a guide to the sky
Visions of the past
By MARCELO DE OLIVEIRA SOUZA
Sky ’ s Up Editor
“ When , at night , the Infinite rises The moonlight , along the fallen paths My tactile intensity is so much That I feel the soul of Cosmos in my fingers !...”* — Augusto dos Anjos ( Brazilian poet )
* Free translation to English of the original poetry in Portuguese
A starry night is a magnificent time for wonder and reflection . Stars are bright spots that sensitize our retina . The history of the Universe is present in the images formed in the mind and soul . In brief daydreams and dream trips to distant worlds . Poets , lovers , and dreamers , at that moment of contemplative observation , cannot imagine that they are peering into relics of the Universe . For a long time , humanity considered the propagation of light to be instantaneous . In the past there are some records of different opinions . Aristotle , despite considering the instantaneous propagation , in “ De Sensu ”, reports that Empedocles ( 492 BC – 432 BC ) said that sunlight should take a finite time to reach Earth . Galileo Galilei ( 1564 – 1642 ) in his book “ Discorsi e dimonstrazioni matematiche intorno a due nuove scienze ” presents a brief discussion of the propagation of light . One of the characters in the book , Simplicio , who defends Aristotle ’ s ideas , states that “ experiences in our daily life show that the propagation of light is instantaneous ”. As an example , he cites that if we are very far away , the light emitted by a shot reaches our eyes before the noise of the shot . Another character shows that from this fact it is not possible to state that the
COURTESY
Above , in his book “ Discorsi e dimonstrazioni matematiche intorno a due nuove scienze ”, Galileo Galilei presented a brief discussion of the propagation of light . Below , illustration from 1676 article by Danish astronomer Ole Römer on the measurement of the speed of light .
propagation of light is instantaneous . One can only deduce that light travels faster than sound . Galileo performed an experiment to determine the speed of propagation of light . Two people with covered lamps stand in positions far apart . One of them discovers the lamp . When the other person perceives the light coming from that lamp , they immediately discover yours . In the experiment carried out by Galileo , it was not possible to notice , with the instruments available at the time , any delay . In 1676 the Danish astronomer Ole Römer estimated for the first time the propagation speed of light . It was based on observation of the movement of Io ( one of its 63 known moons ) around the planet Jupiter . It obtained a value much lower than that considered nowadays . The French physicist Armand Fizeau in 1849 determined this value with good precision , from an experiment carried out in the laboratory and based on Galileo ’ s proposal . Nowadays we consider the value of approximately 300,000km / s . In a second the light travels 300.000km !!! The average distance between the Earth and the Moon is approximately 384,000km . So whenever we see her
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