that the light of Sirius changed iu colour before arriving at the eye at least thirty times in a second.
Scintillation presents phenomena too remarkable to have been allowed to remain without attempts, at least, at explanation. Aristotle, who noticed the scintillation of the fixed stars, and the steady light of the planets, speaks of a want of fixity in our sight for such distant objects, while the planets being comparatively near can be viewed with a steadier gaze. Ptolemy also refers to the trepidation of the organs of sight, and the consequent trembling of the heavenly body. Allmon and Vitellion refer the twinkling of stars to the effects, of refraction which the stellar rays experience in our atmosphere. Aguil lonius explains it by the rapid movement of rotation of the stars. Tycho adopts such an explanation as when a diamond cut into facets rotates, but the planets do not scintillate because be imagined them not to rotate. Carden adopts Aristotle's explanation. Sealiger,
among other attempts at explanation, refers to the intermittent light of an incandescent body, and to the changes in the intensities of the stellar light produced by vapours floating in the atmosphere, Galileo's explanation assumes a vibration which stars impress upon their own light. Kepler compares the stars to diamonds cut into facets so that the least movement produces iridescent colours, or in other words he supposed a star to contain angular surfaces unequally luminous. Scheiuer imagines scintillation to be an optical illusion. Descartes explaius the phenomenon by means of his favourite vortices or tour billons, which he supposed to surround all celestial bodies. Huygbens explains scintillation by means of the vapours of the earth ; and Gassendi attributes it to a vibratory motion in the eye. Riccioli calls in the aid of atmospheric) vapours, and imagines also that particles of ' dust and opaque filaments floating in the air influence the phenomenon. Hooke explains it by the irregular refractions of the stellar rays in our atmosphere, arising from the unequal distribution of heat. Sir Isaac Newton refers to the refractive power of the humours of the eye, and also of the atmosphere, and speaks of a trembling movement in the latter. Jurin endeavours to explain scintillation by means of Newton's theory of fits of easy transmission and reflection. Cassini explains it by supposing a sort of luminous coma to surround the stars, and to undergo refraction and reflection in our atmosphere, but that these scattered rays being united through the intervention of a telescope the twinkling is less apparent. Long explains twinkling as the momentary disappearance of the stars, in consequence of the interposition of motes floating in the air. Mairan refers it to an undulatory move ment like that of the horizon seen over a vast plain illuminated by the sun. 31ichell'a explanation is based ou the corpuscular theory of light. Wanda refers to tho agitation of the air, and to motes float. Ing in it, forgetting, as Long had done, that these motes must be at least equal to the diameter of the pupil of the eye. Muschenbroeck refers to the vivacity of light and to the activity with which it acts on our organ of sight Darwin explains twinkling by means of the theory of accidental colours which was much discussed in his day, and explains it by the Law of contrast Saussure speaks of the oscillation of the luminous rays produced by tho alternate condensations and dilatations of certain part of the atmosphere. Dr. Thomas Young says,—" The cause of the twinkling of the stars is not fully ascertained, but it is referred, a ith some probability, to changes which are per petually taking place in the atmosphere, and which affect its refractive density. It is said that in some climates, where the air is remarkably serene, the stars have scarcely any appearance of twinkling." Arngo remarks on the singularity of the fact, that the author of the doctrine of interference of light, and indeed " of the only realty now experi ment which has been made on scintillation from the amt. of Aristotle to our own day, should not hesitate in facing the difficulties of the problem to say, ' I do not know.' There is more true merit in this candour than in the former unsatisfactory attempts at explanation." Nicholson, however, was equally candid, for he professed his inability to find in any of the known properties of light the Cause of the plea. nomena of twinkling. Mot in his ' A/arena:nisi' bases his explanation on the unequal refraction of the rays by our atmosphere. Forster imagines that the changes in colour which accompany twinkling must be duo to some change in the star itself, or that our atmosphere acts like a prism on the rays of light. Capocei refers the phenomena to our eye and not to any changes in the star, while Kiimtz, in hie ' Matoorologie; refers to a supposed oscillation of the star as a mere point of light about its mean position, while a planet having an apparent diameter of from 30 to 40 seconds, it is more difficult to appreciate its apparent change in volume.