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Subtle Lunar Eclipse

Today’s (Feb. 10) lunar activity comes
in the form of a penumbral
eclipse
. What does that mean and how does this type differ from a total
eclipse? Let’s take a look:

image

First off, what is a penumbra? During a lunar eclipse, two shadows are cast by the Earth. The first is called the umbra (UM bruh). This
shadow gets smaller as it goes away from the Earth. It is the dark center of the
eclipse shadow where the moon is completely in the shadow of the Earth.

image

The second shadow is called the penumbra (pe
NUM bruh). The penumbra gets larger as it goes away from the Earth. The penumbra
is the weak or pale part of the shadow. This occurs because the Earth is covering a portion of the sun.

image

Penumbral eclipses occur when only the outer
shadow (the penumbra) of Earth falls on the moon’s surface. This type of eclipse is much more
difficult to observe than total eclipses or when a portion of the moon passes into the umbra. That said, if you’re very
observant, you may notice a dark shadow on the moon during mid-eclipse on
Friday evening. You may not notice anything at all. It’s likely the moon will
just look at little bit darker than normal…like this: 

image

Earth’s penumbral shadow forms a diverging
cone that expands into space in the opposite direction of the sun. From within
this zone, Earth blocks part but not the entire disk of the sun. Thus, some
fraction of the sun’s direct rays continues to reach the most deeply eclipsed
parts of the moon during a penumbral eclipse.

For most of North America, the penumbral
eclipse will begin at moonrise (sunset) on Friday, Feb. 10 and will be obscured by
evening light. Here’s a guide of when to look up:

image

Fun fact: Aristotle (384 – 322 BCE) first
proved that Earth was round using the curved umbral shadow seen at partial
eclipses. In comparing observations of several eclipses, he noted that Earth’s
shadow was round no matter where the eclipse took place. Aristotle correctly
reasoned that only a sphere casts a round shadow from every angle.

To learn more about lunar eclipses, visit: https://svs.gsfc.nasa.gov/11828

Make sure to follow us on Tumblr for your regular dose of space: http://nasa.tumblr.com

Source: NASA

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Subtle Lunar Eclipse

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