*THE WANDERING ASTRONOMER*
Tuesday, October 24, 2023
Partial Lunar Eclipse?


Eclipses are not solitary.  They occur in pairs. A solar eclipse will
either be followed or preceded by a lunar eclipse by almost exactly two
weeks.       The solar eclipse that dazzled millions on October 14, 2023
will be followed by a partial lunar eclipse on October 28th that, well,
could possibly raise the eyebrows of dozens.      While total lunar
eclipses are capable of instilling awe and rapture in even the most s
partial eclipses, especially those that are largely penumbral, often fail
to grip.    We can be well assured that bustling throngs will not
congregate in New England for this event simply because it won't be visible
here, for the most part.

Let's proceed to the graphic:

[image: Lunar_eclipse_chart_close-2023Oct28.png]

We can see that during this eclipse, the full moon will glide gracefully
through the penumbra, the outer part of Earth's shadow.  During the
mid-eclipse, the moon's southern limb will pass along the uppermost region
of the umbra.   Consequently, a small patch of darkness, like a disquieting
rumor of Mordor, will play about the moon's lower edge.          However,
during most of this eclipse, the moon will only be steeped in the penumbra.
The resultant obscuration shall be so slight as to be imperceptible.

As we can see from this next graphic, this event will be wholly observable
from Europe, as well as most of Africa and Asia.

[image: Visibility_Lunar_Eclipse_2023-10-28.png]


Notice that the northeastern United States is located within the region
between U4-P4.   This designation indicates that the moon will rise toward
the last phase of the eclipse: the time when the moon is leaving the
penumbra.   The moon will have left the umbra before it even rises from our
perspective.     In other words, there will be nothing to see, especially
considering that the atmospheric obscuration is at a maximum along the
horizon.

That having been written, this eclipse is not wholly without interest.
Notice that the moon will be moving along the top of the umbra/penumbra
during this partial eclipse.    This position indicates that it is either
toward the end or the beginning of its Saros Cycle.    A Saros Cycle
consists of a series of eclipses that share the same geometry.  Successive
eclipses within a given Saros are separated by approximately 18 years and
11 days.

If the Saros cycle begins at the upper part of Earth's shadow, each
subsequent eclipse will be progressively lower than the one preceding
it.    Conversely, if the Saros cycle begins with the moon at the lower
part of Earth's shadow, each subsequent eclipse within the cycle will move
progressively higher.     A Saros cycle begins at the top of the shadow
when the moon is at or near the *ascending node, *the intersection point
between Earth's orbit and that of the moon.  When at the ascending node,
the moon moves 'north' of Earth's orbit after moving through the
intersection point.  The Saros cycle begins at the bottom of the shadow
when the moon is at or near the  *descending node*.

This partial lunar eclipse is the 11th eclipse within Saros Cycle
146.        We can therefore conclude that it is within the beginning
phases of the cycle.  First,  Saros cycles generally consist of more than
70 eclipses.    Secondly, Saros cycles occurring at the ascending node are
assigned even designation numbers.            The partial eclipse that we
won't see on Saturday is just one of many Saros cycle 146 eclipses that
haven't yet happened.

The first Saros 146 lunar eclipse  (July 11, 1846) was partially
penumbral.  The moon's lower limb barely touched the upper part of the
penumbra.  The second 146 lunar eclipse (July 21, 1861) was slightly lower,
but still partially penumbral, as was the next one, which occured on August
2, 1879.    The first partial lunar eclipse in the 146 series only happened
on October 17, 2005.  The eclipse this coming Saturday is the one
immediately following that 2005 event in the 146 series.    Over the next
two centuries, each Saros 146 eclipse will shift farther into the umbra
than the one before it.    The first total lunar eclipse in Saros 146 won't
occur until May 25, 2366! That one will not be visible from New England.
The final total lunar eclipse of this series will happen on November 16,
2654.     The following Saros 146 eclipses will continue the descent and
each one will be less covered by the umbra than the previous eclipses in
that sequence.  The final partial lunar eclipse happens on June 12, 2997
and the final penumbral on August 29, 3123.

While this Saturday's event won't captivate us visually, we can reflect
that we're watching (or, in our case, not watching) an eclipse that is part
of a sequence that started the year the Liberty Bell was cracked and won't
end for more than a thousand years from now.

Carpe Diem.