...The darkest portion of the penumbra is its external edge, whichstands out conspicuously against the adjoining bright surface of theSun... 
Thomas Orchard 「The Astronomy of Milton's 'Paradise Lost'」
...One penumbra will sometimes enclose several umbræ whilst the nucleimay be entirely wanting...
Thomas Orchard 「The Astronomy of Milton's 'Paradise Lost'」
...13, in which the dark centralnucleus appears in sharp contrast with the lighter margin or penumbra...
Robert Stawell Ball 「The Story of the Heavens」
...Grover has not only seen theabove-named features, but even a penumbra to the shadow on the ringswith a 2-inch telescope...
Richard A. Proctor 「Half-hours with the Telescope」
...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」
...The two atmospheric layers, which will be of varying thickness about aspot, will account for all the shades of darkness seen in the penumbra...
Edward Singleton Holden 「Sir William Herschel: His Life and Works」
... The time occupied by the Moon in passing throughthe penumbra, before and after a lunar eclipse, will generallyrun to about an hour for each passage...
George Chambers 「The Story of Eclipses」
...The shadow of the fourth, for instance, isnearly all penumbra, the really black part being quite minute bycomparison...
Richard A. Proctor 「Half-hours with the Telescope」
...The shadow of the third has a considerable penumbra, andeven that of the first is not wholly black...
Richard A. Proctor 「Half-hours with the Telescope」
...The penumbra attending the earth's shadow is usually supposed to render this observation uncertain to two or three minutes of time, or more than half a degree of longitude...
Matthew Flinders 「A Voyage to Terra Australis」
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