...The umbra is generally uniformly dark, butat times filmy luminous clouds have been observed floating over it... 
Thomas Orchard 「The Astronomy of Milton's 'Paradise Lost'」
...Thenucleus is believed to be the orifice of a tubular depression in thefloor of the umbra, prolonged downwards to an unknown depth...
Thomas Orchard 「The Astronomy of Milton's 'Paradise Lost'」
...Thepenumbra is brightest at its inner edge, where the filaments present amarked contrast when compared with the dark cavity of the umbra whichthey surround and overhang...
Thomas Orchard 「The Astronomy of Milton's 'Paradise Lost'」
...In some instances the umbra alone has a breadth of 20,000 or30,000 miles—three times the extent of the diameter of the Earth...
Thomas Orchard 「The Astronomy of Milton's 'Paradise Lost'」
...A spot is usually composedof a dark central portion called the umbra, and a less dark fringearound this called the penumbra (, p...
Cecil G. Dolmage 「Astronomy of To-day」
...The umbra ofa good-sized spot could indeed engulf at once many bodies the size ofour earth...
Cecil G. Dolmage 「Astronomy of To-day」
...The umbra of a spot is not actually dark...
Cecil G. Dolmage 「Astronomy of To-day」
...When asun-spot has completed its period of existence, the photospheric matteroverwhelms the penumbra, and rushes into the umbra, which itobliterates, causing the spot to disappear...
Thomas Orchard 「The Astronomy of Milton's 'Paradise Lost'」
...It is bordered onboth sides by the spectrum of the penumbra, which is much brighterthan that of the umbra, but fainter than that of the adjoining regionsof the sun...
Robert Stawell Ball 「The Story of the Heavens」
...From the fact that the spectrum is darkened we learn that there isconsiderable general absorption of light in the umbra...
Robert Stawell Ball 「The Story of the Heavens」
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