Because the umbral shadow's diameter is typically ˜2.7x the Moon's diameter, it appears as though a semi-circular bite has been taken out of the Moon.Īristotle (384-322 BCE) first proved that Earth was round using the curved umbral shadow seen at partial eclipses. This is primarily due to a contrast effect because the remaining portion of the Moon in the penumbra may be brighter by a factor of about 500x. The lunar limb extending into the umbral shadow usually appears very dark or black. In comparison, partial eclipses are easy to see with the naked eye. For more information, see: Partial Lunar Eclipse of 2012 Jun 04 Appearance of Partial Lunar Eclipses Various stages of the partial lunar eclipse of 2012 Jun 04 are captured in this time sequence. The penumbral eclipse only becomes readily apparent when it is within ˜0.05 magnitudes of becoming a partial eclipse. Under such conditions, the Moon remains relatively bright with only a subtle gradient across its disk. A penumbral magnitude greater than ˜0.6 is needed before skilled observers can detect faint shading across the Moon's disk.Įven when one edge of the Moon is 9/10 of the way into the penumbral shadow, approximately 10% of the Sun's rays stillreach the Moon's deepest limb. The primary penumbral contacts (P1 and P4), as well as the early and late stages of a penumbral eclipse, are completelyinvisible to the eye with or without optical aid.
Thus, some fraction of the Sun's direct rays continues to reach the most deeply eclipsed parts of the Moon during a penumbral eclipse. From within this zone, Earth blocks part but not the entire disk of the Sun. (click for larger image) Appearance of Penumbral Lunar EclipsesĮarth's penumbral shadow forms a diverging cone that expands into space in the anti-solar direction. The two photos show the Moon shortly before the eclipse began (left) and at mid-eclipse when 88.9% of the Moon's diameter was immersed in the penumbral shadow (right). This high contrast image of the penumbral lunar eclipse of 2002 Nov 20 shows how subtle these eclipses are.
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