Sky and Telescope - February 2017 - 48

FEBRUARY 2017 OBSERVING
Celestial Calendar by Alan MacRobert

A Deep Penumbral Lunar Eclipse
Watch the Moon flirt with Earth's shadow on the evening of February 10th.

a tease, but it illustrates
principles of light and
shade you may not have
thought about. Watch
them play out on a
grand scale.

S A penumbral eclipse not quite as deep as
the one we're about to get occurred over the
Far East on November 28, 2012.

48

F E B R U A R Y 2 0 1 7 * SK Y & TELESCOPE

N

o shadow that you normally see has a sharp edge. A shadow is
always surrounded by a hazy penumbra, unless the light casting it
comes from a point source. Neither a light bulb nor the Sun qualify.
The penumbra is where, if you were in it, you would see the shadowcasting object cover part of the light source but not all of it. So a penumbra's width, as seen from the shadow-casting object, always equals
the angular width of the light source. Which is why, in the diagram for
the upcoming penumbral lunar eclipse at the top of the facing page, the
penumbra of Earth's shadow is almost exactly as wide as the Moon. As
seen from Earth, both the Sun and Moon appear very nearly the same
width: about ½° across.
A penumbral eclipse is a tease. None of the Moon enters Earth's
interestingly dark-red umbra, as happens during a partial or total lunar
eclipse. But on Friday evening February 10th (for North America), we'll
see just about the best penumbral eclipse possible. The Moon's northern
limb will miss the Earth's umbra by only about 100 miles (160 km), or
5% of the Moon's diameter. Although none of the Moon will go dark,
the penumbral shading will be very plain to see.

HONG KONG SPACE MUSEUM

A penumbral eclipse is



Sky and Telescope - February 2017

Table of Contents for the Digital Edition of Sky and Telescope - February 2017

Contents
Sky and Telescope - February 2017 - Cover1
Sky and Telescope - February 2017 - Cover2
Sky and Telescope - February 2017 - 1
Sky and Telescope - February 2017 - Contents
Sky and Telescope - February 2017 - 3
Sky and Telescope - February 2017 - 4
Sky and Telescope - February 2017 - 5
Sky and Telescope - February 2017 - 6
Sky and Telescope - February 2017 - 7
Sky and Telescope - February 2017 - 8
Sky and Telescope - February 2017 - 9
Sky and Telescope - February 2017 - 10
Sky and Telescope - February 2017 - 11
Sky and Telescope - February 2017 - 12
Sky and Telescope - February 2017 - 13
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