ASTRONOMIA
Urano non era noto agli antichi a causa della sua piccola magnitudine (5,7), quasi al limite della visibilità per l’occhio umano.
La sua scoperta avvenne nel 1781, quando già era in uso il telescopio.
Fu scoperto da W. Herschel, il quale, nel corso delle sue abituali osservazioni del cielo, notò un oggetto insolito che ben presto riconobbe essere un pianeta dal moto lento fra le stelle. Osservato al telescopio, Urano appare come un minuscolo disco verdastro, troppo piccolo perché si possano individuare dettagli significativi della superficie.
Urano ruota intorno al Sole in senso retrogrado e il suo asse è inclinato di 82° rispetto alla perpendicolare del piano dell’orbita.
La sonda Voyager 2, che il 24 gennaio 1986 è arrivata a soli 73.000 km dal pianeta, ha permesso di aggiornare le nostre conoscenze.
Si è accertato che:
il periodo di rotazione del pianeta intorno al proprio asse è compreso fra le 15 e le 17 ore
la temperatura, ai livelli delle nubi esterne, è di -210 °C
il campo magnetico ha una intensità di 0,25 gauss.
Urano presenta un sistema di dieci anelli. Di questi cinque furono scoperti nel 1977 mediante lo studio dell’occultazione, da parte del pianeta, di una stella che, al passaggio di ogni anello, veniva temporaneamente oscurata. Altri quattro furono scoperti nel 1978 con lo stesso metodo. Il decimo fu rivelato nel 1986 dalla sonda Voyager 2.
Miranda, Ariel, Umbriel, Titania, Oberon sono nell’ordine crescente di distanza dal pianeta, i satelliti di Urano noti prima dei dati trasmessi dalla sonda Voyager 2.
Ruotano in senso retrogrado nel piano equatoriale del pianeta. Il più piccolo di essi è Miranda con un diametro di circa 500 km, mentre il maggiore è Oberon che ha un diametro di circa 1600 km. Il numero dei satelliti attualmente conosciuti di Urano è quindici, ma è probabile che tale numero aumenti.
a cura di Cristina tartaglino
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