Solstizio D’estate. Giorno di Massima luce solare

21 2011

Solstizio D’estate. Giorno di Massima luce solare

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21 Giugno: Il solstizio in astronomia è definito come il momento in cui il Sole raggiunge, nel suo moto apparente lungo l’eclittica, il punto di declinazione massima o minima.

Il fenomeno è dovuto alla inclinazione dell’asse di rotazione terrestre rispetto all’eclittica; il valore di declinazione raggiunta coincide con l’angolo di inclinazione terrestre e varia con un periodo di 41000 anni tra 22.1° e 24.5°.

Attualmente è di 23°27’ e l’angolo è in diminuzione.

Il Sole raggiunge il valore massimo di declinazione positiva nel mese di giugno in occasione del solstizio di estate boreale, mentre raggiunge il massimo valore di declinazione negativa in dicembre, in occasione del solstizio di inverno boreale, corrispondente all’estate nell’emisfero australe.

Da un esame di una tabella dei tempi dei solstizi si può verificare che il fenomeno ritarda di circa sei ore ogni anno (5 ore, 48 minuti e 46 secondi per la precisione), salvo subire un nuovo riposizionamento indietro ogni quattro anni, in conseguenza degli anni bisestili, introdotti proprio per evitare un progressivo disallineamento delle stagioni con il calendario. A causa di queste variazioni può capitare che il solstizio astronomico cada il 20 o il 21 giugno per l’estate nell’emisfero nord o il 21 o 22 dicembre per l’inverno nell’emisfero nord. (/Wikipedia) -

Solstizio d’estate (Giugno)
2011 - 21 Giu 17:16
2012 - 20 Giu 23:09

Solstizio d’inverno (Dicembre)
2011 - 22 Dic 05:30
2012 - 21 Dic 11:11

Source by Redazione

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