La meridiana naturale del Col delle Ventidueore a Valstagna

Autore: Angelo Chemin

Il Col delle ventidueore è una piccola giogaia di monti che si dirama dal monte Cornon, alto 1065 mt, partendo da un’altezza di 760 mt slm, fino a raggiungere il fiume Brenta a Ponte Subiolo ad un’altezza di mt 152 slm. Alla quota media di 500 mt. slm si trova il tipico insediamento di mezzacosta del Pian delle Casarette. L’estensione del colle misurata lungo la linea equinoziale congiungente il Piangrande con il Pian delle Casarette e il fiume Brenta è di 1300mt.

Il Col delle Ventidue ore costituisce una meridiana naturale. Deve il suo nome al fatto che, con buona approssimazione, il colle è ancora illuminato dal sole due ore prima del tramonto, nel computo orario all’uso italico, mentre il resto della valle è in ombra.

Il computo all’uso italico conta le ore a partire dal tramonto che coincide con le ore ventiquattro, in quell’istante finisce il giorno in corso e incomincia quello nuovo con la notte e il giorno seguente che si conclude al tramonto successivo. Le ore italiche sono ore diseguali quindi le ore di sole sono computate più corte nell’autunno-inverno e più lunghe nella primavera-estate. Questo sistema fu in uso fino a tutto il secolo XIX, mentre quello ad ore astronomiche (o alla francese) incominciò a prendere piede nella seconda metà del sec. XIX. Il sistema italico era funzionale ad una suddivisione delle ore di luce utile particolarmente per i lavori agricoli.

Nel caso del Col delle Ventidueore lo gnomone è costituito dal pianoro e dalla piccola sella del Piangrande che permettono ai raggi del sole di illuminare il versante Sud anche quando il resto della valle è in ombra. Il luogo d’osservazione e di utilizzo è l’abitato di Valstagna. Questa particolare posizione del Colle e del Pian delle Casarette fa in modo che l’irraggiamento solare sia maggiore rispetto ad altri siti. La tradizione orale riferisce che la legna necessaria per riscaldarsi nell’inverno era “metà” rispetto a quella necessaria nella “busa” di Valstagna, cioè sul fondovalle. Il sito fu abbandonato nei primi anni ’60 del 1900. Le campagne di prospezione degli anni 2005, 2006, 2007 hanno messo in luce in particolare la presenza della vite che ha bisogno di una buona insolazione.

Incidenza dei raggi solari nei vari mesi dell’anno:

Esempio di ombra alle ore 15:00 del primo giorno del mese:

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Alessandro Tessaro e Chiara Lago

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