Sky & Telescope - January 2019 - 19

before the last trace of penumbral shading leaves the eastern
An astronaut standing inside the umbra on the Moon's
(right-hand) edge of the lunar disk?
surface would see the Sun entirely hidden by Earth. But
viewed from inside the penumbra, the Sun's disk would be
Enlargement of the umbra: Observers have long realonly partly covered - varying from a small scallop as the
ized that the umbra's diameter averages about 2% larger than
eclipse begins (with an imperceptible dimcalled for strictly by the geometry of the
ming of the lunar landscape) to only a sliver
The umbra's diameter av- eclipse, and yet it can vary from eclipse to
of sunlight just before totality, with the sureclipse for reasons that are not fully undererages about 2% larger
roundings noticeably darker.
stood. The most likely explanation is that it
than called for strictly
Hence, the outermost part of the penis due to the varying overall transparency of
by the geometry of the
umbra is so lightly shaded that initially
Earth's upper atmosphere.
eclipse, and yet it can
you won't notice any change in the Moon's
The amount of enlargement during this
vary from eclipse to
appearance at all. But by the time the Moon's
upcoming eclipse can be deduced after the
eclipse for reasons that
leading edge gets about two-thirds across
fact from careful telescope-aided timings of
are
not fully understood. when lunar craters enter or leave the umbra.
the penumbra, about 40 minutes later, you
might begin to detect a very weak dimming
If your sky is clear, the edge of the advancing
of the Moon's western limb (the left edge for northerners) -
or retreating umbra is sharp enough to permit crater timings
the ﬁrst visible sign that an eclipse is in progress.
accurate to a few tenths of a minute.
How early can you see this with certainty? As the Moon
If you'd like to try this, Senior Contributing Editor Roger
advances deeper into the penumbra, the shading becomes
Sinnott has prepared predictions of the entrance and exit
much stronger and more obvious. Pay attention to the pentimes for 24 prominent lunar features (listed on page 20).
umbra's color (brown? gray?) and its overall darkness.
Note that these are given in Universal Time, so be sure to
With the full Moon so high up this January as seen from
make the correct offset for your time zone. Also, before the
the U.S. and Canada, an early penumbral sighting could be
eclipse begins, I would advise anyone unfamiliar with the
especially problematic due to the lunar disk's overpowering
Moon's features to identify these locations on a lunar map.
glare. It might help to wear dark glasses - really! One trick
Just before the eclipse, be sure to set your watch accurately
I've used in the past is to project the Moon's image with binfrom a cell-phone time app (such as Emerald Time) that
oculars or a telescope onto white paper or cardboard.
displays to the nearest second. The listed times refer to each
crater's center, so start to watch a given feature several minPartial stages: The partial eclipse begins when the much
utes before the prediction.
darker umbral shadow ﬁrst touches the lunar limb. The
Record your timings to the nearest 5 seconds, even though
change is dramatic: A dark dent forms on the Moon's westyour uncertainty might be 10 to 20 seconds or more. Once
ern side, and as the minutes tick by the dent becomes a big,
the eclipse is over, you can email your timings to Sinnott at
rounded bite. The curved edge of the shadow gives a nakedrsinnott@post.harvard.edu to aid his ongoing analysis of
eye indication that Earth is nearly spherical (no shape other
these events. In your report, mention your full name, telethan a sphere always casts a circular shadow).
scope aperture, magniﬁcation, and sky conditions.
During the eclipse's partial stages, you'll easily see the
penumbra as a dusky border to the shadow's dark core. After
The total eclipse: The umbra appears quite dark during
the last bit of umbra disappears, how much time elapses
the opening stages of partial eclipse (or the late stages after

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Daytime
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t WORLD VIEW
This map shows
where the Moon
will rise or set
during stages of
the eclipse - and,
since an eclipsed
Moon is always
full, the Sun sets
or rises at almost
the same time on
the opposite horizon. For January's
event, the sky is
dark and the Moon
well up across
both North and
South America
during all phases
of the eclipse.

sk yandtele scope.com * JA N UA RY 2 019

http://www.skyandtelescope.com

Sky & Telescope - January 2019

Table of Contents for the Digital Edition of Sky & Telescope - January 2019